Atoms and Molecules

Key Notes

(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.

Chemical Bond

A chemical bond is the force of attraction that holds atoms together to form molecules or compounds. Atoms combine to achieve a stable electronic configuration (usually an octet). In order to attain stable electronic configurations, atoms gain, lose, or share their electrons.

Covalent Bonds

Definition

The bond formed by sharing of electrons between same or different atoms is a covalent bond. In a covalent bond, the shared pair of electrons belongs to the valence shell of both atoms.

Properties of Covalent Bonds

Physical state

Covalent compounds exist as gases, liquids, and soft solids.

Solubility

Generally insoluble in water and other polar solvents, but soluble in organic solvents such as benzene and toluene.

Melting & boiling points

They generally have low melting and boiling points.

Electrical conductivity

They are non-conductors of electricity in solid, molten, or aqueous state.

Types of Covalent Bonds

There are three types of covalent bonds based on the number of electron pairs shared:

Single bond

A — B

Sharing one pair of electrons

Double bond

A = B

Sharing two pairs of electrons

Triple bond

A ≡ B

Sharing three pairs of electrons

Key idea: Greater number of shared electron pairs means a higher bond order.