# Element X has two isotopes. X-15 has 7 protons and 8 neutrons. What is the atomic number and mass number of the second isotope, which has 10 neutrons in its nucleus?

##### 1 Answer
Aug 6, 2016

The second isotope has an atomic number equal to $7$ and a mass number equal to $17$.

#### Explanation:

The most important thing to keep in mind here is that isotopes are atoms that have the same number of protons inside their nuclei.

In other words, an isotope is defined as an atom that shares the number of protons with at least one other atom but differs in the number of neutrons it has inside its nucleus.

In this case, you know that element $\text{X}$ has two isotopes. Right from the start, you know that these isotopes must have the same atomic number, i.e. the same number of protons located inside their nuclei.

Since isotope $\text{X-15}$ has $7$ protons, you can say for a fact that the second isotope will have $7$ protons as well.

Now, the name of the isotope is written using its mass number, which is simply the number of protons and neutrons located inside the nucleus.

The second isotope is said to contain $10$ neutrons. Add this to the $7$ protons to get the isotope's mass number, $A$

$A = \text{7 protons" + "10 neutrons} = 17$

This means that the second isotope will be called $\text{X-17}$, which designates an atom of element $\text{X}$, i.e. an atom that has an atomic number equal to $7$, that has a mass number equal to $17$.