ICSE Class 10 Answered

Beryllium has two valence electrons and accommodate four electrons in Lewis structure.

Boron has three valence electrons and often accommodate only six electrons in Lewis structures.

Let us take BF₃   which is an electron-deficient compound.

Each fluorine has a complete octet but boron has only six electrons. Changing a fluorine lone pair to a bonding pair would alleviate the electron deficiency, but fluorine's high electron affinity means this element is unlikely to share its nonbonding electrons with boron. The electron deficiency means that BF₃ is a highly reactive compound. For example, it reacts readily with NH₃.

In this reaction, the lone pair on N forms a new covalent bond between the compounds. In the new compound, both N and B have full octets and the neither compound is electron deficient.