Chapter 2, Problem 1ACQ

Figure 2.3 How many neutrons do (K) potassium-39 and potassium-40 have, respectively?

Summary Introduction

To calculate:

The number of neutrons in potassium-39 and potassium-40.

Introduction:

All the atoms are composed of electrons, protons, and neutrons. Protons are negatively charged particle and each proton has +1 charge and mass of 1. They are present in the nucleus of the atoms. Electrons are negatively charges particles with charge -1 and negligible mass.

Explanation of Solution

Neutrons are also present in the nucleus with the protons, but do not have any charge. They have mass of 1 and together with protonscomprising the mass of the atom. Potassium-39 and potassium-40 are isotopes of each other. Isotopes of the same element have same atomic number but different atomic masses. That is, they have different number of neutrons but same number of protons.

  $\begin{array}{l}\text{protons}\text{in}\text{potassium}\text{=}\text{19}\\ \text{Atomic}\text{mass}\text{of}\text{potassium-39}\text{=}\text{39amu}\\ \text{Atomic}\text{mass}\text{of}\text{potassium-40=}\text{}\text{40amu}\\ \text{atomic}\text{mass}\text{=}\text{protons}\text{+}\text{neutrons}\\ \text{neutrons}\text{=}\text{atomic}\text{mass-protons}\\ \text{neutrons}\text{in}\text{K-39}\text{=}\text{39-19=20}\\ \text{neutrons}\text{in}\text{K-40=40-19=21}\end{array}$

Conclusion

Neutrons in potassium-39 = 20

Neutrons in potassium-40 = 21