Quick Revision Notes & Formula Sheet

Class 9 Science - Chapter: Atoms and Molecules

MP Board (NCERT Curriculum)

Chemical combination is governed by two fundamental laws established by Antoine L. Lavoisier and Joseph L. Proust.

Law of Conservation of Mass:
- Statement: Mass can neither be created nor destroyed in a chemical reaction.
- Formula: $\text{Total Mass of Reactants} = \text{Total Mass of Products}$
Law of Constant Proportions (Law of Definite Proportions):
- Statement: In a chemical substance, the elements are always present in definite proportions by mass, regardless of the source or method of preparation.
- Example: In water ($H_2O$), the ratio of the mass of hydrogen to the mass of oxygen is always $2:16$ or $1:8$, whether it comes from a river, rain, or well.

John Dalton provided a basic theory about the nature of matter, key points of which include:

All matter is made of tiny particles called atoms (परमाणु).
Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction (Note: Later disproven by discovery of subatomic particles).
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.

Atom (परमाणु): The smallest particle of an element that takes part in a chemical reaction and retains all the chemical properties of that element.
Molecule (अणु): A group of two or more atoms that are chemically bonded together (e.g., $O_2$, $H_2O$).
Atomic Mass Unit (amu / u): Exactly equal to one-twelfth ($1/12^{th}$) the mass of one carbon-12 atom.
Valency (संयोजकता): The combining capacity of an atom. It represents the number of electrons lost, gained, or shared to complete its octet.
Ion (आयन): A charged particle formed by loss or gain of electrons.
- Cation (धनायन): Positively charged ion (e.g., $Na^+$).
- Anion (ऋणायन): Negatively charged ion (e.g., $Cl^-$).

To write the chemical formula of a binary compound:

Example: Formula of Aluminium Oxide

Molecular Mass: The sum of atomic masses of all the atoms in a molecule of a substance. It is expressed in atomic mass units ($u$).
- Example: Molecular mass of Water ($H_2O$) $= (2 \times \text{Atomic mass of H}) + (1 \times \text{Atomic mass of O})$ $= (2 \times 1) + (1 \times 16) = 2 + 16 = 18\text{ u}$
Formula Unit Mass: The sum of atomic masses of all atoms in a formula unit of an ionic compound (used for compounds like $NaCl$ that do not exist as independent molecules).
- Example: Formula unit mass of $NaCl$ $= (1 \times \text{Atomic mass of Na}) + (1 \times \text{Atomic mass of Cl})$ $= 23 + 35.5 = 58.5\text{ u}$

The mole is the basic SI unit for measuring the amount of substance.

Avogadro Constant ($N_A$): $6.022 \times 10^{23}$
Definition of 1 Mole: One mole of any substance contains exactly $6.022 \times 10^{23}$ particles (atoms, molecules, or ions).

Number of moles ($n$) from Mass: $$\text{Number of moles } (n) = \frac{\text{Given Mass } (m)}{\text{Molar Mass } (M)}$$
Number of moles ($n$) from Number of Particles ($N$): $$\text{Number of moles } (n) = \frac{\text{Given Number of Particles } (N)}{\text{Avogadro Number } (N_A)}$$
Mass of atoms/molecules from moles: $$\text{Mass} = \text{Number of Moles } (n) \times \text{Molar Mass } (M)$$
Number of Particles ($N$): $$N = n \times N_A = \frac{m}{M} \times N_A$$