Chapter 5, Problem 106A

Interpretation Introduction

Interpretation: The reason for the atomic mass of chlorine to be 35.453 amu and not a whole number.

Concept Introduction: An atom is composed of protons, neutrons, and electrons. Here, protons and neutrons are present in the nucleus of an atom and electrons are present in atomic orbitals around the nucleus. The atomic number of an atom is equal to the number of protons and electrons in it. Atomic mass is the sum of the number of protons and neutrons.

Answer to Problem 106A

This is due to two isotopes of chlorine.

Explanation of Solution

The mass number is the sum of the number of protons and neutrons in the nucleus of an atom thus, it should be a whole number. Here, the atomic mass of chlorine is 35.453 amu which is not a whole number. This is due to isotopes of chlorine. There are two isotopes with mass numbers 35 and 37. They have the same number of protons but the number of neutrons is different. Since the number of protons is the same, thus, the atomic number is the same.

In chlorine-35, the number of protons and neutrons are 17 and 18 respectively. Similarly, in chlorine-37, the number of protons and neutrons are 17 and 20 respectively. Here, the natural abundance of chlorine-35 and chlorine-37 is 77.5% and 22.5% respectively.

The atomic mass of chlorine is obtained from the average of atomic masses of chlorine-35 and chlorine-37.

  $\begin{array}{c}\text{Atomic mass}=35\left(\frac{77.5}{100}\right)+37\left(\frac{22.5}{100}\right)\\ =27.125+8.325\\ =35.45\end{array}$

Thus, the atomic mass of chlorine is 35.45 amu.

Conclusion

Therefore, the reason for the atomic number of chlorine not being a whole number is isotopes of chlorine.