Bonding and properties of materialsBonding and properties of materials

Atoms can be held together by chemical bonds. When atoms form bonds, they can achieve a stable electron arrangement. To achieve a stable electron arrangement atoms can lose, gain or share electrons. There are different types of bonds that hold atoms together.

Covalent bonding

A covalent bond is a shared pair of electrons between atoms of two non-metal elements.

A covalent bond happens when the positive nuclei from two different atoms are held together by their common attraction for the shared pair of electrons held between them.

Covalent bonds are strong bonds.

Atoms that share pairs of electrons form molecules. A molecule is a group of atoms held together by covalent bonds.

A diatomic molecule is a molecule containing only two atoms. There are seven diatomic elements that you have to remember and a simple mnemonic to help with this. If you remember "I Bring Clay For Our New House” then you will have remembered that the seven diatomic elements are Iodine, Bromine, Chlorine, Fluorine, Oxygen, Nitrogen and Hydrogen.

Diagrams can be used to show how the outer electrons are shared to form the covalent bonds in a molecule.

Hydrogen (H₂)

Both hydrogen atoms have only one electron, but by forming a single covalent bond, both can have a full outer shell. The shape of the molecule formed is called linear.

This can also be shown as H-H.

Methane (CH₄)

Carbon atoms have four outer electrons so need four more for a full outer shell. The carbon forms four single bonds to the hydrogen atoms, so all the atoms now have a full outer shell of electrons. The shape formed is called tetrahedral.

Ammonia (NH₃)

Nitrogen atoms have five outer electrons so needs three more for a full outer shell. Nitrogen forms three single covalent bonds to hydrogen atoms. The shape formed is called trigonal pyramidal.

Water (H₂O)

Oxygen atoms have six outer electrons so need two more for a full outer shell. The oxygen forms two single covalent bonds with the two hydrogen atoms. The shape formed is called angular.

More than one bond can be formed between atoms leading to double and triple bonds. Examples of these are diatomic oxygen (double bond) or nitrogen (triple bond).

This could also be written as:

Substances that consist of covalent molecules are usually gases or liquids at room temperature because the attractions between molecules are weak and easy to overcome.

Covalent substances that are solids with high melting points have much larger molecules. A covalent network structure consists of a giant 3-dimensional lattice of covalently bonded atoms.

Boron, carbon and silicon are all examples of covalent network elements. Diamond and graphite, two forms of carbon and compounds like silicon dioxide and silicon carbide are all covalent networks.