Many elements have non-integral atomic masses because:

Because isotopes have varying masses and occur in nature in variable ratios, elements have non-integral atomic masses.

For instance, Cl;z=17 and A = 35 and 37 They are there in a 3 :1 ratio.

$\therefore \frac{3\times 35+1\times 37}{4}=35.5\left(\mathrm{A}\right)$.