Consider the three gaseous hydrides NH₃, PH₃ and AsH₃ - Leaving Cert Chemistry - Question b - 2016

The dot and cross diagram for AsH₃ illustrates the sharing of electrons between arsenic and hydrogen:

H   H
•   •
 \ /  
  As
•
\\
H

The shape of the arsine molecule is predicted to be pyramidal (trigonal pyramidal). This geometric arrangement arises from the presence of a lone pair on the arsenic atom, causing the hydrogen atoms to be positioned at the vertices of a pyramid.

Among the three hydrides, ammonia (NH₃) is expected to exhibit intermolecular hydrogen bonding due to the high electronegativity of nitrogen, which allows for stronger interactions. Phosphine (PH₃) and arsine (AsH₃) do not form hydrogen bonds of this nature.

Hydrogen bonding in azane (ammonia, NH₃) is stronger than in phosphine (PH₃) and arsine (AsH₃) because nitrogen is small and strongly electronegative, leading to significant hydrogen bonding when bonded to electronegative elements like oxygen, fluorine, or nitrogen.

Ammonia has the highest boiling point among the three hydrides due to the presence of strong hydrogen bonds, which require more energy to break compared to the weaker van der Waals forces present in PH₃ and AsH₃.

Phosphine has a lower boiling point than arsine because it has fewer than two electrons available for bonding and exhibits weaker intermolecular forces compared to arsenic, which has a larger atomic size and consequently can interact more significantly through temporary dipoles.