Ionic bonding: the transfer of electrons

When sharing is not enough

Some atoms have very different electronegativities: one strongly attracts electrons, the other holds onto them very weakly. In this case, instead of sharing electrons, one of the atoms completely gives up one or more electrons to the other. This is ionic bonding, typical between a metal and a non-metal.

Forming ions

This transfer of electrons creates two ions of opposite charge:

• the atom that loses electrons becomes a cation, positively charged,
• the atom that gains electrons becomes an anion, negatively charged.

The two ions then attract each other very strongly through electrostatic attraction (opposite charges attract). This attraction is the ionic bond itself.

Example: sodium chloride (NaCl)

Sodium (Na) has 1 valence electron. Chlorine (Cl) has 7. Sodium gives up its single valence electron to chlorine:

Na  ->  Na+  +  e-
Cl  +   e-   ->  Cl-

Sodium becomes the Na+ ion (a stable configuration with 8 electrons on its new outer shell), and chlorine becomes the Cl- ion (a stable configuration with 8 electrons). The Na+ and Cl- ions then assemble by the millions to form an ionic crystal, the well-known structure of table salt.

A good way to check an ionic equation

An ionic transformation must always be balanced both in atoms and in charges. Check: on the left, Na (charge 0) and Cl (charge 0) give a total charge of zero; on the right, Na+ (charge +1) and Cl- (charge -1) also give a total charge of zero (+1 - 1 = 0). The equation is balanced.

Classic mistake

Never say that an ionic crystal is made of NaCl "molecules": there is no isolated NaCl molecule, only a regular, infinite stacking of Na+ and Cl- ions, called a crystal lattice.