Class 9 Science NCERT Notes – Chapter 3: Atoms and Molecules (PDF, MindMap, Q&A, Quizzes)

Chapter 3 (Chemistry): Atoms and Molecules – Class 9 NCERT Science Detailed Study Notes.

1. Historical Foundations of Atomic Theory

The concept that matter is composed of fundamental, indivisible particles dates back to ancient times.

• Ancient Indian Philosophy (circa 500 BC):
  • Maharishi Kanad: Postulated that matter (padarth) could be divided into progressively smaller particles until a stage is reached where no further division is possible. He named these ultimate, indivisible particles Parmanu.
  • Pakudha Katyayama: Expanded on this idea, suggesting that Parmanu normally exist in a combined form, which gives rise to the various forms of matter we observe.
• Ancient Greek Philosophy (Same Era):
  • Democritus and Leucippus: Independently proposed that if matter is continuously divided, one would eventually reach a particle that cannot be divided further.
  • Democritus: Coined the term atoms (from the Greek word atomos, meaning “indivisible”) to describe these particles.

These early ideas were philosophical in nature and lacked the experimental validation that would come centuries later.

2. The Laws of Chemical Combination

By the late 18th century, experimental science laid the groundwork for modern chemistry. Antoine L. Lavoisier, through experiments conducted with Joseph L. Proust, established two fundamental laws governing chemical reactions.

2.1 Law of Conservation of Mass

This law addresses the question of “whether mass changes during a chemical reaction?”.

• Statement: Mass can neither be created nor destroyed in a chemical reaction.
• Implication: The total mass of the reactants (the substances that react) is equal to the total mass of the products (the substances formed) in any chemical reaction.
• Example: In a reaction where 5.3 g of sodium carbonate reacts with 6.0 g of acetic acid, the total mass of reactants is 11.3 g. The products are 2.2 g of carbon dioxide, 0.9 g of water, and 8.2 g of sodium acetate, for a total product mass of 11.3 g, thus demonstrating the conservation of mass.

2.2 Law of Constant Proportions (or Definite Proportions)

This law describes the composition of chemical compounds.

• Statement (by Proust): “In a chemical substance the elements are always present in definite proportions by mass.”
• Implication: A given compound, regardless of its origin or method of preparation, always contains the same elements combined in the same fixed ratio by mass.
• Examples:
  • Water (H₂O): The ratio of the mass of hydrogen to the mass of oxygen is always 1:8. Decomposing 9 g of water will always yield 1 g of hydrogen and 8 g of oxygen.
  • Ammonia (NH₃): Nitrogen and hydrogen are always present in the ratio 14:3 by mass.

3. Dalton’s Atomic Theory

In 1808, British chemist John Dalton proposed a comprehensive theory about the nature of matter that provided a scientific explanation for the laws of chemical combination. He built upon the ancient Greek idea of atoms.

Postulates of Dalton’s Atomic Theory:

1. All matter is composed of very tiny particles called atoms, which participate in chemical reactions.
2. Atoms are indivisible particles that cannot be created or destroyed in a chemical reaction. (This postulate directly explains the Law of Conservation of Mass).
3. Atoms of a given element are identical in mass and chemical properties.
4. Atoms of different elements have different masses and chemical properties.
5. Atoms combine in the ratio of small whole numbers to form compounds.
6. The relative number and kinds of atoms are constant in a given compound. (This postulate directly explains the Law of Constant Proportions).

4. The Atom: The Building Block of Matter

Atoms are the fundamental building blocks of all matter.

• Size: Atoms are incredibly small, far smaller than anything imaginable. Their size is measured in nanometers (nm).
  • 1 nm = 10⁻⁹ m
  • The radius of a hydrogen atom is approximately 10⁻¹⁰ m.
• Symbols:
  • Dalton: Was the first to use symbols to represent elements, with each symbol also representing one atom of that element.
  • Berzelius: Suggested using one or two letters from the element’s name as its symbol.
  • Modern System (IUPAC): The International Union of Pure and Applied Chemistry approves names and symbols.
    • Symbols are one or two letters from the element’s name (in English, Latin, German, or Greek).
    • The first letter is always uppercase; the second is always lowercase (e.g., Al for Aluminium, not AL; Co for Cobalt, not CO).
    • Examples from Latin names: Iron (Fe, from ferrum), Sodium (Na, from natrium), Potassium (K, from kalium).

4.1 Atomic Mass

• Concept: A key part of Dalton’s theory was that each element has a characteristic atomic mass.
• Atomic Mass Unit (u): Since measuring the mass of a single atom is difficult, a relative scale was developed.
  • Standard Reference: In 1961, the carbon-12 isotope was chosen as the standard.
  • Definition: One atomic mass unit (u) is defined as exactly one-twelfth (1/12th) the mass of one atom of carbon-12.
  • The relative atomic mass of an element is the average mass of its atoms compared to 1/12th the mass of a carbon-12 atom.

5. Molecules and Ions

Atoms of most elements do not exist independently; they combine to form molecules and ions.

5.1 Molecules

• Definition: A molecule is a group of two or more atoms chemically bonded together. It is the smallest particle of an element or compound that can exist independently and retain the substance’s properties.
• Molecules of Elements: Composed of the same type of atoms.
  • Atomicity: The number of atoms in a molecule of an element.
  • Monoatomic: Consist of a single atom (e.g., Helium – He, Argon – Ar).
  • Diatomic: Consist of two atoms (e.g., Oxygen – O₂, Hydrogen – H₂).
  • Tetra-atomic: Consist of four atoms (e.g., Phosphorus – P₄).
  • Poly-atomic: Consist of more than two atoms (e.g., Sulphur – S₈, Ozone – O₃).
• Molecules of Compounds: Formed when atoms of different elements combine in definite, fixed-number ratios.
  • Example (Water): The ratio of the number of hydrogen atoms to oxygen atoms is always 2:1, resulting in the formula H₂O.

5.2 Ions

• Definition: An ion is a charged particle formed from an atom or a group of atoms. Compounds made of metals and non-metals often contain ions.
• Types of Ions:
  • Cation: A positively charged ion (e.g., Sodium ion – Na⁺, Magnesium ion – Mg²⁺).
  • Anion: A negatively charged ion (e.g., Chloride ion – Cl⁻, Oxide ion – O²⁻).
• Polyatomic Ions: A cluster of atoms that acts as a single unit and carries a fixed charge.
  • Examples: Ammonium (NH₄⁺), Hydroxide (OH⁻), Sulphate (SO₄²⁻), Carbonate (CO₃²⁻).

6. Writing Chemical Formulae

A chemical formula is a symbolic representation of the composition of a compound.

• Valency: The combining power or capacity of an element. It determines how many atoms of another element it can combine with.
• Rules for Writing Formulae:
  1. The valencies or charges on the ions must balance, making the overall compound neutral.
  2. For compounds with a metal and a non-metal, the symbol of the metal is written first (e.g., NaCl, CaO).
  3. When a polyatomic ion is used more than once in a formula, it is enclosed in brackets, and the number of ions is written as a subscript outside the bracket (e.g., Ca(OH)₂, Al₂(SO₄)₃). If only one polyatomic ion is present, no brackets are needed (e.g., NaOH).
• Criss-Cross Method:
  1. Write the symbols of the constituent elements or ions side-by-side.
  2. Write the valency or charge above each symbol.
  3. “Criss-cross” the valencies, bringing the numbers down to become the subscripts for the opposite element/ion (ignoring the charges).
  4. Simplify the ratio of subscripts to the smallest whole numbers if possible.
  • Example (Aluminium Oxide): Al has a valency of 3⁺ and O has 2^–. Criss-crossing gives Al₂O₃.
  • Example (Calcium Oxide): Ca has 2⁺ and O has 2^–. Criss-crossing gives Ca₂O₂, which simplifies to CaO.

7. Molecular Mass and Formula Unit Mass

• Molecular Mass: The sum of the atomic masses of all atoms in a single molecule of a substance. It is expressed in atomic mass units (u).
  • Example (H₂O): (2 × Atomic Mass of H) + (1 × Atomic Mass of O) = (2 × 1 u) + (1 × 16 u) = 18 u.
• Formula Unit Mass: Used for substances whose constituent particles are ions (ionic compounds). It is the sum of the atomic masses of all atoms in a formula unit of the compound. The calculation is identical to that of molecular mass.
  • Example (NaCl): (1 × Atomic Mass of Na) + (1 × Atomic Mass of Cl) = (1 × 23 u) + (1 × 35.5 u) = 58.5 u.

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Q&A Section

Short-Answer Quiz

Answer each question in 2-3 sentences.

1. Who were the ancient Indian philosophers who first proposed the idea of indivisible particles of matter, and what did they call these particles?
2. What was the contribution of the Greek philosophers Democritus and Leucippus to the concept of matter?
3. State the Law of Conservation of Mass as established by Antoine L. Lavoisier.
4. Explain the Law of Constant Proportions using the example of water.
5. Which two postulates of Dalton’s Atomic Theory provide a direct explanation for the two main laws of chemical combination?
6. Describe the modern system for creating symbols for chemical elements, including the rule for capitalization.
7. Why was the carbon-12 isotope chosen as the standard reference for atomic mass in 1961?
8. Define the atomic mass unit (u).
9. What is the difference between a molecule of an element and a molecule of a compound?
10. Define “atomicity” and provide an example of a monoatomic and a diatomic element.
11. How are atoms, which are too small to be seen with the naked eye, visualised in the modern era?
12. What is an ion, and what are the names for positively and negatively charged ions?
13. Give an example of a simple ionic compound and identify its constituent cation and anion.
14. What is a polyatomic ion? Provide two examples from the text.
15. Define the term “valency” in the context of chemical combination.
16. Briefly explain the “criss-cross” method for determining the chemical formula of a compound.
17. What are the three main rules to follow when writing a chemical formula?
18. When are brackets used in writing a chemical formula? Give an example.
19. What is a binary compound?
20. Explain the difference between molecular mass and formula unit mass, including when each term is used.
21. Calculate the molecular mass of nitric acid (HNO₃), given the atomic masses H=1u, N=14u, O=16u.
22. How do most atoms exist in nature, and why is this the case?
23. Describe the origins of the names for the elements copper and gold.
24. What is the IUPAC, and what is its role in chemistry?
25. According to Dalton’s theory, how are the atoms of a specific element different from the atoms of any other element?

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Multiple-Choice Quiz

Select the best answer for each question.

1. The concept of Parmanu was postulated by which Indian philosopher?
   • a) Pakudha Katyayama b) Democritus c) Maharishi Kanad d) Antoine L. Lavoisier
2. The Law of Constant Proportions was stated by:
   • a) John Dalton b) Joseph L. Proust c) Leucippus d) Berzelius
3. In ammonia, what is the constant ratio by mass of nitrogen to hydrogen?
   • a) 1:8 b) 3:8 c) 14:3 d) 3:14
4. Which postulate of Dalton’s theory states that atoms cannot be created or destroyed?
   • a) Postulate i b) Postulate ii c) Postulate v d) Postulate vi
5. Atomic radius is typically measured in which unit?
   • a) Micrometres b) Millimetres c) Nanometres d) Picometres
6. Who first suggested that symbols of elements should be made from one or two letters of their name?
   • a) Dalton b) Proust c) Lavoisier d) Berzelius
7. The symbol for potassium is ‘K’ which is derived from its Latin name:
   • a) Kalium b) Plumbum c) Natrium d) Ferrum
8. One atomic mass unit (u) is defined as:
   • a) The mass of one hydrogen atom b) 1/16th the mass of one oxygen-16 atom c) 1/12th the mass of one carbon-12 atom d) The mass of one proton
9. Which of the following elements exists as a monoatomic molecule?
   • a) Oxygen b) Hydrogen c) Helium d) Chlorine
10. The atomicity of sulphur is described as:
    • a) Diatomic b) Tetra-atomic c) Monoatomic d) Poly-atomic
11. A negatively charged ion is called a(n):
    • a) Cation b) Anion c) Polyatomic ion d) Atom
12. The symbol for the sulphate ion is:
    • a) SO₃²⁻ b) S²⁻ c) SO₄²⁻ d) OH⁻
13. What is the valency of the magnesium ion (Mg²⁺)?
    • a) 1 b) 2 c) 3 d) 4
14. Using the criss-cross method, what is the correct chemical formula for aluminium chloride?
    • a) AlCl b) AlCl₂ c) Al₃Cl d) AlCl₃
15. The formula Ca(OH)₂ indicates that for every one calcium atom, there are:
    • a) One oxygen atom and one hydrogen atom b) One oxygen atom and two hydrogen atoms c) Two oxygen atoms and two hydrogen atoms d) Two oxygen atoms and one hydrogen atom
16. What is the correct name for the compound K₂SO₄?
    • a) Potassium sulphite b) Potassium sulphide c) Potassium sulphate d) Calcium sulphate
17. The term used for the mass of an ionic compound like CaCl₂ is:
    • a) Atomic mass b) Molecular mass c) Formula unit mass d) Ionic mass
18. What is the calculated molecular mass of methane (CH₄)? (C=12u, H=1u) a) 13 u b) 16 u c) 24 u d) 48 u
19. Which of the following is an example of a polyatomic ion?
    • a) Cl⁻ b) Na⁺ c) NH₄⁺ d) O²⁻
20. When writing the formula for an ionic compound, which element is written first?
    • a) The non-metal b) The metal c) The element with the lower atomic mass d) The element with the higher valency

Essay Questions

1. Trace the historical development of the concept of the atom, from the philosophical ideas of ancient India and Greece to the scientific theory proposed by John Dalton.
2. Explain the two fundamental laws of chemical combination: the Law of Conservation of Mass and the Law of Constant Proportions. Provide a clear example for each law as described in the source text.
3. List and elaborate on the six postulates of Dalton’s Atomic Theory. For each postulate, explain its significance in understanding the nature of matter and chemical reactions.
4. Discuss the evolution and modern conventions of chemical symbols for elements. Include the contributions of Dalton and Berzelius, and the role of the IUPAC. Use examples like Cobalt, Sodium, and Iron to illustrate the rules.
5. Define atomic mass and the atomic mass unit (u). Explain why a relative mass standard was necessary and describe the reasoning behind choosing carbon-12 as the current standard.
6. Distinguish between atoms, molecules, and ions. Describe the different types of molecules (of elements and compounds) and ions (cations, anions, polyatomic), providing specific examples for each category.
7. What is atomicity? Using a table format, classify the following elements based on their atomicity as mentioned in the text: Argon, Oxygen, Phosphorus, and Sulphur.
8. Explain the concept of valency and its importance in chemistry. Describe, step-by-step, how to use the “criss-cross” method to write the chemical formulae for magnesium chloride and aluminium oxide.
9. What are the key rules for writing chemical formulae for compounds, particularly those involving metals, non-metals, and polyatomic ions? Illustrate these rules with the examples of sodium chloride (NaCl), calcium hydroxide (Ca(OH)₂), and sodium carbonate (Na₂CO₃).
10. Describe the process of calculating the molecular mass and formula unit mass of a substance. Calculate the molecular mass of ethyne (C₂H₂) and the formula unit mass of potassium carbonate (K₂CO₃) using the atomic masses provided in the text (H=1, C=12, O=16, K=39).

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Answer Keys

Answer Key for Short-Answer Quiz

1. The Indian philosopher Maharishi Kanad postulated that matter (padarth) is composed of smaller and smaller particles. He named the ultimate, indivisible particle Parmanu. Pakudha Katyayama further elaborated that these particles exist in a combined form.
2. The Greek philosophers Democritus and Leucippus suggested that if matter is continuously divided, one would eventually reach an indivisible particle. Democritus named these particles “atoms,” meaning indivisible.
3. The Law of Conservation of Mass states that mass can neither be created nor destroyed in a chemical reaction. This means the total mass of the reactants equals the total mass of the products.
4. The Law of Constant Proportions states that elements in a compound are always present in definite proportions by mass. For example, water (H₂O) always consists of hydrogen and oxygen in a mass ratio of 1:8, regardless of its source.
5. Dalton’s second postulate (atoms are indivisible and cannot be created or destroyed) explains the Law of Conservation of Mass. His sixth postulate (the relative number and kinds of atoms are constant in a given compound) explains the Law of Constant Proportions.
6. The modern system uses one or two letters from an element’s name. The first letter is always written as a capital letter (uppercase), and the second letter, if present, is a small letter (lowercase).
7. The carbon-12 isotope was chosen for a universally accepted atomic mass unit. This was considered relevant because it gave the masses of most elements as whole numbers or very close to them.
8. One atomic mass unit (u) is a mass unit defined as being exactly equal to one-twelfth (1/12th) the mass of one atom of the carbon-12 isotope.
9. A molecule of an element is made up of only one type of atom (e.g., O₂). A molecule of a compound is made up of atoms of two or more different elements joined together in a fixed ratio (e.g., H₂O).
10. “Atomicity” is the number of atoms that constitute a molecule of an element. For example, Helium (He) is monoatomic (one atom), while Oxygen (O₂) is diatomic (two atoms).
11. While atoms cannot be seen with the naked eye, modern techniques can produce magnified images of the surfaces of elements that show the individual atoms, such as the surface of silicon.
12. An ion is a charged particle formed from an atom or group of atoms. A positively charged ion is called a cation, and a negatively charged ion is called an anion.
13. Sodium chloride (NaCl) is a simple ionic compound. It is composed of a positively charged sodium ion (Na⁺), which is the cation, and a negatively charged chloride ion (Cl⁻), which is the anion.
14. A polyatomic ion is a group of atoms that are bonded together and carry an overall net charge. Examples from the text include the ammonium ion (NH₄⁺) and the hydroxide ion (OH⁻).
15. Valency is the combining power or capacity of an element. It can be thought of as the number of “arms” an atom has to form chemical bonds with other atoms.
16. The criss-cross method involves writing the symbols and valencies of two combining ions or elements. The numerical value of each ion’s valency is then crossed over and becomes the subscript for the other ion to create a neutral formula.
17. The three main rules are: 1) the valencies or charges on the ions must balance; 2) the symbol of the metal is written first in a metal/non-metal compound; 3) brackets are used for polyatomic ions when there is more than one.
18. Brackets are used when a compound contains two or more of the same polyatomic ion. For example, in magnesium hydroxide, Mg(OH)₂, brackets enclose the OH ion to show two hydroxyl groups are bonded to one magnesium atom.
19. Binary compounds are the simplest type of compounds, which are made up of two different elements. Examples include hydrogen chloride (HCl) and carbon tetrachloride (CCl₄).
20. Molecular mass is the sum of atomic masses in a molecule of a covalent substance. Formula unit mass is the sum of atomic masses in a formula unit of an ionic compound; the calculation is the same, but the terminology reflects the ionic nature of the substance.
21. The molecular mass of HNO₃ is the sum of the atomic masses: (1 × H) + (1 × N) + (3 × O). This equals 1 u + 14 u + (3 × 16 u) = 1 + 14 + 48 = 63 u.
22. Atoms of most elements cannot exist independently. They form molecules and ions, which then aggregate in large numbers to form the matter we can see, feel, and touch.
23. The name “copper” was taken from Cyprus, the place where it was first found. The name “gold” was taken from an English word meaning yellow, referring to its color.
24. IUPAC stands for the International Union of Pure and Applied Chemistry. It is an international scientific organization responsible for approving the names of elements, symbols, and units.
25. According to Dalton’s theory, atoms of different elements have different masses and different chemical properties. This is what distinguishes one element from another.

Answer Key for Multiple-Choice Quiz

1. c) Maharishi Kanad
2. b) Joseph L. Proust
3. c) 14:3
4. b) Postulate ii
5. c) Nanometres
6. d) Berzelius
7. a) Kalium
8. c) 1/12th the mass of one carbon-12 atom
9. c) Helium
10. d) Poly-atomic
11. b) Anion
12. c) SO₄²⁻
13. b) 2
14. d) AlCl₃
15. c) Two oxygen atoms and two hydrogen atoms
16. c) Potassium sulphate
17. c) Formula unit mass
18. b) 16 u
19. c) NH₄⁺
20. b) The metal

Essay Question Answers

1. The concept of the atom began as a philosophical idea. Around 500 BC, Indian philosopher Maharishi Kanad proposed that matter (padarth) was made of indivisible particles he called Parmanu. Around the same time, Greek philosophers Democritus and Leucippus theorized that continuous division of matter would lead to an ultimate, uncuttable particle, which Democritus named the “atom.” These ideas remained philosophical until the late 18th century when experimental science began to provide evidence. The work of Lavoisier and Proust established the laws of chemical combination, setting the stage for a scientific theory. In 1808, John Dalton introduced his atomic theory, which took the philosophical idea of the atom and used it to provide a robust explanation for these experimental laws, marking the transition of the atom from a concept to a cornerstone of chemical science.
2. The two fundamental laws of chemical combination are:
   • The Law of Conservation of Mass: This law states that mass is neither created nor destroyed during a chemical reaction. This means the total mass of all substances before the reaction (reactants) must equal the total mass of all substances after the reaction (products). An example is the reaction of 5.3 g sodium carbonate with 6 g acetic acid (total reactants 11.3 g) to produce 8.2 g sodium acetate, 2.2 g carbon dioxide, and 0.9 g water (total products 11.3 g).
   • The Law of Constant Proportions: Also known as the Law of Definite Proportions, this law states that a pure chemical compound always contains the same elements combined in the same fixed proportion by mass. For instance, water (H₂O) always contains hydrogen and oxygen in a mass ratio of 1:8, regardless of whether it comes from a river or is synthesised in a lab.
3. Dalton’s six postulates are:
   1. All matter is made of atoms: This establishes the atom as the fundamental particle of matter that participates in reactions.
   2. Atoms are indivisible and indestructible: This explains the Law of Conservation of Mass, as reactions only rearrange atoms, not create or destroy them.
   3. Atoms of a given element are identical in mass and properties: This defines an element as a substance composed of only one type of atom.
   4. Atoms of different elements have different masses and properties: This distinguishes one element from another on a fundamental level.
   5. Atoms combine in small whole-number ratios to form compounds: This explains why compounds have discrete, consistent formulae.
   6. The relative number and kinds of atoms are constant in a compound: This explains the Law of Constant Proportions, as the fixed ratio of atoms results in a fixed ratio of mass.
4. The use of symbols began with John Dalton, who used unique pictorial symbols to represent one atom of an element. This system was cumbersome. The chemist Berzelius proposed a more practical system where symbols were derived from one or two letters of the element’s name. This forms the basis of our modern system, which is approved by the IUPAC. The modern convention is that the first letter is always capitalised and the second, if present, is lowercase. For example, Cobalt is ‘Co’ to distinguish it from the compound carbon monoxide ‘CO’. Many symbols come from English names (e.g., Hydrogen, H), while others derive from Latin, Greek or German names, such as Sodium being ‘Na’ (from natrium) and Iron being ‘Fe’ (from ferrum).
5. Atomic mass is the characteristic mass of an atom of a particular element. Since individual atoms are too small to weigh directly, a relative scale was created using a standard reference. The atomic mass unit (u) is the unit for this scale. Initially, 1/16th of the mass of an oxygen atom was used, but in 1961, the carbon-12 isotope was universally adopted as the standard. One atomic mass unit (u) is now defined as exactly 1/12th the mass of a single atom of carbon-12. This standard was chosen because it provided a convenient basis for measurement and resulted in the atomic masses of most other elements being close to whole numbers.
   • Atoms are the smallest, fundamental particles of an element that retain the chemical properties of that element (e.g., a single atom of Carbon, C).
   • Molecules are formed when two or more atoms are chemically bonded together. They are the smallest particle of a substance capable of independent existence. Molecules of elements consist of identical atoms (e.g., O₂), while molecules of compounds consist of different atoms in a fixed ratio (e.g., H₂O).
   • Ions are atoms or groups of atoms that have gained or lost electrons, resulting in a net electrical charge. Positively charged ions are cations (e.g., Na⁺, Ca²⁺), formed by losing electrons. Negatively charged ions are anions (e.g., Cl⁻, S²⁻), formed by gaining electrons. Polyatomic ions are molecules with a net charge (e.g., NH₄⁺, SO₄²⁻).
6. Atomicity is the number of atoms present in a single molecule of an element. It describes how atoms of an element bond with each other to exist in a stable state. | Element | Atomicity | Classification | | :— | :— | :— | | Argon | 1 | Monoatomic | | Oxygen | 2 | Diatomic | | Phosphorus | 4 | Tetra-atomic | | Sulphur | Multiple | Poly-atomic |
7. Valency is the combining capacity of an element’s atom. It determines the ratio in which atoms combine to form compounds. Magnesium Chloride (MgCl₂):
   1. Symbols: Mg and Cl
   2. Valencies: Mg has a valency of 2+; Cl has a valency of 1-.
   3. Criss-Cross: The ‘2’ from Mg crosses to Cl, and the ‘1’ from Cl crosses to Mg.
   4. Formula: Mg₁Cl₂ which is written as MgCl₂. Aluminium Oxide (Al₂O₃):
   5. Symbols: Al and O
   6. Valencies: Al has a valency of 3+; O has a valency of 2-.
   7. Criss-Cross: The ‘3’ from Al crosses to O, and the ‘2’ from O crosses to Al.
   8. Formula: Al₂O₃. This ratio is already in its simplest form.
8. The key rules for writing chemical formulae are:
   1. Balance Charges: The overall charge of the compound must be neutral. The total positive charge from the cations must balance the total negative charge from the anions. For example, in sodium carbonate (Na₂CO₃), two Na⁺ ions (+2 total) balance one CO₃²⁻ ion (-2 total).
   2. Order of Elements: In a compound of a metal and a non-metal, the metal’s symbol is written first. For example, in sodium chloride, sodium (Na) is written before chlorine (Cl).
   3. Use of Brackets: When a formula contains more than one of a specific polyatomic ion, the ion’s formula is placed in brackets with a subscript indicating its number. For example, in calcium hydroxide, there are two hydroxide (OH⁻) ions for every one calcium (Ca²⁺) ion, so the formula is Ca(OH)₂. If there is only one polyatomic ion, no brackets are needed, as in NaOH.
9. The process involves summing the atomic masses of all atoms present in the chemical formula.
   • Molecular Mass of Ethyne (C₂H₂):
     • Formula: C₂H₂
     • Atoms: 2 Carbon atoms, 2 Hydrogen atoms
     • Calculation: (2 × Atomic Mass of C) + (2 × Atomic Mass of H)
     • Result: (2 × 12 u) + (2 × 1 u) = 24 u + 2 u = 26 u.
   • Formula Unit Mass of Potassium Carbonate (K₂CO₃):
     • Formula: K₂CO₃
     • Atoms: 2 Potassium atoms, 1 Carbon atom, 3 Oxygen atoms
     • Calculation: (2 × Atomic Mass of K) + (1 × Atomic Mass of C) + (3 × Atomic Mass of O)
     • Result: (2 × 39 u) + (1 × 12 u) + (3 × 16 u) = 78 u + 12 u + 48 u = 138 u.

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Glossary of Key Terms

• Anion: A negatively charged ion.
• Atom: The smallest particle of an element that retains its chemical properties. They are the building blocks of all matter.
• Atomic Mass: The characteristic mass of an atom of an element, typically expressed in atomic mass units (u).
• Atomic Mass Unit (u): A unit of mass equal to exactly one-twelfth (1/12th) the mass of one atom of carbon-12.
• Atomicity: The number of atoms constituting a molecule.
• Binary Compound: A compound made up of two different elements.
• Cation: A positively charged ion.
• Chemical Formula: A symbolic representation of the composition of a compound, showing its constituent elements and the number of atoms of each.
• Formula Unit Mass: The sum of the atomic masses of all atoms in a formula unit of an ionic compound.
• Ion: An atom or a group of atoms having a net positive or negative charge.
• IUPAC: International Union of Pure and Applied Chemistry, an organization that approves names, symbols, and units in chemistry.
• Law of Conservation of Mass: States that mass can neither be created nor destroyed in a chemical reaction.
• Law of Constant Proportions: States that in a chemical substance, the elements are always present in definite proportions by mass. Also known as the Law of Definite Proportions.
• Molecule: A group of two or more atoms that are chemically bonded together and capable of independent existence. It is the smallest particle of an element or a compound that shows all the properties of that substance.
• Molecular Mass: The sum of the atomic masses of all the atoms in a molecule of a substance.
• Padarth: An ancient Indian term for matter.
• Parmanu: The name given by Maharishi Kanad to the smallest, indivisible particles of matter.
• Polyatomic Ion: A cluster of atoms that acts as a single ion and carries a fixed charge.
• Valency: The combining power or capacity of an element.