Ionic bonding is the electrostatic attraction between oppositely charged ions – it is not the transfer of electrons from one atom to another to form the ions themselves. Cations are positively charged and anions are negatively charged.
The good news is that we are still using dot/cross diagrams to represent bonding at A level, and only the outer shell electrons are shown.
The Na+ ion is isoelectronic (has the same electron configuration) with the preceding noble gas in the Periodic table and the Cl– ion is isoelectronic with the noble gas that follows it. This pattern holds for all simple cations and anions, regardless how many electrons are lost or gained.
Polyatomic ions have more than one element in the ion eg. ammonium ion NH4+, sulphate ion SO42- (the bonding between the atoms in the ion is covalent).
Ions don’t form molecules. Millions of ions form a giant ionic lattice in which the ionic bond or electrostatic attraction is non-directional. The formula NaCl simply represents the simplest ratio of cations and anions in the lattice. The positions of the ions in the lattice depend on their relative size, charge and the playoff between the attractive and repulsive forces.
The coordination number describes the number of oppositely charged ions an ion in the lattice is in contact with – in the case of NaCl the coordination number of both sodium and chlorine is 6.
The strength of an ionic bond is indicated by the lattice enthalpy which is the amount of energy released when one mole of the lattice is formed from gaseous ions.
Na+(g) + Cl–(g) ⇾ NaCl(s) ΔLEH⦵ = -787 kJ mol-1
Cs+(g) + Cl–(g) ⇾ CsCl(s) ΔLEH⦵ = -657 kJ mol-1
NaCl is a stronger lattice than CsCl. Sodium ions are smaller than caesium ions and can pack more closely with the chloride ions – the electrostatic attraction between the ions is stronger and so it takes more energy to pull the lattice apart (+787 kJ mol-1). You can delve into this in much more detail here.
In general, ionic compounds are solid at room temperature and have high melting points – this is a reflection of the overall strength of the electrostatic attraction between ions in the lattice. They are hard, brittle, often soluble in water and conduct electricity when molten or in solution.
If you are ready to push your understanding a little further, this is the exercise for you 😊.