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1. atoms are the smallest particle of an element
2. atoms may or may not be stable
3. N
1. molecules is a group of atoms which is the smallest particle of 2. an element or compound

2. molecules are stable
3. H₂O
(II) 2N means 2 molecules of N atom
while the other means  2 atoms of nitrogen 
(a) 6 is the atomicity of that compound 
(b) 3 is the atomicity of that compound 

i hope it helps.............
2 5 2