Chapter 3: Atoms and Molecules
This comprehensive study material is fully aligned with the KVS Class IX Science Student Support Material (2025-26). It details all foundational concepts of laws of chemical combinations, Dalton's atomic theory, atomic and molecular masses, writing chemical formulas, and polyatomic ions, with complete solved questions from the official bank.
1. Laws of Chemical Combination
The laws of chemical combination were established after much experimentations by Antoine L. Lavoisier and Joseph L. Proust.
(a) Law of Conservation of Mass
"Mass can neither be created nor destroyed in a chemical reaction."
In any chemical reaction, the total mass of the reactants is always equal to the total mass of the products.
Example: When $2.8\mathrm{~g}$ of nitrogen gas reacts completely with $0.6\mathrm{~g}$ of hydrogen gas, it produces exactly $3.4\mathrm{~g}$ of ammonia gas.
(b) Law of Constant Proportions (Law of Definite Proportions)
"In a chemical substance, the elements are always present in definite proportions by mass."
This is also known as Proust's law. No matter from which source or method a compound is obtained, it always contains the same elements combined in the same fixed mass ratio.
- In Water ($\mathrm{H_2O}$): Hydrogen and Oxygen always combine in the ratio of $1:8$ by mass. Thus, if $9\mathrm{~g}$ of water is decomposed, $1\mathrm{~g}$ of hydrogen and $8\mathrm{~g}$ of oxygen are always obtained.
- In Ammonia ($\mathrm{NH_3}$): Nitrogen and Hydrogen are always present in the ratio of $14:3$ by mass.
2. Dalton's Atomic Theory
British chemist John Dalton provided the basic theory about the nature of matter, providing an explanation for both laws of chemical combination. The main postulates of his theory are:
- All matter is made of very tiny particles called atoms, which participate in chemical reactions.
- Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction. (Explains the Law of Conservation of Mass).
- Atoms of a given element are identical in mass and chemical properties.
- Atoms of different elements have different masses and chemical properties.
- Atoms combine in the ratio of small whole numbers to form compounds. (Explains the Law of Constant Proportions).
- The relative number and kinds of atoms are constant in a given compound.
3. Atoms and Symbols
An atom is the smallest unit of an element that retains its chemical identity and participates in chemical reactions. Atoms are highly reactive and generally cannot exist independently (except noble gases).
IUPAC (International Union of Pure and Applied Chemistry) approves element names, symbols, and units. Many symbols are the first letter or first two letters of the element's English name. The first letter is always capitalized, and the second letter is lowercase (e.g., Hydrogen: $\mathrm{H}$, Helium: $\mathrm{He}$, Cobalt: $\mathrm{Co}$).
Elements with Latin/Greek/German Names
| English Name | Latin/German/Greek Origin | Symbol |
|---|---|---|
| Sodium | Natrium | \(\mathrm{Na}\) |
| Potassium | Kalium | \(\mathrm{K}\) |
| Iron | Ferrum | \(\mathrm{Fe}\) |
| Copper | Cuprum | \(\mathrm{Cu}\) |
| Silver | Argentum | \(\mathrm{Ag}\) |
| Mercury | Hydrargyrum | \(\mathrm{Hg}\) |
| Gold | Aurum | \(\mathrm{Au}\) |
4. Atomic Mass
One atomic mass unit is a mass unit equal to exactly one-twelfth (\(1/12^{\text{th}}\)) the mass of one atom of Carbon-12.
Carbon-12 is the universally accepted standard isotope reference for measuring relative atomic masses. Relative atomic mass is expressed in unified mass (\(\mathrm{u}\)) (which replaced 'amu').
Relative Atomic Masses of Common Elements
| Atomic No. (\(Z\)) | Element | Atomic Mass (\(\mathrm{u}\)) | Atomic No. (\(Z\)) | Element | Atomic Mass (\(\mathrm{u}\)) |
|---|---|---|---|---|---|
| 1 | Hydrogen (\(\mathrm{H}\)) | 1 | 10 | Neon (\(\mathrm{Ne}\)) | 20 |
| 2 | Helium (\(\mathrm{He}\)) | 4 | 11 | Sodium (\(\mathrm{Na}\)) | 23 |
| 3 | Lithium (\(\mathrm{Li}\)) | 7 | 12 | Magnesium (\(\mathrm{Mg}\)) | 24 |
| 4 | Beryllium (\(\mathrm{Be}\)) | 9 | 13 | Aluminium (\(\mathrm{Al}\)) | 27 |
| 5 | Boron (\(\mathrm{B}\)) | 11 | 14 | Silicon (\(\mathrm{Si}\)) | 28 |
| 6 | Carbon (\(\mathrm{C}\)) | 12 | 15 | Phosphorus (\(\mathrm{P}\)) | 31 |
| 7 | Nitrogen (\(\mathrm{N}\)) | 14 | 16 | Sulphur (\(\mathrm{S}\)) | 32 |
| 8 | Oxygen (\(\mathrm{O}\)) | 16 | 17 | Chlorine (\(\mathrm{Cl}\)) | 35.5 |
| 9 | Fluorine (\(\mathrm{F}\)) | 19 | 18 | Argon (\(\mathrm{Ar}\)) | 40 |
5. Molecules and Ions
Molecules
A molecule is the smallest particle of an element or compound capable of independent existence under normal conditions, showing all physical and chemical characteristics of that substance. It is a group of two or more atoms chemically bonded together.
- Molecules of Elements: Contain atoms of the same element.
- Monoatomic: \(\mathrm{He}\), \(\mathrm{Ne}\), \(\mathrm{Ar}\) (Noble gases)
- Diatomic: \(\mathrm{H_2}\), \(\mathrm{O_2}\), \(\mathrm{N_2}\), \(\mathrm{Cl_2}\)
- Triatomic: \(\mathrm{O_3}\) (Ozone)
- Tetra-atomic: \(\mathrm{P_4}\) (Phosphorus)
- Polyatomic: \(\mathrm{S_8}\) (Sulphur)
- Atomicity: The number of atoms constituting a molecule is known as its atomicity.
- Molecules of Compounds: Atoms of different elements join together in definite proportions by mass (e.g., \(\mathrm{H_2O}\) in mass ratio \(1:8\); \(\mathrm{CO_2}\) in mass ratio \(3:8\)).
Ions
An ion is a charged particle formed when an atom gains or loses electrons. Ions can be positively charged (cations) or negatively charged (anions). A group of atoms carrying a net charge is called a polyatomic ion.
- Cations (Metal ions usually): \(\mathrm{Na^+}\) (Sodium), \(\mathrm{Mg^{2+}}\) (Magnesium), \(\mathrm{Al^{3+}}\) (Aluminium).
- Anions (Non-metal ions usually): \(\mathrm{Cl^-}\) (Chloride), \(\mathrm{O^{2-}}\) (Oxide), \(\mathrm{N^{3-}}\) (Nitride).
- Polyatomic Ions: \(\mathrm{NH_4^+}\) (Ammonium), \(\mathrm{OH^-}\) (Hydroxide), \(\mathrm{NO_3^-}\) (Nitrate), \(\mathrm{HCO_3^-}\) (Hydrogencarbonate), \(\mathrm{CO_3^{2-}}\) (Carbonate), \(\mathrm{SO_4^{2-}}\) (Sulphate), \(\mathrm{PO_4^{3-}}\) (Phosphate).
6. Writing Chemical Formulae
The chemical formula of a compound is a symbolic representation of its composition. To write formulas, we cross-multiply (criss-cross) the valencies/charges of the combining atoms or polyatomic ions.
Rules:
- The valencies or charges on the ion must balance.
- When a compound consists of a metal and a non-metal, the name/symbol of the metal is written first (e.g., Calcium oxide: \(\mathrm{CaO}\)).
- In compounds formed with polyatomic ions, the ion is enclosed in a bracket before writing the subscript to indicate the ratio (e.g., Magnesium hydroxide: \(\mathrm{Mg(OH)_2}\)). If the polyatomic ion is 1, brackets are omitted (e.g., Sodium nitrate: \(\mathrm{NaNO_3}\)).
| Compound Name | Combining Symbols | Valency/Charge | Criss-Cross Formula |
|---|---|---|---|
| Hydrogen sulphide | \(\mathrm{H}\) \(\mathrm{S}\) | \(+1\) \(-2\) | \(\mathrm{H_2S}\) |
| Magnesium chloride | \(\mathrm{Mg}\) \(\mathrm{Cl}\) | \(+2\) \(-1\) | \(\mathrm{MgCl_2}\) |
| Aluminium oxide | \(\mathrm{Al}\) \(\mathrm{O}\) | \(+3\) \(-2\) | \(\mathrm{Al_2O_3}\) |
| Calcium hydroxide | \(\mathrm{Ca}\) \(\mathrm{OH}\) | \(+2\) \(-1\) | \(\mathrm{Ca(OH)_2}\) |
7. Molecular Mass & Formula Unit Mass
Molecular Mass: The sum of the atomic masses of all the atoms in a molecule of a substance. It is expressed in unified mass (\(\mathrm{u}\)).
Example: Calculation of Molecular Mass of Methyl Alcohol (\(\mathrm{CH_3OH}\)):
Formula Unit Mass: Used for substances whose constituent particles are ions. The calculation method is identical to molecular mass, but we use the term "formula unit" because individual molecules do not exist in ionic networks.
Example: Calculation of Formula Unit Mass of Zinc Oxide (\(\mathrm{ZnO}\)):
8. Chapterwise Question Bank
Multiple Choice Questions (1 Mark Each)
MCQ 1 Which law is supported when the total mass of reactants equals the total mass of products in a chemical reaction?
(a) Law of constant proportions (b) Law of multiple proportions
(c) Law of conservation of mass (d) Law of conservation of energy
MCQ 2 Dalton's atomic theory could explain the Law of Constant Proportions because it proposed that:
(a) Atoms can be divided (b) Atoms of different elements have identical mass
(c) Atoms combine in fixed ratios (d) Atoms can be created or destroyed
MCQ 3 During a classroom discussion on the molecular forms of non-metals, a student identified that elemental phosphorus exists as \(\mathrm{P_4}\). Based on this information, what is the atomicity of phosphorus?
(a) Diatomic (b) Monoatomic (c) Tetra-atomic (d) Polyatomic
MCQ 4 Which of the following compounds has a molecular mass of \(18\mathrm{~u}\)?
(a) \(\mathrm{HNO_3}\) (b) \(\mathrm{CO_2}\) (c) \(\mathrm{H_2O}\) (d) \(\mathrm{CH_4}\)
MCQ 5 While learning about isotopes, a student observed that carbon exists as both Carbon-12 and Carbon-14, which have different masses but are the same element. Based on this observation, which postulate of Dalton's atomic theory is invalidated by the discovery of isotopes?
(A) Atoms combine in fixed ratios
(B) Atoms are indivisible
(C) Atoms of the same element have the same mass
(D) Atoms participate in chemical reactions
MCQ 6 Carbon dioxide (\(\mathrm{CO_2}\)) is a compound made up of one carbon atom and two oxygen atoms. Given that the atomic mass of carbon (C) is \(12\mathrm{~u}\) and oxygen (O) is \(16\mathrm{~u}\), what is the molecular mass of carbon dioxide?
(A) \(32\mathrm{~u}\) (B) \(28\mathrm{~u}\) (C) \(44\mathrm{~u}\) (D) \(30\mathrm{~u}\)
Assertion-Reason Based Questions (1 Mark Each)
Directions: Choose (a) if both A and R are true and R is correct explanation of A; (b) if both A and R are true but R is not correct explanation; (c) if A is true but R is false; (d) if A is false but R is true.
A/R 7
Assertion (A): Atoms always combine to form molecules and ions.
Reason (R): Atoms of most elements are not able to exist independently.
A/R 8
Assertion (A): Atomicity of ozone is three while that of oxygen is two.
Reason (R): Atomicity is the number of atoms constituting a molecule.
A/R 9
Assertion (A): Atomic mass of aluminium is \(14\).
Reason (R): An atom of aluminium is \(27\) times heavier than \(1/12^{\text{th}}\) of the mass of carbon-12 atom.
A/R 10
Assertion (A): The valency of aluminium is \(3\) and oxygen is \(2\).
Reason (R): The chemical formula of aluminium oxide is \(\mathrm{Al_3O_2}\).
Very Short Answer Questions (2 Marks Each)
Q11 \(2.8\mathrm{~g}\) of nitrogen gas was allowed to react with \(0.6\mathrm{~g}\) of hydrogen gas to produce \(3.4\mathrm{~g}\) of ammonia. Show that these observations are in agreement with the law of conservation of mass. State the law.
Q12 Metals such as copper and aluminium are good conductors of electricity, while non-metals like sulphur and oxygen are not. Explain why metals conduct electricity and non-metals do not on the basis of atomic properties.
Q13 As per the law of definite proportions, carbon and oxygen combine in a ratio of \(3:8\). Compute the mass of oxygen gas that would be required to react completely with \(6\mathrm{~g}\) of carbon.
Q14 Find an element that is:
(a) Malleable and ductile
(b) Good conductor of electricity
(c) A constituent of water
(d) Liquid at room temperature
Q15 You are given a list of substances: oxygen (\(\mathrm{O_2}\)), carbon dioxide (\(\mathrm{CO_2}\)), iron (\(\mathrm{Fe}\)), and water (\(\mathrm{H_2O}\)). Which of these are compounds, and why?
Short Answer Questions (3 Marks Each)
Q16 Dalton's atomic theory was an important step in the development of atomic science, but some of its ideas were later found to be incorrect. State any two drawbacks of Dalton's atomic theory.
Q17 In a reaction, \(5.3\mathrm{~g}\) of sodium carbonate reacted with \(6.0\mathrm{~g}\) of ethanoic acid. The products were \(2.2\mathrm{~g}\) of carbon dioxide, \(0.9\mathrm{~g}\) of water, and \(8.2\mathrm{~g}\) of sodium ethanoate. Show that these observations are in agreement with the law of conservation of mass.
Q18 Classify the following molecules as diatomic, triatomic, or polyatomic molecules: \(\mathrm{HCl}\), \(\mathrm{H_2}\), \(\mathrm{H_2O}\), \(\mathrm{NH_3}\), \(\mathrm{CH_3OH}\), \(\mathrm{PCl_5}\).
Q19 Write the chemical formulas of:
(a) Magnesium hydroxide
(b) Hydrogen sulphide
(c) Barium Chloride (valency of Ba is \(+2\))
Long Answer Questions (5 Marks Each)
Q20 (a) You are provided with a fine white colored powder which is either sugar or salt. How would you identify it without tasting?
(b) Calculate the molecular mass of the following:
(i) \(\mathrm{H_2CO_3}\) (Carbonic acid)
(ii) \(\mathrm{C_2H_5OH}\) (Ethanol)
(iii) \(\mathrm{MgSO_4}\) (Magnesium sulphate)
Q21 (a) You are studying substances that are formed through the transfer of electrons between atoms. These substances consist of charged particles that attract each other strongly to form compounds. Describe what such charged particles are called and give three examples of compounds formed by them.
(b) Write the chemical formulas of: (i) Sodium carbonate, (ii) Ammonium chloride.
Case Study Based Questions (4 Marks Each)
Case Study 22 Atoms and molecules are the building blocks of matter. An atom is the smallest unit of an element that retains its chemical properties, while a molecule is a group of two or more atoms held together by chemical bonds. Atoms consist of a positively charged nucleus, which contains protons and neutrons, surrounded by negatively charged electrons in energy levels or shells. The number of protons in an atom determines its atomic number and defines its unique identity as an element. Atoms gain, lose, or share electrons to achieve stable configuration, forming chemical bonds and molecules.
Questions:
1. The smallest unit of an element that retains its chemical properties is:
(a) Proton (b) Electron (c) Nucleus (d) Atom
2. A group of two or more atoms held together by chemical bonds is called:
(a) Element (b) Compound (c) Molecule (d) Nucleus
3. What are the positively charged particles present in the nucleus of an atom called?
(a) Electrons (b) Protons (c) Neutrons (d) Valence electrons
4. Which part of an atom contains electrons in energy levels or shells?
(a) Protons (b) Neutrons (c) Nucleus (d) Valence shell / Energy levels
Case Study 23 A student was asked by his teacher to verify the law of conservation of mass in the laboratory. He prepared 5% aqueous solutions of \(\mathrm{NaCl}\) and \(\mathrm{Na_2SO_4}\). He mixed 12 mL of both these solutions in a conical flask. He weighed the flask on a balance. He then stirred the flask with a rod and weighed it again after some time. There was no change in mass.
Questions:
1. Was the student able to verify the law of conservation of mass?
2. If not, what was the mistake committed by him?
3. In your opinion, what should he have done to successfully verify the law?
4. What is the value-based scientific lesson from this experiment?
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