Covalent bonding  (Page 13/2)

Chemical bonding

When you look at the matter, or physical substances, around you, you will realise that atoms seldom exist on their own. More often, the things around us are made up of different atoms that have been joined together. This is called chemical bonding . Chemical bonding is one of the most important processes in chemistry because it allows all sorts of different molecules and combinations of atoms to form, which then make up the objects in the complex world around us.

What happens when atoms bond?

A chemical bond is formed when atoms are held together by attractive forces. This attraction occurs when electrons are shared between atoms, or when electrons are exchanged between the atoms that are involved in the bond. The sharing or exchange of electrons takes place so that the outer energy levels of the atoms involved are filled and the atoms are more stable. If an electron is shared , it means that it will spend its time moving in the electron orbitals around both atoms. If an electron is exchanged it means that it is transferred from one atom to another, in other words one atom gains an electron while the other loses an electron.

Chemical bond

A chemical bond is the physical process that causes atoms and molecules to be attracted to each other, and held together in more stable chemical compounds.

The type of bond that is formed depends on the elements that are involved. In this chapter, we will be looking at three types of chemical bonding: covalent , ionic and metallic bonding .

You need to remember that it is the valence electrons that are involved in bonding and that atoms will try to fill their outer energy levels so that they are more stable (or are more like the noble gases which are very stable).

Covalent bonding

The nature of the covalent bond

Covalent bonding occurs between the atoms of non-metals . The outermost orbitals of the atoms overlap so that unpaired electrons in each of the bonding atoms can be shared. By overlapping orbitals, the outer energy shells of all the bonding atoms are filled. The shared electrons move in the orbitals around both atoms. As they move, there is an attraction between these negatively charged electrons and the positively charged nuclei, and this force holds the atoms together in a covalent bond.

Covalent bond

Covalent bonding is a form of chemical bonding where pairs of electrons are shared between atoms.

You will have noticed in the above examples that the number of electrons that are involved in bonding varies between atoms. We say that the valency of the atoms is different.

Valency

The number of electrons in the outer shell of an atom which are able to be used to form bonds with other atoms.

In the first example, the valency of both hydrogen and chlorine is one, therefore there is a single covalent bond between these two atoms. In the second example, nitrogen has a valency of three and hydrogen has a valency of one. This means that three hydrogen atoms will need to bond with a single nitrogen atom. There are three single covalent bonds in a molecule of ammonia. In the third example, the valency of oxygen is two. This means that each oxygen atom will form two bonds with another atom. Since there is only one other atom in a molecule of ${\mathrm{O}}_2$ , a double covalent bond is formed between these two atoms.