Question

Which of the following molecules does not have a duplet configuration?
A.Hydride ion
B.Hydrogen molecule
C.Lithium cation
D.$B{{e}^{+3}}$

Answer 1

The word “duplet” is often associated with the number 2. The electronic configuration and the atomic numbers for the elements needs to be known for this.

Complete step by step solution:
The electronic configuration of hydrogen atom is $1{{s}^{1}}$ while after accepting one extra electron it becomes the hydride ion which is $1{{s}^{2}}$. Hence it has two electrons in the valence shell.
In the hydrogen molecules, there are two hydrogen atoms in which each atom has one electron. Therefore when two atoms combine then the molecule has duplet configuration.
The atomic number of lithium is 3 and the electronic configuration is: $1{{s}^{2}}2{{s}^{1}}$. So after losing an electron to become the cation, the configuration becomes $1{{s}^{2}}$. Hence here also the duplet configuration is observed.
Coming to $B{{e}^{+3}}$, the electronic configuration of beryllium $1{{s}^{2}}2{{s}^{2}}$. So after losing three electrons, there is only one electron left in the beryllium atom. Hence the beryllium tri-positive cation does not obey the octet or duplet configuration.

So, the correct answer is option D.

Note:
There are two different types of electronic configuration that are considered to be stable for the atoms, one is the octet and the other is the duplet configuration. In both these configurations, the electrons are balanced and satisfy the number of electrons required to be present in the valence shell of the atom to get stability according to the Hund’s Rule and the Pauli’s exclusion principle.