Chapter 30, Problem 1RQ

To determine

The mass number of a nucleus.

Answer to Problem 1RQ

Themass number of a nucleusis $\underset{\_}{21}$.

Explanation of Solution

In an atom nucleus is present at the center and it is made up of protons and neutrons. The total number of protons and neutrons present in the nucleus of an atom is called as mass number. And it is denoted by the symbol A.

The expression for mass number is,

  $A=n_p+n_n$        (I)

Here, $n_p$ is the number of protons and $n_n$ is the number of neutrons.

Conclusion:

Substitute $10$ for $n_p$ and $11$ for $n_n$ in Equation (I) to find the mass number of the nucleus.

  $\begin{array}{c}A=10+11\\ =21\end{array}$

Therefore, the mass number of a nucleusis $\underset{\_}{21}$.