Question

how to identify electron deficient compound

Answer 1

Some compounds can't reach their octet configuration even after involving in covalent compounds.

Even if there is a mutual share of electrons between the atoms, The participating atoms can't reach their stable configuration.

A best example to this is $BF_{3}$

Boron being an element with atomic number 5 , needs to lose 3 electrons to attain stability. But When it forms 3 covalent bonds with fluorine, It gains 3 electrons partially. But still is not able to reach octet.

Such elements are electron deficient compounds.

The best way to identify them is When the molecules have atoms with very few ( 1 , 2 , 3 ) electrons in their valence shell

Answer 2

This must help you
I got image from google