The element chlorine (cl) has two isotopes: chlorine-35 and chlorine-37. 75.5% of chlorine atoms have 18 neutrons and 17 protons, and the other 24.5% have 20neutrons and 17protons. using the isotopic composition provided, calculate the average atomic mass of chlorine.

35.8 u

Explanation:

The atomic mass of Cl is the weighted average of the atomic masses of its isotopes.

We multiply the atomic mass of each isotope by a number representing its relative importance (i.e., its percent abundance).

Atomic mass of Cl-35 = 17p + 18n = 17 × 1.007 u + 18 × 1.009 u

= 17.119 u + 18.162 u = 35.28 u

Atomic mass of Cl-37 = 17p + 20n = 17 × 1.007 u + 20 × 1.009 u

= 17.119 u + 20.180 u = 37.30 u

Set up a table for easy calculation.

0.755 × 35.28 u =  26.64  u

0.245 × 37.30 u =    9.138 u

              TOTAL = 35.8     u

Note: The actual atomic mass of Cl is 35.45 u.

The calculated value above is incorrect because

(a) the given isotopic percentages are incorrect and

(b) the protons and neutrons have less mass when they are in the nucleus than when they are free. Thus, the calculated masses of Cl-35 and Cl-37 are too high.