Question

A cation with $+2$ charge with atomic number $56$ and atomic mass $137$ contains:
I. $54$ protons
II. $81$ neutrons
III. $54$ electrons

• A. I, II, and III
• B. I and II only
• C. II and III only
• D. I only

Answer 1

Answer: (C)

II and III only

Atomic mass or Mass number of the element is $137$. The mass number is defined as the total number of protons and neutrons.

The atomic number of the given element is $56$ which means, the element has $56$ protons. It is a cation with $+2$ charge. Hence the number of electrons is two less than the number of protons, that is the number of electrons is ($56$$-$$2$), which is equal to $54$.

Mass number is $137$, number of protons is $56$. Hence, the number of neutrons is ($137$$-$$56$), that is $81$.

Thus the element has $81$ neutrons and $54$ electrons.

Option $C$ is the correct answer.