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An ionic, or electrovalent, bond is formed between oppositely charged ions because of electrostatic forces of attraction. All ionic bonding is exothermic.

example: sodium + chlorine --> sodium chloride

How and why we have ionic compounds:

Noble gases like neon or argon have eight electrons in their outer energy levels (or two in the case of helium). These noble gas structures are thought of as being in some way a "desirable" thing for an atom to have.

When other atoms react, they try to organize things so that their outer levels are either completely full or completely empty.

Using ∆H for the formation of an ionic compound

Ionic bonding can be broken down into 3 steps: ex. sodium chloride

...Formation of the sodium ions (ionization)

Na+(g) + energy -----> Na+(g) (∆H = +496 kJ/mol)

...Formation of the chloride ions (electron affinity)

Cl(g) + e- -----> Cl-(g) + energy (∆H = -348 kJ/mol)

...Formation of the ion pair (bond energy)

Na+(g) + Cl-(g) -----> NaCl(g) + energy (∆H = -504 kJ/mol)

According to Hess's Law, the ∆H for the formation of one mole of sodium chloride ion-pairs from one mole of sodium atoms and one mole of chlorine atoms is ∆Hrxn = ∆H1 + ∆H2 + ∆H3 = -356 kJ/mol

EX: Ionic bonding in sodium chloride

Sodium (2,8,1) has 1 electron more than a stable noble gas structure (2,8). If it gave away that electron it would become more stable.

Chlorine (2,8,7) has 1 electron short of a stable noble gas structure (2,8,8). If it could gain an electron from somewhere it too would become more stable.

The answer is obvious:

If a sodium atom gives an electron to a chlorine atom, both become more stable.

The sodium has lost an electron, so it no longer has equal numbers of electrons and protons. Because it has one more proton than electron, it has a charge of 1+. If electrons are lost from an atom, positive ions are formed.

Positive ions are sometimes called cations.

The chlorine has gained an electron, so it now has one more electron than proton. It therefore has a charge of 1-. If electrons are gained by an atom, negative ions are formed.

A negative ion is sometimes called an anion.

The nature of the bond:

The sodium ions and chloride ions are held together by the strong electrostatic attractions between the positive and negative charges.

In conclusion, Electrons are transferred from one atom to another resulting in the formation of positive and negative ions. The electrostatic attractions between the positive and negative ions hold the compound together.