Atomicity is the number of  atoms consisting in a molecule. 
Atomic number is the number of protons. It  is represented by symbol Z.

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 Atomic number references how many proton's that element contains. Valence electron's varies by size of the atom, along with how many proton's it has, and over all charge. This is the case because an atom with more protons(positive charge) can keep more electrons around it. However an atom with more electrons than proton's has a harder time maintaining electrons around it because their repulsive forces start to outweigh the attractive forces of the nucleus. Also electron's orbit atoms in a set of orbitals. These orbitals vary in size and how many electron's they can hold in them. They also increase in energy. Due to this layering of electron's only a certain amount of electron's are considered available for bonding because the rest are to tightly held to the nucleus of the atom to bond. 

Finally, it appears that the amount of proton's in a nucleus of an atom determine the properties for that element. Why it is based on the number of proton's is not known to me. That is apparently just how nature works. Since an elements properties are not based on mass. Since many elements have isotopes. Isotopes are elements with the same amount of proton's but a differing number of neutrons in the core. This can some times affect their chemistry by making them less stable. However it does not effect the basics of the element's properties
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my answer is same as sameer is said !!!
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