Why do elements form an ionic bond

An ionic bond is formed when a metallic element and a non-metallic element form a compound.

Explanation:

As you may know, each atom contains a set amount of electrons. For example, Hydrogen has one electron, helium has two electrons, lithium has three electrons and so on.

Electrons come in shells. The first shell can hold 2 elections at most, and each shell past this point can hold 8 electrons. When an atom has a filled outer shell, it is considered stable. (This so why the noble gases are unreactive- they have a full outer shell of electrons.)

This is why compounds can form- it is the chemical bonding of 2 or more chemical elements: take NaCl, sodium chloride, or SO2, sulphur dioxide.

You specifically asked about ionic bonds. Ionic bonds, as expressed above, form between a compound of a metallic and non metallic element.

Let's take sodium chloride as an example. Sodium has 1 electron in its outer shell and chlorine has 7. Both need a full outer shell to become stable. Because of this, the sodium atom loses its only electron in its outer shell to become a sodium ion and the chlorine atom gains the electron to become a chlorine ion as well. Because of this, both ions have a full outer shell, rendering them stable.

Now, because the sodium ion is short of one electron, it has more protons than electrons, resulting in it having a positive charge. The chlorine ion is the opposite, having a negative charge.