A bond formed between two atoms, where electrons are shared, is called a covalent bond. Covalent compounds, usually formed between non-metal elements, 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 one or more electrons 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.
Sodium is in group 1 of the periodic table, so it has one electron in its outer shell. When sodium forms an ion, it loses this one electron. Oxygen is in group 6 of the periodic table, so it has six electrons in its outer shell. When oxygen forms an ion, it gains two electrons. Therefore, the charge of simple ions is related to the group that an element is in:
As well as simple ions of elements, there are some common ions which contain two elements combined together. These ions are known as compound ions.
Here are the names and formulae of some compound ions:
|Name of ion||Ion formula||Elements combined|
The formula of an ionic compound can be predicted using the formulae of its ions. The numbers of ions in a formula must give an equal number of positive and negative charges.
|Compound||Ions present||Electrical charges||Formula|
|Sodium chloride||Na+ and Cl-||One positive, one negative||NaCl|
|Sodium oxide||Na+ and O2-||One positive, two negative||Na2O|
|Magnesium oxide||Mg2+ and O2-||Two positive, two negative||MgO|
|Magnesium chloride||Mg2+ and Cl-||Two positive, one negative||MgCl2|