Chap 2: Chemical symbols, formulae and valency

# Chap 2: Chemical symbols, formulae and valency ## Names of compounds ### Introduction Welcome to this lesson on naming chemical compounds! In chemistry, compounds are all around us, from the water we drink to the salt we use in our food. Just as people have names, chemical compounds also have specific names that allow scientists worldwide to communicate clearly about them. In this lesson, we will learn the systematic rules for naming different types of chemical compounds, making it easier for you to understand and describe the world of chemistry. ### Key Concepts #### What is a Compound? A **compound** is a substance formed when two or more different chemical elements are chemically bonded together. Unlike mixtures, compounds have a fixed composition and specific properties that are different from the elements they are made from. For example, water (\( H_2O \)) is a compound made from hydrogen and oxygen. Its properties (liquid at room temperature, no taste or smell) are very different from hydrogen (a flammable gas) and oxygen (a gas that supports combustion). #### Types of Compounds We can generally classify compounds into two main types based on the type of chemical bond holding them together: ##### 1. Ionic Compounds * **Formation:** Formed when a metal atom transfers one or more electrons to a non-metal atom. This creates positively charged ions (**cations**) and negatively charged ions (**anions**), which are then attracted to each other. This usually occurs between a metal and a non-metal. * **Characteristics:** Often solids with high melting points. ##### 2. Covalent Compounds * **Formation:** Formed when two non-metal atoms share electrons. This usually occurs between two non-metals. * **Characteristics:** Can be solids, liquids, or gases with relatively lower melting points. #### Naming Ionic Compounds Naming ionic compounds follows a set of straightforward rules: ##### Rule 1: Binary Ionic Compounds (Metal + Non-metal) 1. **Name the metal first.** The metal keeps its original name. 2. **Name the non-metal second.** Change the ending of the non-metal's name to "-ide". * **Examples:** * \( NaCl \): Sodium chloride (Sodium + Chlorine \(\rightarrow\) Chlor*ide*) * \( MgO \): Magnesium oxide (Magnesium + Oxygen \(\rightarrow\) Ox*ide*) * \( KF \): Potassium fluoride (Potassium + Fluorine \(\rightarrow\) Fluor*ide*) * \( Al_2S_3 \): Aluminium sulfide (Aluminium + Sulfur \(\rightarrow\) Sulf*ide*) ##### Rule 2: Ionic Compounds with Transition Metals (Metals with Variable Valency) Some metals, especially transition metals, can form ions with different charges (valencies). To avoid confusion, we use Roman numerals in parentheses after the metal's name to indicate its charge. 1. **Name the metal first**, followed by a Roman numeral in parentheses indicating its charge. 2. **Name the non-metal second**, changing its ending to "-ide". * **Common Roman Numerals:** * I = 1 * II = 2 * III = 3 * IV = 4 * V = 5 * **Examples:** * \( FeCl_2 \): Iron(II) chloride (Iron has a \( +2 \) charge, Chlorine is \( -1 \)) * \( FeCl_3 \): Iron(III) chloride (Iron has a \( +3 \) charge, Chlorine is \( -1 \)) * \( CuO \): Copper(II) oxide (Oxygen is \( -2 \), so Copper must be \( +2 \)) * \( Cu_2O \): Copper(I) oxide (Oxygen is \( -2 \), so each Copper must be \( +1 \)) ##### Rule 3: Ionic Compounds with Polyatomic Ions A **polyatomic ion** is a group of atoms that are covalently bonded together but carry an overall charge, behaving as a single ion. When naming compounds with polyatomic ions, you name the ions involved. 1. **Name the metal (or first element)**. If it's a transition metal, use Roman numerals. 2. **Name the polyatomic ion.** (Do NOT change its ending). * **Common Polyatomic Ions to Know:** * \( OH^- \): Hydroxide * \( NO_3^- \): Nitrate * \( SO_4^{2-} \): Sulfate * \( CO_3^{2-} \): Carbonate * \( PO_4^{3-} \): Phosphate * \( NH_4^+ \): Ammonium (This is the only common polyatomic cation) * **Examples:** * \( NaOH \): Sodium hydroxide * \( Ca(NO_3)_2 \): Calcium nitrate * \( (NH_4)_2SO_4 \): Ammonium sulfate * \( Fe_2(CO_3)_3 \): Iron(III) carbonate #### Naming Covalent Compounds (Binary Covalent Compounds) Covalent compounds are typically formed between two non-metals. We use prefixes to indicate the number of atoms of each element present in the compound. ##### Rule 4: Binary Covalent Compounds (Non-metal + Non-metal) 1. **Name the first non-metal.** 2. **Name the second non-metal**, changing its ending to "-ide". 3. **Use Greek prefixes** before each element's name to indicate the number of atoms of that element. The prefix "mono-" is usually omitted for the first element if there is only one atom. * **Greek Prefixes:** * 1: mono- * 2: di- * 3: tri- * 4: tetra- * 5: penta- * 6: hexa- * 7: hepta- * 8: octa- * 9: nona- * 10: deca- * **Examples:** * \( CO \): Carbon monoxide (One carbon, one oxygen. "mono" is omitted for carbon) * \( CO_2 \): Carbon dioxide (One carbon, two oxygens) * \( SO_2 \): Sulfur dioxide * \( PCl_3 \): Phosphorus trichloride * \( N_2O_4 \): Dinitrogen tetroxide * \( SF_6 \): Sulfur hexafluoride ### Vocabulary * **Compound:** A substance formed when two or more different chemical elements are chemically bonded together. * **Ionic Compound:** A compound formed by the transfer of electrons between a metal and a non-metal. * **Covalent Compound:** A compound formed by the sharing of electrons between two non-metals. * **Cation:** A positively charged ion. * **Anion:** A negatively charged ion. * **Polyatomic Ion:** A group of atoms covalently bonded together that carries an overall electrical charge and acts as a single ion. * **Prefix:** A word part added to the beginning of a word to modify its meaning (e.g., di-, tri-). * **Suffix:** A word part added to the end of a word to modify its meaning (e.g., -ide). * **Valency:** The combining power of an element or radical, especially as measured by the number of hydrogen atoms it can combine with or replace. (Crucial for determining the charge of transition metals). ### Examples Let's put the rules into practice: #### Naming from Formula: 1. \( KCl \): Potassium chloride (Metal + Non-metal) 2. \( Fe_2O_3 \): Iron(III) oxide (Transition metal, oxygen is \( -2 \), so \( 3 imes -2 = -6 \). Thus, \( 2 imes +3 = +6 \) for iron. Hence Iron(III).) 3. \( SO_2 \): Sulfur dioxide (Non-metal + Non-metal, use prefixes) 4. \( CCl_4 \): Carbon tetrachloride (Non-metal + Non-metal, use prefixes) 5. \( Na_2SO_4 \): Sodium sulfate (Metal + Polyatomic ion, \( SO_4^{2-} \) is sulfate) 6. \( Ca(OH)_2 \): Calcium hydroxide (Metal + Polyatomic ion, \( OH^- \) is hydroxide) #### Writing Formula from Name: 1. **Calcium bromide:** Calcium is \( Ca^{2+} \), Bromide is \( Br^- \). To balance charges: \( CaBr_2 \). 2. **Dinitrogen pentoxide:** "Di" means 2 Nitrogen, "pent" means 5 Oxygen. \( N_2O_5 \). 3. **Iron(III) sulfate:** Iron(III) is \( Fe^{3+} \), Sulfate is \( SO_4^{2-} \). To balance charges: \( Fe_2(SO_4)_3 \). 4. **Magnesium nitride:** Magnesium is \( Mg^{2+} \), Nitride (Nitrogen) is \( N^{3-} \). To balance charges: \( Mg_3N_2 \). ### Exercises #### Part A: Name the following compounds: 1. \( LiF \) 2. \( K_2O \) 3. \( CuCl_2 \) 4. \( MnO_2 \) 5. \( Al(OH)_3 \) 6. \( CaCO_3 \) 7. \( N_2O \) 8. \( PCl_5 \) 9. \( SiO_2 \) 10. \( SnO \) #### Part B: Write the chemical formula for the following compounds: 1. Sodium iodide 2. Barium sulfate 3. Nickel(II) nitrate 4. Potassium phosphate 5. Sulfur trioxide 6. Carbon disulfide 7. Diphosphorus pentoxide 8. Iron(II) hydroxide ### Assignment #### Part A: Name the following compounds, paying close attention to type of compound: 1. \( MgBr_2 \) 2. \( Fe_2(SO_4)_3 \) 3. \( P_4O_{10} \) 4. \( AgF \) 5. \( Cu(NO_3)_2 \) 6. \( NH_4Cl \) 7. \( HCl \) (Remember, this can also be named as hydrogen chloride when it's a gas) 8. \( CoO \) 9. \( CBr_4 \) 10. \( Li_2CO_3 \) #### Part B: Write the chemical formula for the following compounds: 1. Calcium phosphide 2. Lead(IV) oxide 3. Ammonium carbonate 4. Nitrogen trichloride 5. Zinc oxide 6. Chromium(III) sulfide 7. Sulfur dichloride 8. Potassium permanganate (Permanganate is \( MnO_4^- \)) ### Summary In this lesson, you've learned the fundamental rules for naming chemical compounds. We distinguished between **ionic compounds** (formed between metals and non-metals) and **covalent compounds** (formed between two non-metals). For ionic compounds, remember to name the metal first and change the non-metal's ending to "-ide". For transition metals, use Roman numerals to indicate their charge. Polyatomic ions keep their special names. For covalent compounds, use prefixes (mono-, di-, tri-, etc.) to indicate the number of atoms of each element. Mastering these naming conventions is a crucial step in understanding and communicating in the language of chemistry! ## Chemical symbols and names of elements. ### Introduction Welcome to our lesson on chemical symbols and names of elements! In chemistry, elements are the basic building blocks of all matter. To make it easier for chemists and scientists around the world to communicate and understand each other, each element has been assigned a unique, short symbol. In this lesson, we will learn why these symbols are used, how they are formed, and identify some of the most common elements and their symbols. ### Key Concepts #### What are Elements? An element is a pure substance consisting of only one type of atom. They cannot be broken down into simpler substances by ordinary chemical means. Examples include hydrogen, oxygen, and gold. #### Why Do We Use Chemical Symbols? Imagine writing out the full name of every element in a complex chemical reaction! It would be very time-consuming and prone to errors. Chemical symbols provide a concise, universal language for scientists worldwide. They act as a shorthand, making it easier to write chemical formulas and equations. #### Rules for Chemical Symbols Chemical symbols were first introduced by the Swedish chemist Jöns Jacob Berzelius in the early 19th century. Here are the main rules for assigning and writing them: * **One-Letter Symbols:** Some elements have a symbol consisting of a single capital letter. For example: * Hydrogen: H * Oxygen: O * Carbon: C * Nitrogen: N * **Two-Letter Symbols:** Most elements have a symbol consisting of two letters. The first letter is always capitalized, and the second letter is always lowercase. For example: * Helium: He * Calcium: Ca * Neon: Ne * **Origin of Symbols:** * **English Names:** Many symbols are derived directly from the English name of the element (e.g., Carbon - C, Oxygen - O, Chlorine - Cl). * **Latin or Greek Names:** Some elements have symbols derived from their older Latin or Greek names. This is important to remember as their symbol might not directly match their English name. For example: * Sodium (English) comes from *Natrium* (Latin), so its symbol is \( \text{Na} \). * Iron (English) comes from *Ferrum* (Latin), so its symbol is \( \text{Fe} \). * Potassium (English) comes from *Kalium* (Latin), so its symbol is \( \text{K} \). * Gold (English) comes from *Aurum* (Latin), so its symbol is \( \text{Au} \). * Silver (English) comes from *Argentum* (Latin), so its symbol is \( \text{Ag} \). #### Common Elements and Their Symbols It is essential to learn the names and symbols of the most common elements. Here is a list you should be familiar with: * **One-Letter Symbols:** * Boron: B * Carbon: C * Fluorine: F * Hydrogen: H * Iodine: I * Nitrogen: N * Oxygen: O * Phosphorus: P * Sulfur: S * **Two-Letter Symbols (English Origin):** * Aluminium: Al * Argon: Ar * Beryllium: Be * Bromine: Br * Calcium: Ca * Chlorine: Cl * Helium: He * Lithium: Li * Magnesium: Mg * Neon: Ne * Silicon: Si * Zinc: Zn * **Two-Letter Symbols (Latin/Greek Origin):** * Copper: Cu (from *Cuprum*) * Gold: Au (from *Aurum*) * Iron: Fe (from *Ferrum*) * Lead: Pb (from *Plumbum*) * Mercury: Hg (from *Hydrargyrum*) * Potassium: K (from *Kalium*) * Silver: Ag (from *Argentum*) * Sodium: Na (from *Natrium*) * Tin: Sn (from *Stannum*) ### Vocabulary * **Element:** A pure substance consisting of only one type of atom. * **Chemical Symbol:** A shorthand notation, usually one or two letters, that represents a chemical element. * **Atom:** The smallest particle of an element that retains the chemical identity of that element. * **Periodic Table:** A chart that organizes all known elements based on their atomic number, electron configuration, and recurring chemical properties. ### Examples 1. **Identifying Elements:** If you see the symbol \( \text{Mg} \), what element does it represent? It represents Magnesium. 2. **Writing Symbols:** What is the chemical symbol for the element Fluorine? It is \( \text{F} \). 3. **Elements in a Compound:** Table salt has the chemical formula \( \text{NaCl} \). What elements are present in table salt? Sodium (\( \text{Na} \)) and Chlorine (\( \text{Cl} \)). 4. **Understanding Origin:** The symbol for Iron is \( \text{Fe} \). This comes from its Latin name, *Ferrum*. ### Exercises 1. Write the chemical symbol for each of the following elements: a. Oxygen b. Helium c. Potassium d. Aluminium e. Silver 2. Name the element represented by each of these symbols: a. \( \text{C} \) b. \( \text{Na} \) c. \( \text{Cl} \) d. \( \text{Cu} \) e. \( \text{H} \) 3. True or False: The second letter in a two-letter chemical symbol is always capitalized. 4. Which of these symbols is derived from a Latin name? a. \( \text{O} \) b. \( \text{Mg} \) c. \( \text{Au} \) d. \( \text{Ne} \) ### Assignment 1. Create a table with two columns. In the first column, list the names of 10 different elements we discussed. In the second column, write their corresponding chemical symbols. 2. Research and list 5 elements whose symbols are derived from their Latin or Greek names (not already covered in the notes). For each, write its English name, chemical symbol, and the Latin/Greek name it came from. 3. Explain in your own words why chemical symbols are important in chemistry. ### Summary In this lesson, we learned that chemical symbols are a concise and universal way to represent elements. Each element has a unique symbol, which can be one or two letters. The first letter is always capitalized, and the second (if present) is always lowercase. We also discovered that some symbols are derived from the element's English name, while others come from older Latin or Greek names. Understanding these symbols is a fundamental step in learning chemistry, as they are used in all chemical formulas and equations. ## Names and formulae of radicals ### Introduction Welcome to this lesson on chemical radicals! In chemistry, elements combine to form compounds. Sometimes, a group of atoms acts like a single atom, carrying an electrical charge and combining with other atoms or groups of atoms. These special groups are called radicals, also known as polyatomic ions. Understanding their names, formulas, and valencies is crucial for writing correct chemical formulas for many compounds. By the end of this lesson, you will be able to identify, name, and write the formulae of common radicals. ### Key Concepts #### What are Radicals? A radical is a group of two or more atoms that are chemically bonded together and carry an overall electric charge. This entire group then behaves as a single unit during chemical reactions. They are also known as polyatomic ions because they are made of multiple atoms and have a charge. * Radicals are not stable on their own and always exist as part of a compound. * Examples include sulfate \( SO_4^{2-} \), nitrate \( NO_3^- \), and ammonium \( NH_4^+ \). #### Types of Radicals Radicals can be broadly classified based on their charge: ##### Acid Radicals * These are usually negatively charged (anions). * They are formed when an acid loses its hydrogen ions. * Most common radicals are acid radicals. * Examples: Carbonate \( CO_3^{2-} \), Phosphate \( PO_4^{3-} \), Sulfate \( SO_4^{2-} \), Nitrate \( NO_3^- \). ##### Basic Radicals * These are usually positively charged (cations). * The most common basic radical you will encounter is Ammonium. * Example: Ammonium \( NH_4^+ \). #### Valency of Radicals The valency of a radical is determined by the magnitude of its electric charge. It represents the combining power of the radical. * If a radical has a charge of -1 or +1, its valency is 1 (monovalent). * If a radical has a charge of -2 or +2, its valency is 2 (divalent). * If a radical has a charge of -3 or +3, its valency is 3 (trivalent). For example, the hydroxide radical \( OH^- \) has a charge of -1, so its valency is 1. The sulfate radical \( SO_4^{2-} \) has a charge of -2, so its valency is 2. #### Common Radicals and their Properties It is important to memorise the names, formulae, and valencies of common radicals. Below is a list: ##### Monovalent Radicals (Valency 1) * **Hydroxide:** \( OH^- \) * **Nitrate:** \( NO_3^- \) * **Nitrite:** \( NO_2^- \) * **Hydrogen Carbonate (Bicarbonate):** \( HCO_3^- \) * **Chlorate:** \( ClO_3^- \) * **Ammonium:** \( NH_4^+ \) (This is the only common basic radical) ##### Divalent Radicals (Valency 2) * **Sulfate:** \( SO_4^{2-} \) * **Sulfite:** \( SO_3^{2-} \) * **Carbonate:** \( CO_3^{2-} \) * **Chromate:** \( CrO_4^{2-} \) * **Dichromate:** \( Cr_2O_7^{2-} \) ##### Trivalent Radicals (Valency 3) * **Phosphate:** \( PO_4^{3-} \) #### Naming Conventions for Radicals * Most acid radicals end in "-ate" or "-ite". * The suffix "-ate" generally indicates that the radical contains more oxygen atoms (e.g., sulfate \( SO_4^{2-} \)). * The suffix "-ite" generally indicates that the radical contains fewer oxygen atoms than the "-ate" version (e.g., sulfite \( SO_3^{2-} \) has one less oxygen than sulfate). * The only common basic radical is named Ammonium. ### Vocabulary * **Radical (Polyatomic ion):** A group of two or more atoms bonded together that carries an overall electric charge and acts as a single unit in chemical reactions. * **Valency:** The combining power of an element or radical, determined by the number of electrons it can lose, gain, or share. * **Acid Radical:** A negatively charged polyatomic ion typically derived from an acid. * **Basic Radical:** A positively charged polyatomic ion, such as ammonium. ### Examples Let's see how we use the valencies of radicals to write chemical formulae: 1. **Calcium Sulfate:** * Calcium (Ca) has a valency of 2 (\( Ca^{2+} \)). * Sulfate (\( SO_4 \)) has a valency of 2 (\( SO_4^{2-} \)). * Since both valencies are 2, they cancel out, resulting in the formula: \[ CaSO_4 \] 2. **Magnesium Hydroxide:** * Magnesium (Mg) has a valency of 2 (\( Mg^{2+} \)). * Hydroxide (\( OH \)) has a valency of 1 (\( OH^- \)). * We need two hydroxide radicals to balance the valency of one magnesium ion. When more than one radical is needed, we enclose the radical in parentheses. * Formula: \[ Mg(OH)_2 \] 3. **Ammonium Chloride:** * Ammonium (\( NH_4 \)) has a valency of 1 (\( NH_4^+ \)). * Chloride (Cl) has a valency of 1 (\( Cl^- \)). * Since both valencies are 1, they cancel out. * Formula: \[ NH_4Cl \] 4. **Aluminium Nitrate:** * Aluminium (Al) has a valency of 3 (\( Al^{3+} \)). * Nitrate (\( NO_3 \)) has a valency of 1 (\( NO_3^- \)). * We need three nitrate radicals to balance the valency of one aluminium ion. Enclose nitrate in parentheses. * Formula: \[ Al(NO_3)_3 \] ### Exercises 1. Give the chemical formula and valency for the following radicals: a) Carbonate b) Ammonium c) Phosphate d) Hydroxide 2. Write the chemical formulae for the following compounds: a) Sodium Carbonate (Sodium, Na, valency 1) b) Potassium Nitrate (Potassium, K, valency 1) c) Calcium Phosphate (Calcium, Ca, valency 2) d) Ammonium Sulfate e) Zinc Hydroxide (Zinc, Zn, valency 2) ### Assignment 1. Identify whether the following radicals are acid or basic radicals and state their valency: a) \( PO_4^{3-} \) b) \( NH_4^+ \) c) \( SO_3^{2-} \) d) \( HCO_3^- \) 2. Write the names of the following radicals: a) \( NO_2^- \) b) \( OH^- \) c) \( Cr_2O_7^{2-} \) 3. Write the chemical formulae for the compounds formed between: a) Copper(II) and Sulfate b) Iron(III) and Nitrate c) Lead(II) and Carbonate d) Ammonium and Phosphate ### Summary In this lesson, we explored chemical radicals, which are groups of atoms that carry a charge and act as a single unit in chemical reactions. We learned that radicals can be acid (negatively charged) or basic (positively charged), and their valency is determined by the magnitude of their charge. We familiarized ourselves with the names, formulae, and valencies of common radicals like sulfate, nitrate, hydroxide, and ammonium. Finally, we practiced using this knowledge to correctly write chemical formulae for various compounds. Mastering radicals is an essential step in understanding chemical bonding and formulae in chemistry. ## Valency of elements and Radicals ### Introduction Welcome to this lesson on Valency of elements and Radicals! In chemistry, atoms combine in specific ways to form compounds. The 'combining power' of an atom is a very important concept that helps us understand why elements combine in certain ratios. This combining power is known as valency. We will also learn about radicals, which are groups of atoms that behave like a single unit with their own combining power. Understanding valency and radicals is crucial for writing correct chemical formulae and predicting how substances react. ### Key Concepts #### What is Valency? Valency is defined as the combining power of an element. It tells us how many hydrogen atoms (or atoms of any other element with a valency of 1) an atom of that element can combine with. It is essentially related to the number of electrons an atom needs to gain, lose, or share to achieve a stable electron configuration (like that of a noble gas). * **Elements with Fixed Valency:** Some elements always exhibit the same valency. * **Group 1 Elements (Alkali Metals):** Valency of 1 (e.g., Sodium, Na; Potassium, K). * **Group 2 Elements (Alkaline Earth Metals):** Valency of 2 (e.g., Magnesium, Mg; Calcium, Ca). * **Group 13 Elements:** Valency of 3 (e.g., Aluminium, Al). * **Group 14 Elements:** Valency of 4 (e.g., Carbon, C; Silicon, Si). * **Group 15 Elements:** Valency of 3 (e.g., Nitrogen, N; Phosphorus, P). * **Group 16 Elements:** Valency of 2 (e.g., Oxygen, O; Sulphur, S). * **Group 17 Elements (Halogens):** Valency of 1 (e.g., Chlorine, Cl; Bromine, Br). * **Elements with Variable Valency:** Some elements, especially transition metals, can show more than one valency. * **Iron (Fe):** Can have valency 2 (ferrous) or 3 (ferric). * **Copper (Cu):** Can have valency 1 (cuprous) or 2 (cupric). #### Radicals (Polyatomic Ions) A radical is a group of atoms of different elements that behave as a single unit and carry an electrical charge. This charge determines its combining power or valency. * **Common Radicals and their Valencies:** * **Hydroxide:** \( \text{OH}^- \) - Valency 1 * **Nitrate:** \( \text{NO}_3^- \) - Valency 1 * **Ammonium:** \( \text{NH}_4^+ \) - Valency 1 * **Sulphate:** \( \text{SO}_4^{2-} \) - Valency 2 * **Carbonate:** \( \text{CO}_3^{2-} \) - Valency 2 * **Phosphate:** \( \text{PO}_4^{3-} \) - Valency 3 #### How Valency Helps in Forming Compounds Understanding valency is essential for writing chemical formulas. Atoms combine in such a way that their total valencies balance out. For example, if element A has a valency of 1 and element B has a valency of 2, two atoms of A will combine with one atom of B to form a compound (e.g., \( A_2B \)). This is often illustrated using the 'criss-cross' method, where the valency of one element becomes the subscript of the other. ### Vocabulary * **Valency:** The combining power of an element. * **Element:** A pure substance consisting of only one type of atom. * **Compound:** A substance formed when two or more different elements are chemically bonded together. * **Radical (Polyatomic Ion):** A group of atoms that are covalently bonded and carry an overall electrical charge, behaving as a single unit during chemical reactions. * **Cation:** A positively charged ion (e.g., \( \text{Na}^+ \), \( \text{Mg}^{2+} \)). * **Anion:** A negatively charged ion (e.g., \( \text{Cl}^- \), \( \text{O}^{2-} \)). ### Examples 1. **Determining Valency from Group Number:** * Sodium (Na) is in Group 1, so its valency is 1. * Magnesium (Mg) is in Group 2, so its valency is 2. * Oxygen (O) is in Group 16, so its valency is 2. * Chlorine (Cl) is in Group 17, so its valency is 1. 2. **Identifying Valency of Radicals:** * The charge on Sulphate radical (\( \text{SO}_4^{2-} \)) is 2-, so its valency is 2. * The charge on Nitrate radical (\( \text{NO}_3^- \)) is 1-, so its valency is 1. * The charge on Ammonium radical (\( \text{NH}_4^+ \)) is 1+, so its valency is 1. 3. **Elements with Variable Valency:** * Iron can be \( \text{Fe}^{2+} \) (Iron(II) or ferrous) with valency 2, or \( \text{Fe}^{3+} \) (Iron(III) or ferric) with valency 3. * Copper can be \( \text{Cu}^+ \) (Copper(I) or cuprous) with valency 1, or \( \text{Cu}^{2+} \) (Copper(II) or cupric) with valency 2. ### Exercises 1. What is the definition of valency? 2. State the valency of the following elements: a. Calcium (Ca) b. Sulphur (S) c. Aluminium (Al) d. Potassium (K) 3. Name two elements that exhibit variable valency. 4. Identify the following radicals and state their valency: a. \( \text{NO}_3^- \) b. \( \text{NH}_4^+ \) c. \( \text{CO}_3^{2-} \) d. \( \text{OH}^- \) ### Assignment 1. Research and list five more common radicals, along with their chemical formulae and valencies. 2. Explain in your own words why understanding valency is important in chemistry. 3. Can an element have zero valency? If yes, give an example and explain why. 4. Draw a table listing the first 20 elements, their symbols, and their most common valencies. ### Summary In this lesson, we explored the concept of valency, which is the combining power of an element. We learned that some elements have a fixed valency, while others, particularly transition metals, exhibit variable valency. We also discovered radicals, which are charged groups of atoms that act as single units with their own specific valency. Understanding valency and radicals is fundamental for predicting how atoms and groups of atoms combine to form chemical compounds, laying the groundwork for writing and understanding chemical formulae. Keep practicing to master these important concepts! ## Chemical symbols and formulae ### 1. Introduction Welcome to a fascinating journey into the language of chemistry! Just like we use letters to form words, chemists use special shorthand notations to represent elements and compounds. These are called **chemical symbols** and **chemical formulae**. Learning them is essential because they allow scientists all over the world to communicate clearly and concisely about chemical substances, no matter what language they speak. In this lesson, we will explore what these symbols and formulae are, how they are formed, and why they are so important. ### 2. Key Concepts #### 2.1 What is a Chemical Symbol? A **chemical symbol** is a one- or two-letter abbreviation used to represent a chemical element. Think of it as the element's unique identifier. * **Rules for Writing Symbols:** * If the symbol has only one letter, it is always a capital (uppercase) letter. E.g., H for Hydrogen, O for Oxygen, C for Carbon. * If the symbol has two letters, the first letter is always capital, and the second letter is always a small (lowercase) letter. E.g., He for Helium, Na for Sodium, Cl for Chlorine. * **Origin of Symbols:** Many symbols come from the English name of the element (e.g., H for Hydrogen). However, some are derived from their ancient Latin or Greek names. For example: * Sodium: \( \text{Na} \) (from Latin _natrium_) * Iron: \( \text{Fe} \) (from Latin _ferrum_) * Copper: \( \text{Cu} \) (from Latin _cuprum_) * Lead: \( \text{Pb} \) (from Latin _plumbum_) #### 2.2 Significance of Chemical Symbols A chemical symbol means more than just a name: * It represents the **name** of the element. * It represents **one atom** of that element. * It can also represent the **relative atomic mass** of the element (which we will learn about later). #### 2.3 What is a Chemical Formula? A **chemical formula** is a shorthand notation that represents a chemical compound (or a molecule of an element). It shows which elements are present and the exact number of atoms of each element in the smallest unit of that substance. * **Components of a Chemical Formula:** * **Chemical Symbols:** To show which elements are present. * **Subscripts:** Small numbers written below and to the right of a chemical symbol. They indicate the number of atoms of that element in the molecule or compound. If there is no subscript, it means there is only one atom of that element. * **Parentheses:** Used when there is more than one group of atoms (called a polyatomic ion or radical). The subscript outside the parentheses applies to everything inside the parentheses. * **Examples:** * \( \text{H}_2\text{O} \) (Water): Contains 2 atoms of Hydrogen and 1 atom of Oxygen. * \( \text{CO}_2 \) (Carbon Dioxide): Contains 1 atom of Carbon and 2 atoms of Oxygen. * \( \text{O}_2 \) (Oxygen molecule): Contains 2 atoms of Oxygen. * \( \text{NaCl} \) (Sodium Chloride): Contains 1 atom of Sodium and 1 atom of Chlorine. * \( \text{Ca(OH)}_2 \) (Calcium Hydroxide): Contains 1 atom of Calcium, 2 atoms of Oxygen, and 2 atoms of Hydrogen. #### 2.4 Understanding Valency To write chemical formulae correctly, we need to understand **valency**. * **Definition:** Valency is the **combining power** of an element or a radical (a group of atoms that act as a single unit). It tells us how many bonds an atom can form or how many hydrogen atoms it can combine with (or replace). * **Why Valency is Important:** Atoms combine with each other in specific ratios to form stable compounds. Valency helps us determine these ratios. * **Common Valencies (for Form Two):** | Element/Radical | Symbol | Valency | | :-------------- | :----- | :------ | | Hydrogen | \( \text{H} \) | 1 | | Sodium | \( \text{Na} \) | 1 | | Potassium | \( \text{K} \) | 1 | | Chlorine | \( \text{Cl} \) | 1 | | Oxygen | \( \text{O} \) | 2 | | Calcium | \( \text{Ca} \) | 2 | | Magnesium | \( \text{Mg} \) | 2 | | Carbon (in CO2) | \( \text{C} \) | 4 | | Aluminium | \( \text{Al} \) | 3 | | Ammonium | \( \text{NH}_4 \) | 1 | | Hydroxide | \( \text{OH} \) | 1 | | Carbonate | \( \text{CO}_3 \) | 2 | | Sulfate | \( \text{SO}_4 \) | 2 | | Nitrate | \( \text{NO}_3 \) | 1 | #### 2.5 Writing Chemical Formulae using the Criss-Cross Method The criss-cross method is a simple way to write chemical formulae by using valencies. **Steps:** 1. **Write the symbols** of the elements or radicals side-by-side, with the more metallic (or positively charged) element/radical usually first. 2. **Write their valencies** as superscripts above the symbols. 3. **Criss-cross the valencies**: Bring the valency of the first element down as the subscript for the second element, and vice versa. 4. **Simplify the subscripts**: If the subscripts can be divided by a common factor, simplify them to the lowest whole number ratio. 5. **Omit '1'**: If a subscript is '1', do not write it. 6. **Use parentheses**: If a radical (polyatomic ion) gets a subscript greater than 1, enclose the radical in parentheses. ### 3. Vocabulary * **Chemical Symbol:** A one- or two-letter abbreviation for a chemical element. * **Chemical Formula:** A notation showing the types and numbers of atoms present in a compound or molecule. * **Element:** A pure substance consisting of only one type of atom. * **Compound:** A substance formed when two or more different elements are chemically bonded together in fixed proportions. * **Atom:** The smallest unit of an element that retains the chemical properties of that element. * **Molecule:** Two or more atoms held together by chemical bonds, forming the smallest independent unit of a pure substance. * **Subscript:** A small number written below and to the right of a chemical symbol in a formula, indicating the number of atoms of that element. * **Valency:** The combining power of an element or radical. * **Radical (Polyatomic Ion):** A group of atoms that are covalently bonded together and carry an overall electrical charge, acting as a single unit in chemical reactions (e.g., sulfate ion \( \text{SO}_4^{2-} \)). ### 4. Examples Let's apply the criss-cross method to write some chemical formulae: **Example 1: Sodium Chloride** 1. Symbols: \( \text{Na} \) \( \text{Cl} \) 2. Valencies: \( \text{Na}^1 \) \( \text{Cl}^1 \) 3. Criss-cross: \( \text{Na}_1 \) \( \text{Cl}_1 \) 4. Formula: \( \text{NaCl} \) **Example 2: Magnesium Oxide** 1. Symbols: \( \text{Mg} \) \( \text{O} \) 2. Valencies: \( \text{Mg}^2 \) \( \text{O}^2 \) 3. Criss-cross: \( \text{Mg}_2 \) \( \text{O}_2 \) 4. Simplify: Divide by 2. \( \text{Mg}_1 \) \( \text{O}_1 \) 5. Formula: \( \text{MgO} \) **Example 3: Aluminium Oxide** 1. Symbols: \( \text{Al} \) \( \text{O} \) 2. Valencies: \( \text{Al}^3 \) \( \text{O}^2 \) 3. Criss-cross: \( \text{Al}_2 \) \( \text{O}_3 \) 4. Formula: \( \text{Al}_2\text{O}_3 \) **Example 4: Calcium Hydroxide** 1. Symbols: \( \text{Ca} \) \( \text{OH} \) 2. Valencies: \( \text{Ca}^2 \) \( \text{OH}^1 \) 3. Criss-cross: \( \text{Ca}_1 \) \( \text{(OH)}_2 \) (Parentheses are needed because the \( \text{OH} \) radical has a subscript of 2). 4. Formula: \( \text{Ca(OH)}_2 \) **Example 5: Ammonium Sulfate** 1. Symbols: \( \text{NH}_4 \) \( \text{SO}_4 \) 2. Valencies: \( \text{NH}_4^1 \) \( \text{SO}_4^2 \) 3. Criss-cross: \( \text{(NH}_4)_2 \) \( \text{(SO}_4)_1 \) (Parentheses for \( \text{NH}_4 \) as it gets a subscript of 2). 4. Formula: \( \text{(NH}_4)_2\text{SO}_4 \) ### 5. Exercises 1. Write the chemical symbol for each of the following elements: a. Gold b. Silver c. Potassium d. Zinc e. Sulphur 2. Name the element represented by each of these symbols: a. \( \text{P} \) b. \( \text{N} \) c. \( \text{Fe} \) d. \( \text{Cu} \) e. \( \text{Mg} \) 3. State the number of atoms of each element present in the following chemical formulae: a. \( \text{H}_2\text{SO}_4 \) (Sulphuric acid) b. \( \text{NH}_3 \) (Ammonia) c. \( \text{Ca(NO}_3)_2 \) (Calcium Nitrate) d. \( \text{K}_3\text{PO}_4 \) (Potassium Phosphate) 4. Using the criss-cross method and the valencies learned, write the chemical formulae for the compounds formed between the following: a. Potassium and Iodine (Iodine valency is 1) b. Magnesium and Bromine (Bromine valency is 1) c. Sodium and Carbonate d. Aluminium and Chloride e. Copper (valency 2) and Sulphate ### 6. Assignment 1. Define the following terms in your own words: a. Chemical symbol b. Chemical formula c. Valency 2. Complete the following table: | Element/Radical | Symbol | Valency | | :-------------- | :----- | :------ | | Calcium | | | | Chlorine | | | | | \( \text{Na} \) | | | Oxygen | | | | | \( \text{OH} \) | | | Aluminium | | | | | \( \text{SO}_4 \) | | 3. Write the chemical formulae for the following compounds: a. Zinc chloride (Zinc valency is 2) b. Iron(II) oxide (Iron(II) valency is 2) c. Sodium hydroxide d. Potassium nitrate e. Aluminium carbonate 4. Explain the difference between \( \text{H}_2 \) and \( 2\text{H} \). ### 7. Summary In this lesson, we have uncovered the fundamental building blocks of chemical communication: **chemical symbols** and **chemical formulae**. We learned that symbols are a universal shorthand for elements, representing their name and a single atom. Chemical formulae, on the other hand, show us the exact composition of compounds, using subscripts to indicate the number of atoms of each element. Crucially, we explored **valency**, the combining power of elements, and how the **criss-cross method** allows us to predict and write correct chemical formulae. Mastering these concepts provides you with the essential language to understand and discuss the vast world of chemistry! ## Use of tables to derive chemical formulae ### Introduction Welcome to this lesson on deriving chemical formulae! In chemistry, a chemical formula is like a shorthand notation that tells us exactly which elements are present in a compound and in what ratios. Understanding how to write these formulae correctly is fundamental to understanding chemical reactions and the composition of substances. Today, we will learn how to use the valencies of elements and polyatomic ions, often found in tables, to construct correct chemical formulae using a simple method. ### Key Concepts #### What is a Chemical Formula? A chemical formula uses chemical symbols to represent the atoms of each element in a compound and subscripts to indicate the number of atoms of each element. For example, \( H_2O \) is the chemical formula for water, telling us that each molecule of water contains two hydrogen atoms and one oxygen atom. #### What is Valency? Valency is the combining power of an element. It tells us how many bonds an atom of an element can form. Valency is usually a small whole number (e.g., 1, 2, 3, 4). For example, hydrogen has a valency of 1, meaning it can form one bond, while oxygen has a valency of 2, meaning it can form two bonds. Valencies are often found in tables or can be determined from the group number of an element in the periodic table. #### Using Valency to Write Formulae: The Criss-Cross Method The criss-cross method is a simple way to derive chemical formulae using the valencies of the combining elements or ions. ##### Steps for the Criss-Cross Method: 1. **Write the symbols:** Write the symbols of the elements or ions side-by-side, with the metal or positive ion (cation) first, followed by the non-metal or negative ion (anion). 2. **Write the valencies:** Write the valency of each element or ion above its symbol as a superscript, without the positive or negative signs. 3. **Criss-cross the valencies:** Swap the valencies and write them as subscripts to the opposite element's symbol. These subscripts indicate the number of atoms or ions needed. 4. **Simplify (if necessary):** If the subscripts have a common factor, divide them by the largest common factor to get the simplest whole-number ratio. 5. **Remove '1' and add brackets for polyatomic ions:** If a subscript is '1', it is usually omitted. If a polyatomic ion (an ion made of two or more atoms that acts as a single unit, e.g., \( CO_3^{2-} \)) requires more than one unit in the formula, enclose the polyatomic ion in brackets before writing the subscript. #### Common Valencies and Polyatomic Ions It's useful to know the valencies of common elements and ions. These can often be found in a periodic table (for elements) or a valency table. | Element/Ion Name | Symbol | Valency | | :---------------- | :----------------- | :------ | | Sodium | Na | 1 | | Potassium | K | 1 | | Silver | Ag | 1 | | Hydrogen | H | 1 | | Chlorine | Cl | 1 | | Oxygen | O | 2 | | Magnesium | Mg | 2 | | Calcium | Ca | 2 | | Zinc | Zn | 2 | | Iron (II) | Fe | 2 | | Copper (II) | Cu | 2 | | Aluminium | Al | 3 | | Iron (III) | Fe | 3 | | Carbonate | \( CO_3 \) | 2 | | Sulphate | \( SO_4 \) | 2 | | Nitrate | \( NO_3 \) | 1 | | Hydroxide | \( OH \) | 1 | | Ammonium | \( NH_4 \) | 1 | ### Vocabulary * **Chemical Formula:** A shorthand notation representing the types and numbers of atoms present in a compound. * **Valency:** The combining power of an element, indicating how many bonds an atom can form. * **Subscript:** A number written slightly below and to the right of a chemical symbol, indicating the number of atoms of that element in a molecule or compound. * **Polyatomic Ion:** An ion composed of two or more atoms covalently bonded together that carries an overall electric charge and acts as a single unit in chemical reactions. * **Cation:** A positively charged ion. * **Anion:** A negatively charged ion. ### Examples Let's apply the criss-cross method to derive some chemical formulae. #### Example 1: Sodium Chloride (Salt) 1. **Symbols:** Na Cl 2. **Valencies:** Na¹ Cl¹ 3. **Criss-cross:** Na₁Cl₁ 4. **Simplify/Omit '1':** NaCl * Formula: NaCl #### Example 2: Magnesium Oxide 1. **Symbols:** Mg O 2. **Valencies:** Mg² O² 3. **Criss-cross:** Mg₂O₂ 4. **Simplify:** Divide by the common factor 2 \( \rightarrow \) Mg₁O₁ * Formula: MgO #### Example 3: Aluminium Oxide 1. **Symbols:** Al O 2. **Valencies:** Al³ O² 3. **Criss-cross:** Al₂O₃ * Formula: Al₂O₃ #### Example 4: Sodium Carbonate (using a polyatomic ion) 1. **Symbols:** Na \( CO_3 \) 2. **Valencies:** Na¹ \( (CO_3)^2 \) 3. **Criss-cross:** Na₂ \( (CO_3)_1 \) 4. **Omit '1':** Na₂CO₃ * Formula: Na₂CO₃ #### Example 5: Calcium Hydroxide (using brackets for a polyatomic ion) 1. **Symbols:** Ca \( OH \) 2. **Valencies:** Ca² \( (OH)^1 \) 3. **Criss-cross:** Ca₁ \( (OH)_2 \) 4. **Omit '1' and add brackets:** Ca(OH)₂ * Formula: Ca(OH)₂ ### Exercises Derive the chemical formulae for the following compounds using the criss-cross method and the valency table: 1. Potassium Chloride 2. Zinc Oxide 3. Hydrogen Sulphide 4. Aluminium Sulphate 5. Ammonium Chloride 6. Iron (II) Nitrate 7. Copper (I) Oxide (Copper (I) has valency 1) ### Assignment For further practice, derive the chemical formulae for these compounds: 1. Magnesium Nitrate 2. Potassium Carbonate 3. Silver Oxide 4. Calcium Phosphate (Phosphate ion, \( PO_4 \), has valency 3) 5. Lead (II) Oxide (Lead (II) has valency 2) 6. Sodium Sulphate 7. Iron (III) Hydroxide ### Summary In this lesson, we learned that chemical formulae are vital for representing compounds. We explored the concept of valency as the combining power of an element or ion. Most importantly, we mastered the criss-cross method, a systematic approach to derive chemical formulae using valencies. Remember to write symbols, assign valencies, criss-cross them, simplify if possible, and use brackets for polyatomic ions when needed. This method is a powerful tool for accurately representing the composition of chemical compounds.