Question

Why does ionization energy decrease as you move down a group in the periodic table?
A) because shielding decreases
B) because the atoms get closer to noble gas configuration
C) because nuclear charge decreases
D) because the outer electrons get further from the nucleus
E) because effective nuclear charge increases

Answer 1

Hint: Ionization energy is the minimum energy required, when an electron is removed from the outermost orbital of an atom, in its isolated gaseous state. For example, if we remove an electron from a Sodium atom, to form Sodium Ion ( $N{a}^{+}$), the energy required for this conversion is 496kJ/mol, this is the ionization energy .

Complete answer:
It was found that ionization energy decreases as we move down the group. Let us see which reason justifies this observation.
A) Shielding effect: As we move down the group the atomic size increases, the no. of electrons in the orbit also increases. Hence, the distance between the nucleus and the valence electron is more. The nuclear force of attraction reduces as we move down the group. Hence, the electron is easily removed as the shielding due to the inner electron makes it difficult for the nucleus to attract the outermost electron. Statement A is correct.
B) All the atoms in a group will have the same valence shell electronic configuration and all the elements will lose identical no. of electrons during ionization to attain the Noble gas configuration. Hence electronic configuration doesn’t affect the ionization energy. Statement B is wrong.
C) As we move down the group the no. of protons also increases with the no. of electrons. Hence the nuclear charge tends to increase as we move down the group. Because of the shielding the outermost electrons are less attracted but this doesn’t mean that the ionization energy increases for the inner shell electrons. Statement C is incorrect.
D) As we move down the group the size of the atom increases with addition of new orbitals. Hence the electrons tend to move farther away from the nucleus. The Further away the electron, the less attractive force of the nucleus on it. Hence the electron is loosely bound and can be easily removed. Statement D is correct.
E) As we move down the group the effective nuclear charge increases for sure, but the shielding of the electrons overshadows the nuclear charge. The force of attraction should increase down the group, but since the shielding of the electrons dominates the ionization energy decreases down the group and not increases.

Note:
This question can be simply solved by keeping in mind the period trends of the groups. The size of the atom increases as we move down the group and the shielding effect also increases. Across the period the size decreases (except for Noble gases ) and the shielding decreases.