Chlorine is a non meta element. Atomic Mass of it is 35.
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Because chlorine atoms occur as two isotopes: 35Cl and 37Cl. The first accounts for 75.76% of chlorine and the second for the rest so the [weighted] average mass is(75.76*35 + 24.24*37)/100 = 35.48 = 35.5, approx.
The average atomic mass of an element is the average of the atomic masses of its isotopes (that is a weighted average). You have to take into account the abundance of each isotope when they do your averaging.
What is weighted average atomic number
The average atomic mass is the ratio of the average mass of one atom of an element to one twelfth of the mass of an atom of carbon-12. It is, the mass of an atom of the substance, rescaled so that carbon-12 would have a mass of 12.
Magnesium is a meta element. Atomic mass of it is 24.