Number of electron deficient molecules among the following $$PH_3, B_2H_6, CCl_4, NH_3, LiH$$ and $$BCl_3$$ is

An electron deficient molecule is one in which the central atom has fewer than 8 electrons in its valence shell (incomplete octet).

Let us check each molecule:

$$PH_3$$: Phosphorus has 5 valence electrons, forms 3 bonds with H and has 1 lone pair = 8 electrons around P. Not electron deficient.

$$B_2H_6$$: Boron has only 3 valence electrons. In $$B_2H_6$$, each boron is involved in 4 bonds but through 3-centre 2-electron bonds. Each B effectively has only 6 electrons in its valence shell. Electron deficient.

$$CCl_4$$: Carbon has 4 valence electrons, forms 4 bonds = 8 electrons around C. Not electron deficient.

$$NH_3$$: Nitrogen has 5 valence electrons, forms 3 bonds and has 1 lone pair = 8 electrons around N. Not electron deficient.

$$LiH$$: This is an ionic compound ($$Li^+$$ and $$H^-$$). It is not considered electron deficient in this context as $$H^-$$ has a complete 1s shell and $$Li^+$$ has a complete 1s shell.

$$BCl_3$$: Boron has 3 valence electrons, forms 3 bonds = only 6 electrons around B. Electron deficient.

The electron deficient molecules are $$B_2H_6$$ and $$BCl_3$$, giving a count of 2.

The correct answer is Option C.