Atomic Number and Atomic Weight

The atomic number of an element is the number of protons in an atom of that element.  Recall; it is the atomic number, the number of protons, that determines the elemental nature of any given atom.

The atomic weight (mass) is the mass of an atom of that element.  Protons and neutrons weight 1 dalton each.  Electrons are weightless or almost so, and they can be ignored in calculating atomic weight (mass).
 

Element
Atomic number (of protons)
Atomic weight (mass)
H
1
1
He
2
4
Li
3
6
     
     
C
6
12
N
7
14
O
8
16
   
You may ask why the atomic weights above increase faster than the atomic numbers; the reason is "neutrons," particals that weight as much as a proton, but are neutral and do not change the nature of the atom, only the weight.

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