Question

Which of the following molecules doesn’t have a lone pair?
a) $BeC{{l}_{2}}$
b) ${{H}_{2}}O$
c) $HF$
d) $N{{H}_{3}}$

Answer 1

Hint : Lone pair of electrons is the valence shell electron pair that is not shared with the other atoms. It does not take part in bond formation, hence termed as non-bonded electron pair. The valency of elements should be known and the sharing of electrons leads to the formation of a covalent bond.

Complete Step By Step Answer:
Let’s see each of the options.
$BeC{{l}_{2}}$ - In beryllium chloride, the central atom is beryllium with 2 valence electrons. The central atom is beryllium as it is less electronegative than chlorine. The chlorine atoms acquire side positions. Each chlorine atom is short of 1 electron to complete the octet. So, Beryllium will share one electron each with a chlorine atom so that both the atoms can complete the octet. Thus the valence electrons of beryllium are used up and no lone pair of electrons is present on the molecule.
Beryllium has 0 lone pairs of electrons.
${{H}_{2}}O$ - In a water molecule, oxygen is the central atom. It has 6 valence electrons. So to complete the octet oxygen needs two electrons. The two hydrogen atoms present to share their one electron each to complete the octet of oxygen and the water molecule is formed. Out of the 6 valence electrons, two are used for bonding which leaves us with 4 electrons that form 2 lone pairs.
Water has 2 lone pairs of electrons.
$HF$ - In hydrogen fluoride, fluorine is the central atom consisting of 7 valence electrons. It needs one electron to complete the octet. The hydrogen atom shares one electron to complete the octet. Thus, fluorine is left with 6 unshared electrons. These form 3 lone pairs on fluorine.
Therefore, HF has 3 lone pairs of electrons.
$N{{H}_{3}}$ - In ammonia, nitrogen is the central atom having 5 valence electrons. To complete the octet, it requires 3 electrons. This is provided by the 3 hydrogen atoms present. After sharing 3 electrons with hydrogen, nitrogen is left with two electrons. This means it has one lone pair on it.
Hence, $N{{H}_{3}}$ has one lone pair on it.
Final answer: The correct answer is Option A- $BeC{{l}_{2}}$ .

Note :
To easily find the lone pair of electrons on an atom, draw the Lewis dot structure. It shows the presence of lone pairs on the atom. In the above molecules as the electrons were shared the bond formed between the atoms is a covalent bond.