Question

Why ionization energy of lithium is more than sodium?

Answer 1

In general, the ionization energy decreases down the group. This is because down the group, there is an increasing number of principal quantum shells and this results in increasing distance between the nucleus and valence electrons of the element. Thus, forces of attraction between nucleus and valence electrons become weaker and less energy is required to remove the valence electron. (aka decrease in IE)

In the case of Li, it is located above Na in the Periodic Table so the valence electron is closer to the nucleus and more energy is required to overcome the strong forces of attraction between its valence electrons and the nucleus→ higher IE