The element chlorine (Cl) has two isotopes: chlorine-35 and chlorine-37. 75.8% of chlorine atoms have 18 neutrons and 17 protons. The rest of the...

The atomic mass of an atom is the sum of number of protons and neutrons. For example, if there are 18 neutrons and 17 prorons, the atomic mass is 35 g/mole. Similarly, the atom with 20 neutrons and 17 protons, will have an atomic mass of 37 g/mole. Thus, 75.8% of chlorine is in the form of chlorine-35 isotope, and the rest 24.2% is in the form of chlorine-37. The average atomic mass of chlorine atom can be calculated by using weighted averages. 

75.8% of chlorine has the atomic mass of 35 g/mole and 24.2% has an atomic mass of 37 g/mole.

Thus, the average atomic mass = (35 x 75.8/100  + 37 x 24.2/100)= 35.48 g/mole.

Thus, the chlorine atom has an average atomic mass of 35.48 g/mole.

Hope this helps.