Question

: In comparison to boron, beryllium has:
A) Lesser nuclear charge and greater first ionisation enthalpy
B) Lesser nuclear charge and leaser first ionisation enthalpy
C) Greater nuclear charge and greater first ionisation enthalpy
D) Greater nuclear charge and lesser first ionisation enthalpy

Answer 1

Hint: The elements beryllium and boron both are present in the same period of the periodic group. The first ionisation enthalpy is the energy required to remove an electron from a neutral atom. One can think about their position in the periodic table and decide the factors about them which are given in options.

Complete step by step answer:
1) First of all we will learn about these two elements separately, the element Beryllium is a chemical element that is denoted by the symbol Be in the periodic table and it has the atomic number $4$. The element Boron is a chemical element that is denoted by symbol B in the periodic table and it has the atomic number $5$.
2) Now the atom beryllium has atomic number four which means it has four electrons revolving in its orbital. The four electrons are present in the $1{s^2}2{s^2}$ orbitals which are fully filled and all electrons are paired. Hence, it will have less nuclear charge and have greater ionisation enthalpy as all electrons are paired and orbitals are fully filled.
3) Now the atom boron has atomic number five that means it has five electrons revolving in its orbital. The five electrons are present in the $1{s^2}2{s^2}2{p^1}$ orbitals in which $1{\text{s}}$ and $2{\text{s}}$ are fully filled and all electrons are paired but in $2{\text{p}}$ there is only one electron that too unpaired. Now that the unpaired electron is there it is easy to remove the electron from the atom hence it has less ionisation enthalpy and greater nuclear charge.
4) Therefore, beryllium has a lesser nuclear charge and greater first ionisation enthalpy in comparison with boron

So, the correct answer is Option A .

Note:
The atomic radius of elements decreases across a period and increases going down across the group in a periodic table. In the period the nuclear charge increases as electron shielding remains constant and the first ionisation enthalpy decreases as the number shells get added. Remember that the elements with fully filled orbitals have the highest first ionisation enthalpy.