# Question #09e19

Dec 4, 2016

$\text{_15^31"P}$

#### Explanation:

Isotope notation, which is used to describe isotopes of various elements, requires

• the atomic number of the element, $\textcolor{b l u e}{Z}$
• the mass number of the isotope, $\textcolor{red}{A}$
• the chemical symbol of the element

So, the first thing to do here is figure out the chemical symbol of the element. Grab a periodic table and look for the element that has

$\textcolor{b l u e}{Z} = 15$

This element is phosphorus, $\text{P}$, located in period 3, group 15.

Now, you know that its atomic number is equal to $15$ and that its mass number is equal to $31$, so set up your symbol and add the two values as shown in the above image

$\text{_(color(Blue)(15))^color(red)(31)"P } \to$ this is the phosphorus-31 isotope

This particular isotope of phosphorus has $15$ protons, just like all isotopes of phosphorus, and

$31 - 15 = \text{16 neutrons}$

inside its nucleus. The number of neutrons is given by the difference between the mass number and the atomic number.