A stronger electrostatic force between opposing ions strengthens the stability of ionic compounds. An ionic compound is formed when two ions join together.
The process releases lattice energy or lattice enthalpy. When an ionic compound is made from a cation and an anion, lattice energy is released.
A+ + B– → A+B– + Lattice energy
Lattice Energy Examples
Sodium Chloride (NaCl)
787.3 kJ of energy is released when sodium ions (Na+) combine with chloride ions (Cl–).
Na+(g) + Cl–(g) → NaCl(s) ΔHlattice = -787.3 kJ/mol
Calcium Oxide (CaO)
An oxide ion (O2) is formed when calcium ion (Ca2+) combines with calcium ion (Ca2+). This creates calcium oxide (CaO) crystals, which release 3414 kJ of energy.
Ca2+ (g) + O2- (g) → CaO (s) ΔHlattice = -3414 kJ/mol
Calcium Chloride (CaCl2)
Calcium chloride (CaCl2) crystallizes when calcium ion (Ca2+) and chloride ion (Cl-) combine, releasing 2195 kJ of energy.
Ca2+ (g) + 2 Cl– (g) → CaCl2 (s) ΔHlattice = -2195 kJ/mol
Lattice Energy Trend
lattice energy decreases down the group
As we move down the group, the lattice energy continues to decrease. This is because as the size of ions gets larger, the distance between their nuclei grows larger, as well.
So, the attraction between them gets weaker. In the end, the process releases less lattice energy.
lattice energy increase across a period
As we move from one period to the next, the lattice energy keeps rising. Usually, two main things need to happen to increase lattice energy.
- By reducing the distance between atoms. So, as we move from one period to the next, the atomic radius of elements gets smaller. This means that the protons pull on the electrons more and more. So, as the radius gets smaller, the lattice energy goes up.
- When the charge on an ion gets higher. As we move across a period, the charge on ions like K+, Ca2+, Ga3+, etc., increases. Because of this, the lattice energy keeps rising as well.
Factors that affect lattice energy trend
The strength of the ionic bond increases with the increase of lattice energy. And lattice energy depends on two factors: size or radius of ions and charge of ions.
a) Radius of ions
As the distance between ions grows larger, the lattice energy goes down. The lattice energy of lithium halide is shown on the graph below.
As you move down the group, the size of the halide grows larger and the lattice energy starts decreasing.
This is because as the size of ions grows, so does the distance between their nuclei. The result is a reduction in their attraction, which in turn releases less lattice energy.
b) Charge of ions
Because of the stronger attraction between the ions, lattice energy increases as the charge on the ions increases.
As a result, ions with a charge of +2 or -2 will release more lattice energy than ions with a charge of +1 or -1.
Our results show that the lattice energy of MgO is higher than that of NaCl.