Chapter U1.14, Problem 11E

Interpretation Introduction

Interpretation:

Whether an atom with 60 protons and mass number of 155 would be stable or not needs to be determined.

Concept introduction:

An atom or an element exists in many isotopic forms. Some isotopes have stable nuclei while some have unstable nuclei. 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.

Answer to Problem 11E

An atom with 60 protons and mass number of 155 would not be stable.

Explanation of Solution

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.

For the given atom, the atomic number is equal to the number of protons that is 60. The mass number is 155 thus, the number of neutrons will be as follows:

$\text{Number of neutrons}=155-60=95$

From the isotope graph, one can see that there is no square on the graph that corresponds to 60 protons and 95 neutrons. With 60 protons only atoms with 82, 83, 85, 86 and 88 neutrons form stable isotopes. More than 88 neutrons will make the atom unstable.

Hence atom with 60 protons and mass number of 155 would not be stable.

Conclusion

An atom with 60 protons and mass number of 155 would not be stable.