Why did mendeleev leave gaps in his early periodic table, and what was the ultimate importance of the gaps?

Mendeleev grouped elements according to similar physical properties, so he theorized that some elements had not yet been discovered. This is why he left gaps for those elements, along with possible descriptions and properties such as their melting points or appearances. These helped future chemists discover those elements.

Answer the same molecular formula;

The mole ratio of hydrogen to ammonia is 3: 2.