Atomicity of an atom in a compound is the total number of that atom present in that compound. In the question, 3 molecules of Iron(II)nitrate [Fe(NO₃)₂] contains the following number of oxygen.
In one molecule of Fe(NO₃)₂, number of oxygen atoms present is 3 × 2 = 6
If 6 atoms of oxygen are present in one molecule of Fe(NO₃)₂, in three molecules, 6 × 3 = 18 atoms of oxygen will be present in three molecules of Fe(NO₃)₂.
There are 18. In each molecule of Fe(NO3)2 there are 6 oxygen atoms because you have 2 nitrate (NO3) components which each contain 3 oxygen atoms. 2 x 3 = 6. Because you have 3 molecules of Fe(NO3)2, you need to multiply 6 by 3, which gives you 18 oxygen atoms.
