Chap1:The Atom.

# Chap1:The Atom. ## Dalton’s Atomic theory ### Introduction Welcome to our lesson on Dalton's Atomic Theory! In Form Two Chemistry, we begin by understanding the fundamental building blocks of all matter: atoms. Have you ever wondered what everything around us is made of? Long ago, scientists also asked this question. One of the most important early ideas about atoms came from an English scientist named John Dalton. His theory, developed in the early 19th century, laid the foundation for modern chemistry. Today, we will explore his revolutionary ideas that helped us understand the nature of matter and chemical reactions. ### Key Concepts #### What is an Atom? (Review) Before we dive into Dalton's theory, let's remember what an atom is. An **atom** is the smallest unit of an element that retains the chemical identity of that element. Everything you see, touch, or feel is made up of countless tiny atoms. #### Who was John Dalton? John Dalton (1766-1844) was an English chemist, meteorologist, and physicist. He is best known for pioneering the development of modern atomic theory, which marked a major turning point in the history of chemistry. #### Dalton's Atomic Theory Postulates Dalton's theory is based on several key ideas, also known as **postulates**. These postulates helped explain many observations about matter and chemical reactions known at the time. Let's look at each one: ##### 1. All matter is composed of extremely small particles called atoms. * This means that whether it's a rock, water, or air, everything is ultimately made of tiny, individual units we call atoms. ##### 2. Atoms of a given element are identical in mass, size, and other properties; atoms of different elements differ in mass, size, and other properties. * Imagine you have a pile of gold atoms. According to Dalton, every single gold atom in that pile would be exactly the same. However, if you compare a gold atom to an oxygen atom, they would be different in every way (mass, size, etc.). ##### 3. Atoms cannot be created or destroyed, nor can they be subdivided into smaller particles. * This postulate states that atoms are unbreakable and cannot be split. Also, during a chemical reaction, atoms don't just appear or disappear; they are simply rearranged. This idea supports the Law of Conservation of Mass. ##### 4. Atoms of different elements combine in simple whole-number ratios to form chemical compounds. * When atoms from different elements join together to form a compound, they always do so in fixed, simple ratios. For example, water is always made of 2 hydrogen atoms and 1 oxygen atom (H\(_2\)O). You would never find water made of 1.5 hydrogen atoms. ##### 5. Chemical reactions involve the rearrangement, combination, or separation of atoms. * This means that during a chemical reaction, atoms are not changed into other types of atoms. Instead, they just get shuffled around to form new combinations. For example, when hydrogen and oxygen combine to form water, the hydrogen and oxygen atoms themselves don't change; they just link up differently. #### Limitations of Dalton's Atomic Theory (Briefly) While Dalton's theory was groundbreaking, later discoveries showed some parts of it needed adjustment: * **Atoms are not indivisible:** We now know that atoms can be split into smaller subatomic particles (protons, neutrons, and electrons). * **Atoms of the same element are not always identical:** We now know about isotopes, which are atoms of the same element that have different masses due to a different number of neutrons. Despite these limitations, Dalton's theory was a monumental step forward in understanding the fundamental nature of matter. ### Vocabulary * **Atom:** The smallest unit of an element that retains the chemical identity of that element. * **Element:** A pure substance consisting only of atoms that all have the same numbers of protons in their atomic nuclei. * **Compound:** A substance formed when two or more chemical elements are chemically bonded together. * **Chemical Reaction:** A process that involves rearrangement of the molecular or ionic structure of a substance, as opposed to a change in physical form or a nuclear reaction. * **Indivisible:** Unable to be divided or separated. * **Identical:** Exactly alike; precisely the same. * **Postulate:** A statement that is assumed to be true without proof; a basic principle. * **Subatomic Particles:** Particles smaller than atoms, such as protons, neutrons, and electrons. * **Isotopes:** Atoms of the same element that have the same number of protons but different numbers of neutrons, and therefore different masses. ### Examples **1. Law of Conservation of Mass:** Dalton's third postulate (atoms cannot be created or destroyed) explains the Law of Conservation of Mass. If you burn a piece of wood, the ash, smoke, and gases produced will have the same total mass as the original wood and the oxygen used in the burning process. The atoms are just rearranged, not lost or gained. **2. Formation of Water (H\(_2\)O):** This demonstrates Dalton's fourth postulate (atoms combine in simple whole-number ratios). Water is always formed from hydrogen and oxygen atoms in a 2:1 ratio. No matter where or how water is formed, this ratio remains constant. **3. Different Compounds from the Same Elements:** Carbon and oxygen can form carbon monoxide (CO) or carbon dioxide (CO\(_2\)). In CO, there's one carbon atom for every one oxygen atom. In CO\(_2\), there's one carbon atom for every two oxygen atoms. This shows that the same atoms can combine in different *simple whole-number ratios* to form different compounds, illustrating Dalton's fourth postulate. ### Exercises 1. Who proposed the atomic theory that states all matter is made of atoms? a) Isaac Newton b) John Dalton c) Albert Einstein d) Marie Curie 2. According to Dalton, are atoms of the same element identical or different? a) Identical b) Different c) Sometimes identical, sometimes different d) Only identical in compounds 3. Dalton's theory states that atoms can be subdivided into smaller particles. True or False? 4. When hydrogen and oxygen combine to form water (H\(_2\)O), which postulate of Dalton's atomic theory is best illustrated? a) All matter is made of atoms. b) Atoms of the same element are identical. c) Atoms combine in simple whole-number ratios. d) Atoms cannot be created or destroyed. 5. Explain why Dalton's atomic theory helps us understand the Law of Conservation of Mass. ### Assignment 1. Research and write a short paragraph about another scientist who contributed to our understanding of the atom after John Dalton. How did their discoveries refine or challenge Dalton's ideas? 2. Imagine you have a box of red marbles and a box of blue marbles. If the red marbles represent atoms of element A and blue marbles represent atoms of element B, describe how you would use them to illustrate Dalton's fourth postulate regarding the formation of a compound with the formula A\(_2\)B. 3. Why was Dalton's atomic theory considered revolutionary at the time it was proposed? 4. Discuss a real-life example (other than water or carbon compounds) where elements combine in fixed, simple whole-number ratios to form a compound. ### Summary Today, we explored John Dalton's groundbreaking Atomic Theory, which revolutionized our understanding of matter. We learned that Dalton proposed that all matter is made of tiny, indivisible particles called atoms. He stated that atoms of the same element are identical, while atoms of different elements are different. Furthermore, Dalton's theory explained that atoms are rearranged during chemical reactions and combine in simple whole-number ratios to form compounds. Although later discoveries led to some refinements, Dalton's atomic theory remains a cornerstone of modern chemistry, providing a fundamental framework for how we view the structure and behavior of matter. ## Composition of the atom ### Introduction Everything around us, from the air we breathe to the chair you're sitting on, is made of matter. But what is matter made of? The answer lies in tiny, fundamental particles called atoms. In this lesson, we will explore the basic building blocks of matter, understanding what atoms are made of and how their components determine their identity. ### Key Concepts #### What is an Atom? An atom is the smallest unit of an element that retains the chemical identity and properties of that element. For a long time, atoms were believed to be indivisible, but we now know they are made up of even smaller, 'subatomic' particles. #### Subatomic Particles Atoms are composed of three main types of subatomic particles: #### Protons * **Symbol:** \( p^+ \) * **Location:** Found in the nucleus (the dense center of the atom). * **Charge:** Positive (+1). * **Mass:** Approximately 1 atomic mass unit (amu). Protons are relatively heavy. #### Neutrons * **Symbol:** \( n^0 \) * **Location:** Also found in the nucleus, alongside protons. * **Charge:** Neutral (0), meaning they have no electrical charge. * **Mass:** Approximately 1 atomic mass unit (amu). Neutrons have a mass very similar to protons. #### Electrons * **Symbol:** \( e^- \) * **Location:** Orbiting the nucleus in regions called electron shells or energy levels. * **Charge:** Negative (-1). * **Mass:** Extremely small, almost negligible (approximately \( \frac{1}{1836} \) of a proton's mass). Electrons are very light compared to protons and neutrons. #### The Nucleus * The nucleus is the small, dense, and positively charged center of an atom. * It contains both protons and neutrons, which are collectively called nucleons. * The nucleus contains almost all of the atom's mass. #### Electron Shells * Electrons move rapidly in specific energy levels or shells around the nucleus. * These shells determine the size of the atom and play a crucial role in how atoms interact with each other to form compounds. #### Atomic Number (Z) * The **atomic number** (symbolized as Z) is defined as the number of protons in the nucleus of an atom. * It is the most important identifying characteristic of an element. Every atom of a specific element has the same unique atomic number. * For a neutral atom, the number of protons is equal to the number of electrons to balance the charges. \[ \text{Atomic Number (Z)} = \text{Number of Protons} \] For a neutral atom: \[ \text{Number of Protons} = \text{Number of Electrons} \] #### Mass Number (A) * The **mass number** (symbolized as A) is the total number of protons and neutrons in the nucleus of an atom. * Since protons and neutrons have a mass of approximately 1 amu each, the mass number gives us an idea of the atom's approximate mass. \[ \text{Mass Number (A)} = \text{Number of Protons} + \text{Number of Neutrons} \] * From this, we can also calculate the number of neutrons: \[ \text{Number of Neutrons} = \text{Mass Number (A)} - \text{Atomic Number (Z)} \] #### Representing an Atom An atom of an element can be represented using its symbol, mass number, and atomic number: \[ ^A_Z X \] Where: * X is the chemical symbol for the element (e.g., O for Oxygen, Na for Sodium). * A is the Mass Number. * Z is the Atomic Number. ### Vocabulary * **Atom:** The smallest unit of an element that retains the chemical identity of that element. * **Subatomic Particles:** Particles smaller than an atom, namely protons, neutrons, and electrons. * **Proton:** A subatomic particle with a positive (+1) charge, found in the nucleus. * **Neutron:** A subatomic particle with no charge (neutral), found in the nucleus. * **Electron:** A subatomic particle with a negative (-1) charge, found orbiting the nucleus in electron shells. * **Nucleus:** The dense, positively charged center of an atom, containing protons and neutrons. * **Atomic Number (Z):** The number of protons in an atom's nucleus, which defines the element. * **Mass Number (A):** The total number of protons and neutrons in an atom's nucleus. ### Examples #### Example 1: Carbon Atom Consider a neutral Carbon atom with an atomic number (Z) of 6 and a mass number (A) of 12. Its symbol is \( ^{12}_6 C \). * **Number of Protons:** The atomic number (Z) is 6, so it has 6 protons. * **Number of Electrons:** For a neutral atom, the number of electrons equals the number of protons, so it has 6 electrons. * **Number of Neutrons:** Mass Number (A) - Atomic Number (Z) = \( 12 - 6 = 6 \) neutrons. #### Example 2: Sodium Atom Consider a neutral Sodium atom represented as \( ^{23}_{11} Na \). * **Atomic Number (Z):** From the symbol, Z = 11. * **Mass Number (A):** From the symbol, A = 23. * **Number of Protons:** Z = 11 protons. * **Number of Electrons:** Since it's a neutral atom, it has 11 electrons. * **Number of Neutrons:** A - Z = \( 23 - 11 = 12 \) neutrons. ### Exercises 1. List the three main subatomic particles and state the electrical charge of each. 2. In which part of an atom would you find the protons and neutrons? 3. An unknown neutral atom has 17 protons, 18 neutrons, and 17 electrons. * What is its atomic number (Z)? * What is its mass number (A)? * Which element is this? (Hint: Refer to the periodic table for Z=17) 4. For a neutral atom of Helium (He) with \( ^4_2 He \): * How many protons does it have? * How many electrons does it have? * How many neutrons does it have? ### Assignment 1. Complete the following table for neutral atoms: | Element Symbol | Atomic Number (Z) | Mass Number (A) | Number of Protons | Number of Neutrons | Number of Electrons | |---|---|---|---|---|---| | \( ^{14}_7 N \) | | | | | | | \( ^{32}_{16} S \) | | | | | | | \( ^{20}_{10} Ne \) | | | | | | | \( ^{39}_{19} K \) | | | | | | 2. If an atom has 8 protons and 9 neutrons, determine its: * Atomic number. * Mass number. * Number of electrons (assuming it is a neutral atom). * Chemical symbol for the element. 3. Explain why all atoms are electrically neutral even though they contain charged particles. ### Summary * Atoms are the fundamental building blocks of all matter. * They are composed of a central, dense nucleus and electrons orbiting the nucleus. * The nucleus contains positively charged **protons** and neutral **neutrons**. Together, they determine the atom's mass. * Negatively charged **electrons** orbit the nucleus and have a negligible mass. * The **atomic number (Z)** is the number of protons and uniquely identifies an element. For neutral atoms, Z is also the number of electrons. * The **mass number (A)** is the sum of protons and neutrons in the nucleus. * Understanding these components helps us to define and differentiate between different elements and predict their behavior. ## Structure of the Atom (Bohr’s model) ### Introduction Welcome to our lesson on the structure of the atom! Everything around us, from the air we breathe to the solid ground beneath our feet, is made up of tiny particles called atoms. For many years, scientists have tried to understand what an atom looks like and how it works. Different models have been developed over time, each improving our understanding. Today, we will explore a very important model: **Bohr's Model of the Atom**, which helps us visualize how electrons are arranged around the nucleus. ### Key Concepts #### The Atom: A Basic Review Recall that atoms are the fundamental building blocks of all matter. They are incredibly small and are made up of even smaller subatomic particles. #### Rutherford's Model (Brief Recap) Before Bohr, Ernest Rutherford proposed a model where the atom has a tiny, dense, positively charged nucleus at its center, with electrons orbiting it like planets around the sun. However, this model had a problem: according to classical physics, orbiting electrons should continuously lose energy and spiral into the nucleus, making atoms unstable. This is where Bohr's model provided a crucial improvement. #### Bohr's Model of the Atom Niels Bohr, a Danish physicist, improved Rutherford's model in 1913 by introducing the idea of **quantized energy levels** for electrons. His model proposed the following key points: 1. **Central Nucleus:** The atom has a small, dense, positively charged nucleus at its center, containing **protons** and **neutrons**. * **Protons (p+):** Positively charged particles. Each proton has a relative mass of approximately 1 atomic mass unit (amu). * **Neutrons (n0):** Neutral particles (no charge). Each neutron also has a relative mass of approximately 1 amu. 2. **Electrons in Fixed Orbits (Electron Shells):** Electrons (e-) do not orbit randomly. Instead, they revolve around the nucleus in specific, fixed circular paths called **electron shells** or **energy levels**. * Each electron shell has a definite amount of energy associated with it. Electrons in shells closer to the nucleus have lower energy, while electrons in shells further away have higher energy. * Electrons can move from one shell to another by absorbing or emitting a specific amount of energy. This energy is **quantized**, meaning it comes in discrete packets. 3. **Electron Configuration:** The arrangement of electrons in these shells is called electron configuration. Each shell can hold a maximum number of electrons: * **First shell (K-shell):** Maximum of 2 electrons. * **Second shell (L-shell):** Maximum of 8 electrons. * **Third shell (M-shell):** Maximum of 18 electrons (though for the first 20 elements, it often fills with 8 electrons before the next shell starts to fill). #### Valence Electrons * **Valence electrons** are the electrons found in the outermost electron shell of an atom. * These electrons are very important because they determine an atom's chemical properties and how it will react with other atoms. #### Atomic Number and Mass Number * **Atomic Number (Z):** This is the number of **protons** in the nucleus of an atom. It uniquely identifies an element. In a neutral atom, the number of electrons is equal to the number of protons. * \( Z = \text{number of protons} \) * For a neutral atom: \( Z = \text{number of electrons} \) * **Mass Number (A):** This is the total number of **protons and neutrons** in the nucleus of an atom. * \( A = \text{number of protons} + \text{number of neutrons} \) * Therefore, the number of neutrons can be calculated as: \( \text{number of neutrons} = A - Z \) #### Representing an Atom We can represent an atom using its chemical symbol along with its atomic and mass numbers: \[ ^{A}_{Z}X \] Where: * \( \text{X} \) is the chemical symbol of the element. * \( \text{A} \) is the Mass Number. * \( \text{Z} \) is the Atomic Number. ### Vocabulary * **Atom:** The smallest unit of an element that retains the chemical identity of that element. * **Nucleus:** The dense, positively charged center of an atom, containing protons and neutrons. * **Proton:** A positively charged subatomic particle found in the nucleus, with a relative mass of 1 amu. * **Neutron:** A neutral (uncharged) subatomic particle found in the nucleus, with a relative mass of 1 amu. * **Electron:** A negatively charged subatomic particle that orbits the nucleus in electron shells, with a very small relative mass (approximately \( \frac{1}{1836} \) amu). * **Electron Shell (Energy Level):** A fixed region around the nucleus where electrons orbit, each with a specific energy. * **Valence Electron:** An electron in the outermost shell of an atom, involved in chemical bonding. * **Atomic Number (Z):** The number of protons in an atom's nucleus, defining the element. * **Mass Number (A):** The total number of protons and neutrons in an atom's nucleus. * **Bohr's Model:** A model of the atom where electrons orbit the nucleus in specific, quantized energy levels. * **Quantized Energy:** The idea that energy can only exist in discrete, specific amounts rather than in a continuous range. ### Examples 1. **Drawing Bohr's Model for Lithium (Li):** * Lithium has an Atomic Number (Z) of 3 and a Mass Number (A) of 7. * **Protons:** Z = 3 * **Electrons:** Z = 3 (for a neutral atom) * **Neutrons:** A - Z = 7 - 3 = 4 * **Electron Configuration:** The first shell can hold 2 electrons, leaving 1 electron for the second shell. So, (2, 1). * **Valence Electrons:** 1 (in the outermost shell). * *Imagine a central nucleus with 3 protons and 4 neutrons, surrounded by two electron shells. The first shell has 2 electrons, and the second (outermost) shell has 1 electron.* 2. **Determining Subatomic Particles for Carbon (C):** * Carbon is represented as \( ^{12}_{6}C \). * **Atomic Number (Z) = 6.** This means there are 6 protons. * Since it's a neutral atom, there are 6 electrons. * **Mass Number (A) = 12.** * **Number of Neutrons = A - Z = 12 - 6 = 6 neutrons.** * **Electron Configuration:** 6 electrons will fill the first shell with 2, and the second shell with the remaining 4. So, (2, 4). * **Valence Electrons:** 4. ### Exercises 1. Fill in the blanks for a neutral atom: * The atomic number is equal to the number of __________. * The mass number is equal to the number of protons plus the number of __________. * In a neutral atom, the number of electrons is equal to the number of __________. 2. For a Sodium atom (Na), which has an Atomic Number (Z) of 11 and a Mass Number (A) of 23: * How many protons does it have? * How many electrons does it have? * How many neutrons does it have? * Draw the Bohr's model for a Sodium atom, showing the nucleus and electron shells. 3. What is the maximum number of electrons that can be held in: * The first electron shell? * The second electron shell? ### Assignment 1. Draw the Bohr's model for the following elements. For each, identify the number of protons, neutrons, electrons, and valence electrons: * Oxygen (O): Z = 8, A = 16 * Magnesium (Mg): Z = 12, A = 24 * Chlorine (Cl): Z = 17, A = 35 2. Explain in your own words why valence electrons are important in chemistry. 3. Research and briefly describe one limitation of Bohr's model of the atom. ### Summary Today, we explored **Bohr's Model of the Atom**, which provides a clear picture of how atoms are structured. We learned that an atom consists of a central **nucleus** containing positively charged **protons** and neutral **neutrons**. Negatively charged **electrons** orbit this nucleus in specific, fixed energy levels called **electron shells**. The number of protons is the **atomic number** (Z), which defines the element. The sum of protons and neutrons is the **mass number** (A). Electrons in the outermost shell, known as **valence electrons**, are crucial for determining an atom's chemical behavior. Bohr's model was a significant step forward in understanding the quantized nature of electron energy within atoms. ## Relationship between subatomic particles ### Introduction Welcome to this lesson on the fascinating world inside an atom! You've learned that atoms are the basic building blocks of all matter. But what are atoms made of? In this lesson, we will explore the tiny particles that make up an atom – protons, neutrons, and electrons – and understand how their numbers and arrangement define an atom's identity and properties. ### Key Concepts #### What are Subatomic Particles? Atoms are not indivisible; they are composed of even smaller particles called subatomic particles. The three main subatomic particles are: * **Protons:** Positively charged particles found in the nucleus. * **Neutrons:** Neutrally charged particles (no charge) found in the nucleus. * **Electrons:** Negatively charged particles that orbit the nucleus. #### The Nucleus: The Atom's Core The **nucleus** is the dense central part of an atom. It contains protons and neutrons. The nucleus accounts for almost all of an atom's mass. #### Characteristics of Subatomic Particles Let's look at the properties of each subatomic particle: | Particle | Location | Relative Charge | Relative Mass (amu) | | :------- | :----------------- | :-------------- | :------------------ | | Proton | Nucleus | +1 | 1 | | Neutron | Nucleus | 0 (neutral) | 1 | | Electron | Orbiting the nucleus | -1 | 1/1836 (negligible) | #### Atomic Number (Z) – The Element's ID Card * The **Atomic Number (Z)** is the number of **protons** in an atom's nucleus. * It is a unique identifier for each element. Every atom of a specific element has the same atomic number. * For example, all carbon atoms have 6 protons (Z=6), and all oxygen atoms have 8 protons (Z=8). #### Mass Number (A) – The Atom's Weight * The **Mass Number (A)** is the total number of **protons and neutrons** in an atom's nucleus. * Electrons have such a small mass that they are not included when calculating the mass number. * The formula for mass number is: \[ A = \text{Number of Protons} + \text{Number of Neutrons} \] * This also means the number of neutrons can be calculated as: \[ \text{Number of Neutrons} = A - Z \] #### Neutral Atoms: A Balance of Charges * In a neutral atom, the number of **protons** (positive charges) is equal to the number of **electrons** (negative charges). * This balance results in a net charge of zero for the atom. #### Ions: Atoms with a Charge * An **ion** is an atom (or group of atoms) that has gained or lost one or more electrons, resulting in a net electrical charge. * **Cation:** A positively charged ion. Formed when an atom loses electrons. (e.g., \( \text{Na}^+ \) has lost one electron). * **Anion:** A negatively charged ion. Formed when an atom gains electrons. (e.g., \( \text{Cl}^- \) has gained one electron). * The number of protons does *not* change when an atom forms an ion; only the number of electrons changes. #### Isotopes: Same Element, Different Neutrons * **Isotopes** are atoms of the same element (meaning they have the same atomic number, Z, and thus the same number of protons) but have different numbers of neutrons. * Because they have different numbers of neutrons, isotopes of the same element have different mass numbers (A). * For example, Carbon-12 and Carbon-14 are isotopes of carbon. Both have 6 protons, but Carbon-12 has 6 neutrons, while Carbon-14 has 8 neutrons. ### Vocabulary * **Atom:** The smallest unit of an element that retains the chemical properties of that element. * **Subatomic Particle:** Particles smaller than an atom (protons, neutrons, electrons). * **Proton:** A positively charged subatomic particle found in the nucleus. * **Neutron:** A neutrally charged subatomic particle found in the nucleus. * **Electron:** A negatively charged subatomic particle that orbits the nucleus. * **Nucleus:** The dense, positively charged center of an atom, containing protons and neutrons. * **Atomic Number (Z):** The number of protons in an atom's nucleus; defines the element. * **Mass Number (A):** The total number of protons and neutrons in an atom's nucleus. * **Element:** A pure substance consisting only of atoms that all have the same numbers of protons in their atomic nuclei. * **Ion:** An atom or molecule with a net electrical charge due to the loss or gain of one or more electrons. * **Cation:** A positively charged ion. * **Anion:** A negatively charged ion. * **Isotope:** Atoms of the same element (same number of protons) that have different numbers of neutrons. ### Examples **Example 1: A Neutral Carbon Atom** Carbon has an atomic number (Z) of 6 and a mass number (A) of 12. * Number of Protons (Z) = 6 * Number of Electrons (in a neutral atom) = Number of Protons = 6 * Number of Neutrons = Mass Number (A) - Atomic Number (Z) = 12 - 6 = 6 **Example 2: A Sodium Ion (Na+)** Sodium (Na) has an atomic number (Z) of 11 and a mass number (A) of 23. The \( \text{Na}^+ \) ion has a +1 charge, meaning it has lost one electron. * Number of Protons (Z) = 11 * Number of Neutrons = Mass Number (A) - Atomic Number (Z) = 23 - 11 = 12 * Number of Electrons = Number of Protons - Charge = 11 - (+1) = 10 **Example 3: A Chloride Ion (Cl-)** Chlorine (Cl) has an atomic number (Z) of 17 and a mass number (A) of 35. The \( \text{Cl}^- \) ion has a -1 charge, meaning it has gained one electron. * Number of Protons (Z) = 17 * Number of Neutrons = Mass Number (A) - Atomic Number (Z) = 35 - 17 = 18 * Number of Electrons = Number of Protons - Charge = 17 - (-1) = 18 **Example 4: Isotopes of Hydrogen** Hydrogen has three common isotopes: * **Protium (\( \text{}_1^1\text{H} \)):** 1 proton, 0 neutrons, 1 electron. (Mass Number = 1) * **Deuterium (\( \text{}_1^2\text{H} \)):** 1 proton, 1 neutron, 1 electron. (Mass Number = 2) * **Tritium (\( \text{}_1^3\text{H} \)):** 1 proton, 2 neutrons, 1 electron. (Mass Number = 3) Notice that all three have 1 proton (Z=1), making them all hydrogen, but their neutron counts differ, leading to different mass numbers. ### Exercises 1. What defines the identity of an element? a) Number of electrons b) Number of neutrons c) Number of protons d) Mass number 2. An atom has 9 protons, 10 neutrons, and 9 electrons. What is its atomic number and mass number? 3. Calculate the number of protons, neutrons, and electrons for a neutral atom of Oxygen (O) if its atomic number is 8 and its mass number is 16. 4. Consider an atom with 19 protons, 20 neutrons, and 18 electrons. a) What is the atomic number? b) What is the mass number? c) Is this atom neutral or an ion? If it's an ion, what is its charge? 5. Explain the difference between \( \text{}^{35}\text{Cl} \) and \( \text{}^{37}\text{Cl} \) in terms of their subatomic particles. (Chlorine, Z=17). ### Assignment 1. Complete the following table: | Element/Ion | Atomic Number (Z) | Mass Number (A) | Protons | Neutrons | Electrons | Charge | | :---------- | :---------------- | :-------------- | :------ | :------- | :-------- | :----- | | Potassium | 19 | 39 | | | | 0 | | Bromide | 35 | 80 | | | | -1 | | \( \text{Al}^{3+} \) | 13 | 27 | | | | | | | 8 | 18 | | | | 0 | | | | | 20 | 20 | 18 | | 2. Research and write a short paragraph about a practical application of isotopes in medicine or dating ancient artifacts. 3. Draw a simple diagram of a Lithium atom (Z=3, A=7). Clearly label the nucleus, protons, neutrons, and electrons. Indicate the charges for each particle. ### Summary In this lesson, we have explored the fundamental relationship between subatomic particles within an atom. We learned that **protons** define the element's identity through the **atomic number (Z)**. The combined count of **protons and neutrons** gives us the **mass number (A)**. In neutral atoms, the number of protons equals the number of **electrons**. When atoms gain or lose electrons, they become charged **ions**. Finally, we discovered that **isotopes** are variations of the same element, sharing the same number of protons but differing in their number of neutrons. Understanding these relationships is crucial for comprehending chemical properties and reactions.