Question

SeH2 molecule -- How many lone-pairs of electrons are on the central Se atom? H3O+ ion -- How many lone-pairs of electrons are on the central O atom? SeO2 molecule -- How many lone-pairs of electrons are on the central Se atom?

Answer 1

Se as in SeH₂: Two lone pairs.

O as in H₃O⁺: One lone pair.

Explanation

Se is a group 16 element. Both Se and O have six valence electrons in each neutral atom. The structure of an SeH₂ molecule resembles that of a water H₂O molecule. Two Se-H bonds connect the three atoms in each SeH₂ molecule.

Two valence electrons on the central Se atom would go to the two Se-H bonds. The four valence electrons remaining would form two lone pairs.

H₃O⁺ has a trigonal pyramidal geometry.

• Each oxygen atom demands 2 electrons to form an octet.
• Each hydrogen atom demands 1 electrons to form an octet. There are three hydrogen atoms in each H₃O⁺ ion.
• The charge of +1 indicates that the ion lacks one electron. It would take an extra electron to fill that gap.

It takes 2 + 3 × 1 + 1 = 6 extra electrons to ensure an octet for all four atoms in a H₃O⁺ ion. The ion would achieve that configuration by forming 6 / 2 = 3 O-H bonds. The central oxygen atom has six valence electrons when it is neutral. Now it has five in this cation after having lost one to acquire the charge of +1. Three of the valence electrons go to three O-H bonds. The remaining two become a lone pair.