The boiling point of hydrides decreases from ammonia to phosphine and afterward increments from phosphine to bismuth. A similar pattern is watched for their melting points. Please Why is this?

Boiling point depends on intermolecular forces of attraction. In ammonia, there is intermolecular hydrogen bonding, so it has higher boling point as compared to phosphine. In all other hydrides BP(boiling point) depends on Vander Waal forces which are directly proportinal to molecular size. So bismuth hydride has max BP as its size is maximum.