Chapter U1.14, Problem 7E

Interpretation Introduction

Interpretation:

Whether an atom with 31 protons and 31 neutrons is a stable isotope or not needs to be explained.

Concept introduction:

An atom or an element exists in many isotopic forms. Some isotopes are found naturally on Earth while some exist due to human activity. Isotopes differ in the number of neutrons. Some isotopes have stable nuclei while some have unstable nuclei.

Answer to Problem 7E

The atom with 31 protons and 31 neutrons will not be a stable isotope because it has more than 20 protons, so it will need more than 31 neutrons to make it stable.

Explanation of Solution

Elements are found in one or more isotopic forms. Some isotopes have stable nuclei while some have unstable nuclei.

Stability of nucleus determines the stability of an isotope. Unstable nucleus will disintegrate. There should be a balance between the number of protons and number of neutrons to have a stable nucleus. Large atoms need more neutrons to make a stable nucleus.

The number of neutrons is approximately equal to number of protons for atoms up to atomic number 20. Beyond atomic number 20, elements need more neutrons to hold the protons in the nucleus. Atoms with atomic number between 20 and 84 need more neutrons to achieve nuclear stability. Unstable isotopes are radioactive.

Hence, the isotope with 31 protons will need more than 31 neutrons to have nuclear stability. Fewer neutrons will also make nucleus unstable and it will soon disintegrate.

Conclusion

The isotope with 31 protons will need more than 31 neutrons to have nuclear stability. It will not be stable if it has 31 neutrons as it has more than 20 protons.