Question

Which of the following has maximum ionization enthalpy?
A) $K$
B) $Na$
C) $Mg$
D) $Be$

Answer 1

We need to know that the enthalpy change associated with the removal of the first electron from an isolated gaseous atom in its ground state is called first ionization enthalpy. The shielding effect increases down the group as the number of shells increases which changes ionization energy.

Complete answer:
As we know that ionization enthalpy decreases down the group and increases across a period. Ionization enthalpy of an element can be defined as the amount of energy required to remove an electron from an isolated gaseous atom in its gaseous state. Ionization enthalpy depends on the following factors:
Penetration effect
Shielding effect
Electronic configuration
Penetration means the proximity of an electron in an orbital to the nucleus. For each shell and subshell, it can be observed as the relative density of electrons near the nucleus of an atom.
Shielding effect can be defined as the effect in which the inner electrons develop a shield for the electrons in outer shells which does not let the appropriate nuclear charge towards the outermost electrons. Due to this, the outermost electrons experience a low effective nuclear charge and not the actual nuclear charge.
Elements having half-filled and fully filled orbitals are stable. So if we try to remove an electron from these orbitals, then it will make them less stable. Hence, more energy is required to remove an electron from these orbitals. Thus, higher ionization energy.
Since Be and Mg have electronic configuration $ns^2$ while Na and K has $ns^1$ hence ionisation enthalpy of Be , Mg is greater than that of Na and K because it is difficult to extract an electron from a fully filled s orbital. Among Be and Mg , Be has the highest ionisation enthalpy because of its smaller size.

Option D is the correct answer.

Note:
We need to know that while moving left to right in a period, the atomic radius decreases. So if the size of an atom decreases, the attractive force between the nucleus and the outermost electrons increases due to which across a period in the periodic table ionization energy generally increases.