Answer:
In general, the first ionization energy will increase upon moving left to right in a period and it will decrease from coming top to bottom in a group in a periodic table.
Explanation:
The ionization energy is defined as the amount of energy required to remove a valence electron from a gaseous atom or ion.
When we move inside a period from left to right, the atomic radius decreases and so the effective nuclear charge increases. Hence, the electrons are tightly held by nucleus which makes it difficult to remove the electron. Thus, the ionization energy increases along a period from left to right.
When we move down a group from top to bottom, the principal quantum number (number of shells) increases and so the distance between outer electrons and nucleus increases. The protons of the nucleus hold the core electrons more tightly. But, the core electrons repel the valence shell electrons.
Due to this screening effect, the outermost electrons experience less hold of nucleus which makes it easier to remove the electrons. Thus, the ionization energy decreases when coming down the group.
