A bond formed between two atoms, where electrons are shared, is called a covalent bond.Covalent compounds usually form between non-metal elements and are held together by covalent bonds. All compounds that exist as molecules are covalent. Here are some examples:
The subscript numbers in formulae show how many atoms of that element appear in the molecule.
So, the formula NH3 shows that ammonia contains one nitrogen and three hydrogen atoms.
A bond formed between two atoms where an electron is transferred from one atom to the other is called an ionic bond. Ionic compounds are made up of atoms joined together by ionic bonds. They usually (but not always) contain at least one metal element and one non-metal element.
Names and formulae of some common ions:
|Name of ion||Formula of ion||Electrical charge(s)|
The formula of an ionic compound can be predicted by using the formulae of its ions. The numbers of ions in a formula must give an equal number of positive and negative charges.
|Name of compound||Formula||Electrical charges|
|Sodium cloride||NaCl||One positive, one negative|
|Sodium oxide||Na2O||Two positive, two negative|
|Magnesium oxide||MgO||Two positive, two negative|
|Magnesium chloride||MgCl2||Two positive, two negative|