Question

Boron forms covalent compound due to:-
(A)Small size
(B)Higher ionization energy
(C)Lower ionization energy
(D)Both (A) and (B)

Answer 1

Boron is a chemical element with atomic number 5 which means it is small in size. Also the first three ionization enthalpies of boron are very high hence it cannot lose electrons to form a cation and therefore it shares electrons to form a covalent compound.

Complete answer: Let us understand that why boron is bound to make only covalent bonds:-

-Boron (denoted by B) is an element of the 13th group and its atomic number is 5. Its electronic configuration is: $1{{s}^{2}}2{{s}^{2}}2{{p}^{1}}$ and their electrons observe a very high nuclear attraction, hence being the smallest element in its group.

-also being close to the nucleus, a high amount of energy is required for the removal of electrons in the element. Thus boron does not form a tripositive ion easily. This is also considered to be one of the anomalous behaviors (different nature from its respective group) of boron.

-The sum of first three ionization enthalpies of boron is very high due to which it doesn’t form +3 cation which is explained as follows:-
To form ${{B}^{+}}$ ion, only one electron has to remove from 2p orbital (outermost shell) .This electron is at a large distance from the nucleus compared to other 2 electrons, so the force of attraction is less and this can be removed easily. Therefore, first ionization energy of B is small.
Next step is to form ${{B}^{2+}}$ ion. For this, electron has to be removed from fully filled 2s orbital and removal of an electron from fully filled orbital is difficult due to which the second ionization energy is very high. Similarly, after this, ${{3}^{rd}}$electron has to be removed from a half-filled orbital which also requires lots of energy.

-Due to these reasons, first three ionization energies of boron are very high and it becomes difficult to form ${{B}^{3+}}$ ion which brings us to the conclusion that boron can only form covalent bonds

Hence the right option is (D).

Note: -We must know that Boron can never form metallic bonds but aluminum tends to form both covalent as well as metallic bonds.
-Due to its small size, it has more melting and boiling point than other elements of the 13-group. Also it is comparatively very hard.