Explain fractional atomic mass. What is the fractional mass of chlorine?

Fractional atomic mass is the weighted average of the atomic masses of all the naturally occurring isotopes of that element.

Fractional mass of chlorine is 35.5.

Any natural sample of chlorine contains 75% of ³⁵₁₇Cl and 25% ³⁷₁₇Cl, i.e., these isotopes are in the ratio of 3:1 respectively. Thus, average atomic mass of chlorine is calculated as

$\dfrac{(35\times3) + (37\times1)}{4}$ = 35.5. Therefore, atomic mass of chlorine atom is 35.5