Chlorine (Cl) has relative atomic mass 35.5 this half number is due to:

Explanation

The Half number in relative atomic mass of Chlorine (i.e 35.5) is due to two naturally occurring isotopes (Chlorine-35 and Chlorine-37). Chlorine-35 is more abundant isotope(accounting for 75.77% of naturally occurring Chlorine) and it's Atomic Mass is 34.97 amu. Chlorine-37 is less abundant isotope(accounting for remaining 24.33% of naturally occurring Chlorine) and it's Atomic Mass is 36.97 amu. Average atomic mass=(Abundance of Chlorine-35 x Atomic mass of Chlorine-35) + (Abundance of Chlorine-37 x Atomic Mass of Chlorine-37) (0.7577 x 34.97 amu) + (0.2423 x 36.97 amu) =26.52 amu + 8.96 =35.48 amu (Rounded of as 35.5 amu).