# A certain negative ion "X"^(2-) has 18 neutrons in its nucleus and 18 electrons in its extra-nuclear structure. What is the mass number of the most abundant isotope of "X" ?

Sep 4, 2017

$32$

#### Explanation:

You know that this negative ion has a total of $18$ electrons surrounding its nucleus.

As you know, the net charge of an ion is given by the difference that exists between the number of protons located inside its nucleus and the number of electrons surrounding its nucleus.

$\text{net charge = no. of protons " - " no. of electrons}$

You can use the fact that the ion has a $2 -$ net charge to determine the number of protons located inside its nucleus.

$\text{no. of protons} = \left(2 -\right) + 18 = 18 - 2 = 16$

So, you know that element $\text{X}$ has $16$ protons inside its nucleus, i.e. an atomic number, $Z$, equal to $16$.

The mass number of an atom, $A$, is given by the number of nucleons--protons and neutrons--located inside its nucleus

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

This means that this particular isotope of $\text{X}$ has

$A = 16 + 18 = 34$

Now, in order to check whether or not this particular isotope is also the most abundant, you must grab a Periodic Table and look for the element that has $Z = 16$.

You will find that element $\text{X}$ is sulfur, $\text{S}$, and this particular isotope is sulfur-34, $\text{^34"S}$.

Notice that sulfur has an atomic mass of $\text{32.066 u}$. The trick here is the fact that you can get the mass number of the most abundant isotope of sulfur by rounding its atomic mass to the nearest integer.

In this case, you have

$32.066 \approx 32$

This means that the isotope that you're dealing with here, i.e. sulfur-34, is not the most abundant isotope of sulfur. The most abundant isotope of sulfur is, in fact, sulfur-32, $\text{^32"S}$, which has a mass number equal to $32$ because it contains $16$ protons and $16$ neutrons inside its nucleus.