Question

Although boron (B) is smaller in size than barellium (Be). But Ist ionisation enthalpy of Barellium (Be) is higher than Boran (B) Why.

Answer 1

Although Boron is smaller in size than Be. But 1st ionization of Be is greater than B for the following reasons:
Ionization energy means energy required to remove an electron from neutral gaseous atom.
Electronic configuration of Beryllium is 1S2 2S2(since the orbitals are filled completely, its going to be become stable and when atom is stable its very hard to remove electron from it.
Where as B electronic configuration Is 1s2 2s22p1(since the last p orbital is not filled completely then its going to stay unstable and also the energy of P orbital is higher than S orbital so it will be easy to remove an electron from it.

Answer 2

Be has the configuration of :1s2 2s2

And B has the configuration of :1s2 2s2 2p1

When we make a look at these two subshell electronic configurations;we got that the outermost subshell that filled 1st in Be is S-subshell and B is P-Subshell. S subshell has higher penetration power than that of P-Subshell ,When peneteration poweris higher then ionisation enthalpy also will be higher