# The isotope sodium-20 has how many neutrons?

Dec 27, 2016

$\text{9 neutrons}$

#### Explanation:

The trick here is to realize that the name of the isotope includes its mass number.

As you know, the mass number of an atom, $A$, is equal to the number of nucleons, i.e. protons and neutrons, present inside its nucleus

$\textcolor{red}{\underline{\textcolor{b l a c k}{A = Z + \text{no. of neutrons}}}}$

Here

• $Z$ is the atomic number, i.e. the number of protons located inside the nucleus

In isotope notation, the mass number of the isotope is added after a minus sign that follows the name of the element

$\textcolor{red}{\underline{\textcolor{b l a c k}{\text{name of element"-"mass number}}}}$

"sodium"-color(blue)(20) -> {("sodium is the element"), ("the mass number is equal to" color(white)(.)color(blue)(20)) :}
Now, a quick look in the Periodic Table of Elements will reveal that sodium has an atomic number equal to $11$. This means that you have
$\textcolor{b l u e}{20} = 11 + \text{no. of neutrons}$
$\textcolor{\mathrm{da} r k g r e e n}{\underline{\textcolor{b l a c k}{\text{no. of neutrons} = 20 - 11 = 9}}}$