Chapter U1.14, Problem 10E

Interpretation Introduction

Interpretation:

Number of protons present in a stable atom with 90 neutrons is to be determined and the elements have to be named.

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 10E

An atom with 90 neutrons will be stable if it has less than 90 protons. Such atoms will have 62, 63, 64 or 66 protons to form stable nuclei.

The elements would be:

1. Samarium (Sm) with 62 protons
2. Europium (Eu) with 63 protons
3. Gadolinium (Gd) with 64 protons
4. Dysprosium (Dy) with 66 protons

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. Neutrons provide stability to a nucleus because they attract protons and hold them inside 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 90 neutrons will need less than 90 protons to have nuclear stability. From the isotope graph one can see that atoms with 90 neutrons have protons in the range of 60 to 66. Out of these, the element with 60 protons is radioactive, so it is unstable. Other elements have 62, 63, 64 and 66 protons. There is no element with 65 protons. From the periodic table, one can find out the names of elements which have atomic numbers 62, 63, 64 and 66. These are:

1. Samarium (Sm) with 62 protons
2. Europium (Eu) with 63 protons
3. Gadolinium (Gd) with 64 protons
4. Dysprosium (Dy) with 66 protons

Conclusion

The atoms with 90 neutrons will have fewer protons to have nuclear stability. These are atoms with 62, 63, 64 and 66 protons.