Boiling points of hydrides graph question. PLease explain answers for revision purposes?

a)
X: NH₃
Y: CH₄


b)
The four Group 4 hydrides are CH₄, SiH₄, GeH₄ and SnH₄. They all consist of non-polar molecules held together by van der Waals' forces. When going down the group, the molecular sizes are increasing leading to an increase in the strength of van der Waals' forces between molecules. Therefore, there is a fairly steady increase of the Group 4 hydrides going down the group.


c)
Among the four Group 6 hydrides, molecules of water (H₂O) are held together by intermolecular hydrogen bonds, while the other three (H₂S, H₂Se and H₂Te) by van der Waals' forces.

Among the four Group 5 hydrides, molecules of compound X (NH₃) are held together by intermolecular hydrogen bonds, while the other three hydrides (PH₃, AsH₃ and SbH₃) by van der Waals' forces.

For small molecules, hydrogen bonds are much stronger than van der Waals' forces. Hence, water and compound X have boiling points considerably higher than expected.