• Most elements exist naturally as a mixture of isotopes. Therefore, the value we use for the atomic mass of an element is an average mass. This takes into account the proportions (abundance) of all the naturally occurring isotopes. If a particular isotope is present in a high proportion, it will make a large contribution to the average.

• This average value for the mass of an atom of an element is known as the relative atomic mass (Ar).

Chlorine has two isotopes, chlorine-35 and chlorine-37, in an approximate ratio of 3 : 1 (or 75.0% : 25.0%). If the ratio was 1 : 1 (or 50% : 50%), then the relative atomic mass of chlorine would be 36. The actual value is 35.5.

Mass of 100 atoms of chorine = (35 × 75.0) + (37 × 25.0) = 3550

Then the average mass of one atom = 3550/100 = 35.5

Thus, for chlorine Ar (Cl) = 35.5

Electronic configuration of elements

• Electrons move in electron shells , they have a lot of electron shells in a electron

• The circle around the nucleus is the electron shell

• Those shells and electron shells do actually not exist , they are just a metaphor to describe the electron rotation , thus they don't exist.

  
  
  
  

This is a a nucleus explaining the number of shells electron contain in each one of them