First ionization energy is the energy required to remove an electron from an isolated, neutral gaseous atom of an element resulting in the formation of a positive ion.
M(g) + Energy ------> M+(g) + e-
For example, Li(g) + 520 kJ mol-1 ------> Li+(g) + e-
Thus, the first ionization energy of lithium is 520 kJ mol-1. If another electron is to be removed, much more energy (called second ionization energy) will be needed.
Li+ + 7297 kJ mol-1 -------> Li2+ + e-
This is because of the fact that it is more difficult to remove an electron if the atom already possesses a positive charge. Thus, the successive values of the ionization energy of an element will show an increasing trend.
The below figure shows a plot of first ionization energies of some elements against their respective atomic numbers. Certain trends are quite evident from the plot:
- Noble gases occupy peaks because of their extremely stable configurations.
- Alkali metals possess lowest values of ionization energy showing thereby that they are highly reactive.
- In general the values increase across a period (e.g. Li to Ne). This is due to the increasing nuclear charge and the electrons are more strongly held by the attractive force of the nucleus.
- Within a group (e.g. Li to Cs), ionization energies become progressively smaller as we move down the column. This is because of the fact that the outer electron, which is being removed lies farther from the nucleus and thus it becomes easier to pull it out.