CBSE Class 10 Chemistry Chemical Bonding Notes Set 11

Why Atoms Combine

Atoms combine to reach a stable arrangement of electrons like that of the nearest noble gas: an octet of eight electrons (a duet of two for hydrogen and helium). They do this by losing, gaining or sharing electrons. This is the electronic theory of valency of Kossel and Lewis.

Electrovalent (Ionic) Bond

Formed by complete transfer of electrons from a metal to a non-metal. The metal becomes a positive ion and the non-metal a negative ion, held together by electrostatic attraction. Example: Na (2, 8, 1) gives one electron to Cl (2, 8, 7), forming Na⁺ (2, 8) and Cl⁻ (2, 8, 8) in NaCl. In MgCl₂ magnesium gives two electrons, one to each chlorine.
Properties: crystalline solids with high melting and boiling points; conduct electricity when molten or dissolved but not as solids; usually soluble in water; the bond is non-directional.

Covalent Bond

Formed by sharing of electron pairs between atoms of non-metals. A single bond is one shared pair (H–H, HCl), a double bond is two pairs (O=O, CO₂) and a triple bond is three pairs (N≡N, C₂H₂). Examples: H₂O has two O–H bonds with two lone pairs on oxygen; CH₄ has four C–H bonds; CCl₄ has four C–Cl bonds.
Properties: gases, liquids or soft solids with low melting and boiling points; do not conduct electricity; mostly insoluble in water but soluble in organic solvents; the bond is directional. A bond between unlike atoms is polar because the more electronegative atom attracts the shared pair more strongly.

Coordinate (Dative) Covalent Bond

A covalent bond in which both shared electrons come from one atom (the donor) while the other atom (the acceptor) provides an empty orbital. It is shown by an arrow from donor to acceptor. Examples: the ammonium ion NH₄⁺ (N donates its lone pair to H⁺) and the hydronium ion H₃O⁺. Ammonium chloride therefore contains ionic, covalent and coordinate bonds together.

Comparison

1. Ionic: transfer of electrons; ions; high melting point; conducts when molten.
2. Covalent: sharing of electrons; molecules; low melting point; non-conducting.
3. Coordinate: sharing of a pair given by one atom; properties like covalent compounds.

Isoelectronic Species

Atoms, ions or molecules having the same total number of electrons are isoelectronic. Examples: Na⁺, Ne and F⁻ (10 electrons); N₂ and CO (14 electrons); CO₂ and N₂O (22 electrons).