Ionization energy is the amount of energy required to remove one or more electrons from the outermost shell of an isolated atom in the gaseous state.
Thus, the ionization energy gives the ease with which the electron can be removed from an atom.The smaller the value of the ionization energy, the easier it is to remove the electron from the atom.
Then the electron is removed by supplying energy equivalent to the ionization potential. Qmong the factors affecting ionization energy of an element inculdes:
1.The inert gases have very high ionization energy, due to the stability of the outer shell. Helium has the highest ionization energy.
2. Within a group, the ionization energy generally decreases with increasing atomic number. Increasing atomic number results in increasing atomic radii. Thus, the electrons of the outer shell are further away than those of the previous element and can be removed easily.
3. Ionization energy decreases down the group because of increase in the number of shells. The effective nuclear charge decreases as atomic size increases. Thus it is easier to pull one electron from the outermost shell of the atom.