## Q1. What are the postulates of Dalton’s atomic theory of matter?

Ans. According to Dalton’s atomic theory, all matter, whether an element, a compound or a mixture is composed of small particles called atoms. Important points of Dalton’s atomic theory are,

-> All matter is made of very tiny particles called atoms.
-> Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
-> 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.
-> The relative number and kinds of atoms are constant in a given compound.

## Q2. Define and explain atomic mass of an element.

Ans. The atomic mass of an element is the relative mass of its atom as compared with the mass of a particular atom of carbon-12 ($${ C }^{ 12 }$$) isotope taken as 12 units. Thus the atomic mass of an element indicates the number of times one atom of an element is heavier than 1/12th of a carbon-12 ($${ C }^{ 12 }$$) isotope atom. For example, the atomic mass of oxygen is 16 which indicates that an atom of oxygen is 16 times heavier than 1/12th of a carbon-12 ($${ C }^{ 12 }$$) isotope atom.

## (i). Atomicity    (ii). Ion    (iii). Molecular mass    (iv). Relative formula mass.

Ans.

(i). Atomicity: Number of atoms in a single molecule of an element is known as atomicity. This can also be defined as the number of atoms constituting a molecule.

(ii). Ion: An ion is a charged particle which can be positively or negatively charged. A negatively charged ion is called an anion and a positively charged ion is called a cation. Ions may consist of a single charged atom or a group of atoms that have a net charge on them known as polyatomic ion.

(iii). Molecular mass: Molecular mass of a substance is the relative mass of its molecule expressed in atomic mass unit (u). It is equal to the sum of atomic masses of all the atoms present in one molecule of the substance. For example, one molecule of water ($${ H }_{ 2 }O$$) contains two atoms of hydrogen and one atom of oxygen.

Molecular mass of water = 2 x atomic mass of hydrogen + 1 x atomic mass of oxygen

= 2 + 16 = 18 u

(iv). Relative formula mass (or formula mass): The formula mass of a substance is the sum of atomic masses of all atoms in a formula unit of a compound. Formula mass is used for substances whose constituent particles are ions.

## How is the molecular formula of a compound written?

Ans. In order to write formula of a compound comprised of cations and anions, the valencies of the ion must be known. Let us consider a compound composed of cation A with valency $${ x }^{ + }$$ and anion B with valency $${ y }^{ – }$$. Then the molecular formula can be written by following the rules given below:

(i). Write down the symbols of the cation and anion side by side.

$$A$$   $$B$$

(ii). Write their valencies at the top corners as

$${ A }^{ x+ }$$    $${ B }^{ y- }$$

(iii). The valencies or charges on the ions must balance after combining. So, interchange between the ions their valencies and these are placed on the lower side of each radical or used as subscripts.

$${ A }_{ y }$$    $${ B }_{ x }$$

(iv). If a radical is multi-atomic, use a small bracket around it before writing the valency number to indicate the ratio. For example, calcium phosphate, a compound of calcium ion with valency $${ x }^{ + }$$ and phosphate ion with valency $${ y }^{ – }$$ is written as $$C{ a }_{ 3 }{ (P{ O }_{ 4 }) }_{ 2 }$$

(v). Eliminate the common factor, if any, from the numbers used in subscripts.