# Question dae51

Apr 21, 2017

$\text{120 neutrons}$

#### Explanation:

The thing to remember about an atom's mass number, $A$, is that it gives you the number of protons and neutrons present inside said atom's nucleus.

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

For any atom, the atomic number, $Z$, gives you the number of protons located inside the nucleus. You can thus say that

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

Using $A$ and $Z$, you can say that

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

This means that the number of neutrons will be equal to

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

In your case, this isotope of element $\text{X}$ has

$\text{no. of neutrons} = 168 - 48 = 120$

neutrons located inside its nucleus. In isotope notation, you can write this isotope as

""_(color(white)(1)color(green)(48))^color(blue)(168)"X" -> {( color(green)(48)color(white)(.)"protons"), (color(blue)(168)color(white)(.)"protons and neutrons"), (168 - 48 = "120 neutrons") :}#