# Question #29f82

Feb 24, 2017

See explanation.

#### Explanation:

The mass number, $A$, gives you the number of nucleons, i.e. protons and neutrons, present inside the nucleus of an atom.

In other words, the mass number tells you how many protons and neutrons are located inside the nucleus of a given atom.

$\textcolor{b l u e}{\underline{\textcolor{b l a c k}{\text{mass number = no. of protons + no. of neutrons}}}}$

As you know, the number of protons present inside the nucleus is given by the atomic number, $Z$.

You can thus rewrite the above equation as

$\textcolor{b l u e}{\underline{\textcolor{b l a c k}{A = Z + \text{no. of neutrons}}}}$

The mass number of the most common isotope of an element can be calculated by using the value of the atomic mass, which is listed in the element's box in the Periodic Table of Elements.

Take, for example. silver, $\text{Ag}$

As you can see, silver has an atomic mass of $\text{107.87 u}$. To find the mass number of the most common isotope of silver, round this value to the nearest whole number.

In this case, you'd have

$107.87 \approx 108 \to$ rounded to the nearest whole nubmer

This means that the most common isotope of silver is silver-108. This isotope has a mass number equal to $108$.

You know that silver has an atomic number equal to $47$, and since the number of neutrons can be found using the mass number and the atomic number

$A = Z + \text{no. of neutrons" implies "no. of neutrons} = A - Z$

you can say that the silver-108 isotope has

$\text{no. of neutrons} = 108 - 47 = 61$

neutrons located inside its nucleus.