Covalent Bonding – A Level Chemistry B OCR Revision

Covalent Bonding Basics

Covalent bonding involves sharing of electrons between two atoms, often non-metals.
The shared electrons circulate in the electron cloud around both nuclei, bonding the two atoms together.
Covalent bonding occurs to satisfy the octet rule, i.e., each atom seeks to achieve an outer shell capacity of eight electrons.
In a single covalent bond, each atom provides one electron for sharing. In a double covalent bond, two electrons are shared, and so on.

The Valence Shell Electron Pair Repulsion (VSEPR) theory is a key principle explaining the shapes of molecules formed by covalent bonding.
Molecular shape is dictated by the need to minimise repulsion between electron pairs in the valence shell of the central atom.
Shared pairs (bonding pairs) and unshared pairs (lone pairs) of electrons both contribute to the structure of molecules. Lone pairs repel more strongly than bonding pairs.
Common molecular shapes include linear, triangular planar, tetrahedral, trigonal bipyramidal and octahedral.

Bond dissociation energy is the energy required to break one mole of a bond in a molecule into separate atoms, usually expressed in kJmol^-1.
Stable covalent bonds have high bond dissociation energies. They are formed when the energy of the bonded atoms is lower than the energy of the unbonded atoms.
For any specific atom combination, the more electrons that are shared (i.e. double or triple bonds), the stronger and shorter the covalent bond.

Dative covalent bonding (also known as coordinate bonding) occurs when both bonding electrons come from the same atom. An example is the formation of the ammonium ion.
Resonance structures apply when multiple valid Lewis structures can be drawn for a molecule. The actual structure of the molecule is an average of these structures.
Delocalised electrons feature in some covalently bonded structures such as benzene. In these cases, electrons are shared among more than two atoms, resulting in enhanced stability.