Question

Amongst the elements B, Al, C and Si, The element having highest first ionization enthalpy is:
(A) C
(B) Al
(C) B
(D) Si

Answer 1

Ionization enthalpy is the energy required to remove the outermost electron from a neutral gaseous atom and convert it into a positive gaseous ion. Ionization enthalpy is always positive. It depends on stability, nuclear charge and electronic configuration of the atoms. The more the electronegativity of the atom, more is the ionisation enthalpy

Complete Step by step solution
Ionization enthalpy depends upon electronic configuration. Half filled and fully filled orbitals are stable as compared to others. Hence, ionization enthalpy of half filled and fully filled orbitals is higher. Also, ionization enthalpy depends on the shielding effect. In shielding effect, the outermost electron does not get appropriate nuclear charge due to repulsion from the inner electrons. Due to this, the outermost electrons experience low effective nuclear charge. The atomic radius decreases while moving from left to right in a period. So, if the atomic size decreases, the attractive force between the nucleus and outermost electron increases. Hence, the ionization enthalpy across a period generally increases. While moving down the group, the outermost electron goes away from the nucleus as the number of shells in an atom increases and hence effective nuclear charge will be decreased. So, down the group ionization energy usually decreases.
In the periodic table, group 13 includes boron and aluminium. Boron is placed above aluminium in group 13. Ionization enthalpy decreases down the group and hence ionization enthalpy of boron is higher than that of aluminium.
Also, in group 14 carbon and silicon are included. Carbon is placed above silicon in group 14. Here too, ionization enthalpy of carbon is greater than that of silicon.
Across the period, boron is placed before carbon. As the size of a carbon atom is less in comparison with boron, its effective nuclear charge increases. Hence the ionization energy of carbon is greater than that of boron. Amongst the elements B, Al, C and Si; the element having highest first ionization enthalpy is C.
Therefore, the correct answer is option A.

Note
For most of the elements the successive ionization enthalpies are higher, i.e. the energy of removing the first electron is less than the energy to remove the second electron and so on.