Answer: The reactants (initial) contain 6 atoms of hydrogen and 2 atoms of nitrogen
The products (final) contain two atoms of nitrogen and six atoms of hydrogen
The number of atoms is the same in the initial and final boxes
Step-by-step explanation:
Atom is the smallest unit of an element which may or may not exist independently.
Molecule is the smallest unit of a pure substance that consists of two or more atoms held together by a chemical bond .
Molecule of a Compound is a pure substance which is made from atoms of different elements combined together in a fixed ratio by mass. Example:

Molecule of an element is a pure substance formed by combination of same types of atoms bonded by a chemical bond. Example:
and

Here one molecule of nitrogen (
) combines with 3 molecules of hydrogen (
) to give 2 molecules of ammonia. (
).