Question

Question #09e19

Answer 1

`""_15^31"P"`

Explanation:

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

• the atomic number of the element, `color(blue)(Z)`
• the mass number of the isotope, `color(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

`color(blue)(Z) = 15`

This element is phosphorus, `"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

`""_(color(Blue)(15))^color(red)(31)"P " ->` this is the phosphorus-31 isotope

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

`31 - 15 = "16 neutrons"`

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