ATOMS AND MOLECULES

- The properties of matter depend on the properties of atoms and molecules from which it is made.

Laws of chemical combination

(I.) Law of conservation of mass: It states that mass can neither be
Created nor destroyed in a chemical reaction.

- It means that in a chemical reaction, the total mass of products is equal to the total mass of reactants. There is no change in mass during a chemical reaction.

(ii) Law of constant proportions: In a chemical substance the elements are always present in definite proportions by mass.
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Dalton’s atomic theory

- The postulates of his theory are as follows:

(I.) All matter, whether an element, a compound, or a mixture is
Composed of very tiny particles called atoms.

(ii.) Atoms are indivisible particles, which cannot be created or
Destroyed in a chemical reaction.

(iii.) Atoms of a given element are identical in mass and chemical
Properties.

(iv.) Atoms of different elements have different masses and chemical
Properties.

(v.) Atoms combine in the ratio of small whole numbers to form compounds.

(vi.) The relative number and kinds of atoms are constant in a given compound.

- Drawbacks of Dalton’s atomic theory:

(I.) Atoms were indivisible according to Dalton. We now know that under special circumstances, atoms can be further divided into still smaller particles called electrons, protons and neutrons.

(ii.) Daltons atomic theory says that all the atoms of an element have exactly the same mass. It is, however, now known that atoms of the same element can have slightly different masses as in isotopes.

(iii.) His theory says that atoms of different elements have different It is, however, now known that even atoms of different elements can have the same mass as in