The molar enthalpy change accompanied by the removal of an electron from an atom or an ion in its ground state is called ionization enthalpy. In general terms, ionization enthalpy represents the energy required to remove one electron from an atom. The ionization enthalpy is expressed in the units of KJ/mol. Since energy is always required to remove an electron from an atom, the ionization enthalpy is always positive.
First ionization potential is the amount of energy required to convert an isolated gaseous atom to an ion carries one positive charge.
M (g) + Energy → M+ (g) + e–, ΔH = (+ )
Second ionization potential is the amount of energy required to remove an electron from a positive ion carries one positive charge.
M+ (g) + Energy → M2+ (g) + e–, ΔH = (+)
Third ionization potential is the amount of energy required to remove an electron from a positive ion carries two positive charges.
M2+ (g) + Energy → M3+ (g) + e–, ΔH = (+)
The first ionization enthalpy < the second ionization enthalpy < the third ionization enthalpy
The ionization enthalpy decreases moving down the group and increases moving left to right in a period which leads to an increase in the attraction force of the nucleus on the valence electrons.
In order to understand this trend consider two factors:
- The attraction of electrons towards the nucleus
- Repulsion of electrons from each other.
Therefore, as the atomic size increases, the screening effect increases and the nuclear charge decreases which causes the ionization enthalpy to decrease.
For example, the first ionization energy of nitrogen is greater than that of oxygen due to stable half-filled 2p orbitals. In 3d series, chromium and copper have higher second ionization energy due to completely filled and fully filled orbitals.
When we compare magnesium and aluminium, magnesium has its outermost electrons in 3s orbital while aluminium has its outermost electron in the 3p orbital. Electrons in p orbital at a higher energy level than s orbital which means that these electrons require less energy to be removed from the atom. Also, electrons present in 3s2 shield the electrons in 3p1 which makes the p electron be easily removed in the case of aluminium. This is the reason why the 1st ionization enthalpy all aluminium is lesser than that of magnesium. Also, the second ionization energy of magnesium is greater than the first ionization energy due to an increase in the effective nuclear charge.
In the case of potassium, 19K and calcium, 20Ca, the first ionization enthalpy of potassium are lower than that of calcium. However, 2nd ionization enthalpy of potassium is much greater than that of calcium due to difficulty in the loss of electrons in a completely filled shell.
Lastly, the first ionization potential of noble gases is very high because it is difficult to remove an electron from a completely filled shell.
Figure 1: Periodic trends – Ionisation energy