Nuclide notation

By the end of this lesson

By the end you can read a nuclide such as $^{30}_{14}\text{Si}$ aloud in terms of protons and neutrons, and write the notation correctly for a nuclide described in words.

What each part of the notation means

A nuclide is the nucleus of one particular isotope, and

$^{A}_{Z}\text{X}$

is how we name it. Read it from the inside out:

$X$ is the chemical symbol of the element.
$Z$ , the subscript written before the symbol, is the proton number.
$A$ , the superscript before the symbol, is the nucleon number.

For $^{30}_{14}\text{Si}$ : 14 protons, 30 nucleons in total, so $30 - 14 = 16$ neutrons. The symbol is Si because $Z = 14$ is silicon; if you were given $A = 30$ alone you could not name the element.

The same notation is used for the particles that come out of a nucleus, and each has its own fixed numbers:

particle	notation	$Z$	$A$
proton	$^{1}_{1}\text{p}$	1	1
neutron	$^{1}_{0}\text{n}$	0	1
electron	$^{0}_{-1}\text{e}$	$-1$	0
alpha particle	$^{4}_{2}\text{He}$	2	4
gamma photon	$^{0}_{0}\gamma$	0	0

The notation is a compact statement of both conserved quantities, which is why every decay equation is written with it.

Superscript up, subscript down

The visual cue is worth fixing now, because swapping the two is the most common slip:

The bigger number is usually the nucleon number, and it sits on top, where there is room for three digits — $235$ , not $92$ .
The subscript is the proton number, always smaller than or equal to the superscript.

A useful check when you write a nuclide: the superscript minus the subscript must not be negative. If it is, the two numbers are in the wrong places.

Worked example: Writing a nuclide from a description

A nucleus of silicon contains 14 protons and 16 neutrons. Write it in nuclide notation and state its proton and nucleon numbers.

Have a go first?

Quick check

One-line answerQuestion 1 · Recall

Multiple choiceQuestion 2 · Recall

Fill in the blanksQuestion 3 · Apply

Answer the 3 questions above to continue.