Question

Unit of ionization energy is
A.${\text{J/m}}$
B.${\text{kJ}}{\text{.mol}}$
C.${\text{J}}{\text{.mol}}$
D.${\text{kJ/mol}}$

Answer 1

To answer this question, you must recall the concept and actual definition of ionization energy. Ionization energy is a measure of the amount of energy needed to withdraw an electron from the valence shell of an atom. The energy supplied must be high enough so as to surpass the already existing forces of attraction in the element.

Complete step by step answer:
The ionization energy can be defined as the energy which is required to move an electron from its existing energy level in the atom to infinity. We can also define the first ionization energy of an element as the energy required to remove one mole of valence electrons from one mole of atoms in gaseous state resulting into the formation of one mole of gaseous ions carrying a single positive charge each.
We can conclude from the definition that when writing the ionization energy we refer to one mole of a given substance. Thus, ionization energy can be written as the energy required per mole.
The SI unit of energy is Joule but for ionization energy we use it in kilojoules, or $kJ$.
Thus, the unit of ionization energy is ${\text{kJ/mole}}$

Thus, the correct answer is D.

Note:
Ionization energy gives an idea of how strongly an atom clings onto its valence electrons. More is the ionization energy, greater is the strength of attraction of the electrons. Ionization energy is inversely proportional to the radius of the atom as with increasing size, the electrons become more loosely held. As a result, ionization energy decreases as we move down a group and increases as we move across a period.