Atomic Mass of Chlorine

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

Because of something called isotopes, chlorine has an atomic weight of 35.5 instead of 35.

Isotopes are atoms that have the same number of protons (17 in the case of chlorine) but variable numbers of neutrons. Chlorine comes in two forms: chlorine-35 and chlorine-37.

Chlorine-35, which has 17 protons and 18 neutrons, is found in nature roughly 75% of the time. Chlorine-37 has 17 protons and 20 neutrons and is found about 25% of the time in nature. That means there is 75 percent Chlorine-35 and 25% Chlorine-37 in any pure chlorine mixture that can be isolated from all other elements. When the mass of chlorine is determined with a mass spectrometer, the result is 35.5. The atomic weight, or relative  atomic mass, is a weighted average of the weights of all chlorine isotopes.

Relative atomic mass = (7510035) + (2510037) =35.5

Because neutrons have no charge, all isotopes of any element act chemically the same. Because both isotopes of chlorine have 17 positive protons in the nucleus and 17 negative protons from electrons in the surrounding orbitals, it doesn’t matter which isotope is present in a chemical reaction.

Consider a chlorine atom, which has 17 protons and has two isotopes: chlorine 35 and chlorine 37.

There are 17 protons and 20 neutrons in the universe. It happens roughly a quarter of the time in nature. The sum of the masses of protons, neutrons, and electrons in an atom or group of atoms is called atomic mass. The average mass of the naturally occurring isotopes of an element in comparison to the mass of an atom of 12C is called relative atomic mass. 

Relative atomic mass = (7510035) + (2510037) =35.5

As a result, the atomic mass of chlorine is 35.5u rather than a whole number like 36u or 35u.