Because of the presence of isotopes. ... Consider a chlorine atom, it will have 17 protons and hs two isotopes they are chlorine – 35 and chlorine – 37.

The reason for the atomic mass of chlorine taken as 35.5u and not as 35u because of the presence of isotopes.

Isotopes are defined as variants of a particular element where these variants will have the same number of protons but differ in the number of neutrons in the atom..

A number of protons are 17 and the number of neutrons are 20. It happens in nature about 25 per cent of the time.

Atomic mass in an atom or group of an atom is the sum of the masses of protons, neutrons and electrons. For an element relative atomic mass is the average mass of the naturally occurring isotopes of that element relative to the mass of an atom of 12C.

Relative atomic mass = (75/100 x 35) + (25/100 x 37)

Relative atomic mass = 35.5 u.

Thus the atomic mass of chlorine is taken as 35.5u and not a whole number like 36u or 35u