ChemTeam: A Brief Tutorial About Writing Nuclear Symbols

A Brief Tutorial About Writing Nuclear Symbols

First, an example of a nuclear symbol:

$_{6}^{14} C$

Make sure you know that the lower number is the atomic number and the upper number is the mass number.

The atomic number is the number of protons.
The mass number is the sum of the protons and the neutrons.

Here's another nuclear symbol:

$_{atomic number ---> 92}^{mass number ---> 238} U$

Here's a generic nuclear symbol:

$_{Z}^{A} X$

Remember:

A = commonly used symbol for the mass number.
Z = commonly used symbol for the atomic number.

Lastly, remember that you have to do a subtraction to get the number of neutrons:

14 − 6 = 8
238 − 92 = 146

Sometimes, you see the required subtraction explained using symbols:

A − Z = N (when N is the number of neutrons)

Oh, and remember: some textbooks or teachers call it the isotopic symbol. You have to know both names.

Special Note on the neutron and the electron:

$_{atomic number ---> 0}^{mass number ---> 1} n$
$_{"atomic number" ---> −1}^{mass number ---> 0} e$

The atomic number is defined as the number of protons, so you can't have negative one protons. The −1 is identified as the charge and it is used in counting up the total atomic number, but it is not called the atomic number of an electron.

On many Internet answer boards, you can't format isotopic symbols like above. Here is what you often see:

3-Li-7
or
Li-7 (the 3 is implied in the symbol Li)
This last one is sometimes called "symbol-mass number" style. It is not a term that is standard across the chemistry world.

Other people do something like this:

7/3 Li

Also, really old materials (from when the ChemTeam was in school), show nuclear symbols like this:

₉₂U²³⁵

Be prepared!