# Question #0a5fe

Aug 19, 2016

$\text{7 neutrons}$

#### Explanation:

The thing to remember here is that the mass number, $A$, gives you the number of protons and the number of neutrons present inside a given atom's nucleus.

$\textcolor{b l u e}{| \overline{\underline{\textcolor{w h i t e}{\frac{a}{a}} A = Z + \text{no. of neutrons} \textcolor{w h i t e}{\frac{a}{a}} |}}}$

Here

$Z$ - the atomic number of the atom

In your case, the ${\text{X}}^{3 -}$ anion is said to have a mass number equal to $14$. This tells you that this particular ion has a total of $14$ nucleons, which as you know is a term used to describe a proton or a neutron, inside its nucleus.

You can thus say, using the above equation, that your anion has

$14 = Z + \text{no. of electrons}$

Now, you know that this anion has $10$ electrons. The charge of an ion can be calculated by subtracting the number of electrons that surround the nucleus from the number of protons that are located inside the nucleus

${\text{charge" = Z - "no. of e}}^{-}$

In your case, a $3 -$ charge tells you that this anion contains $3$ additional electrons than a neutral atom of $\text{X}$ would contain. This means that you have

$- 3 = Z - 10 \implies Z = 7$

You now know that an atom of $\text{X}$ has $7$ protons inside its nucleus. As a result, the number of protons located inside the nucleus of $\text{X}$ will be

$\textcolor{g r e e n}{| \overline{\underline{\textcolor{w h i t e}{\frac{a}{a}} \textcolor{b l a c k}{\text{no. of neutrons} = 14 - 7 = 7} \textcolor{w h i t e}{\frac{a}{a}} |}}}$

Moreover, you can say that you're dealing with the nitrogen-14 isotope, $\text{^14"N}$, the most abundant of the two stable isotopes of nitrogen.

The ${\text{X}}^{3 -}$ anion is the nitride anion, ${\text{N}}^{3 -}$.