The isotope sodium-20 has how many neutrons?

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Answer 1 · 2016-12-27T01:10:29

$"9 neutrons"$

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

$color(red)(ul(color(black)(A = Z + "no. of neutrons")))$

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

$color(red)(ul(color(black)("name of element"-"mass number")))$

In your case, you have

$"sodium"-color(blue)(20) -> {("sodium is the element"), ("the mass number is equal to" color(white)(.)color(blue)(20)) :}$

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

$color(blue)(20) = 11 + "no. of neutrons"$

Consequently, the number of neutrons present in the nucleus of a sodium-20 isotope will be

$color(darkgreen)(ul(color(black)("no. of neutrons" = 20 - 11 = 9)))$