No. The atmosphere is roughly 80% N2, 20% O2, and about 1% of other trace gases. The molecular mass of N2 is about 28, the molecular mass of O2 is about 32. So, no O2 is not lighter than air, it is...
To answer the question, pure ammonia has a density of ~0.7-0.9 g/L, depending on temperature. The density of air is ~1.2 g/L, again depending on temperature and pressure. So, yes, ammonia gas is...