Atomic Symbol

Information in an Atomic Symbol

An atomic symbol records three pieces of information in a compact form, the element symbol, atomic number, and mass number. Together, they identify the element and state the numbers of protons and nucleons in the atom.

The atomic number is the number of protons. The mass number is the number of protons plus neutrons.

The notation commonly used in basic chemistry is:

{}^{A}_{Z}\mathrm{X}

Read it like this:

$\mathrm{X}$ is the element symbol
$Z$ is the atomic number
$A$ is the mass number

Reading Mass and Atomic Numbers

Choose an atom below, then match the two numbers in its symbol. The upper-left number is the mass number $A$ , while the lower-left number is the atomic number $Z$ . Their positions determine which proton, neutron, or electron count to calculate.

Atomic Symbol Reader

Change the atom example, then match the upper-left and lower-left numbers with the particle counts.

{}^{12}_{6}\mathrm{C}

{}^{12}_{6}\mathrm{C}

Mass number: $A = 12$
Atomic number: $Z = 6$
Element symbol: $X = \mathrm{C}$

Protons: $6$
Neutrons: $6$
Electrons: $6$ in a neutral atom

Counting Protons Electrons and Neutrons

The atomic number is written as $Z$ . This number equals the number of protons in the nucleus.

Z = p^+

For a neutral atom, the number of electrons equals the number of protons. Neutral means the total positive and negative charges balance.

\text{neutral atom} \Rightarrow e^- = p^+ = Z

The mass number is written as $A$ . This number counts protons and neutrons in the nucleus.

A = p^+ + n^0

Once the mass number and atomic number are known, the neutron count is just their difference:

n^0 = A - Z

Carbon Example

For a carbon atom with mass number $12$ , the symbol is written:

{}^{12}_{6}\mathrm{C}

From that symbol, we read:

\begin{aligned} Z &= 6 \Rightarrow p^+ = 6 \\ A &= 12 \Rightarrow p^+ + n^0 = 12 \\ n^0 &= 12 - 6 = 6 \end{aligned}

If that ${}^{12}_{6}\mathrm{C}$ atom is neutral, it also has $6$ electrons.

Mass Number and Relative Atomic Mass

On the periodic table, you often see decimal values such as carbon's relative atomic mass of about $12.01$ . That decimal is not the mass number of one specific carbon atom.

An element's relative atomic mass depends on isotope masses and isotopic composition. That composition can vary with a material's origin and treatment.

So keep these two ideas separate:

What you read	Common form	Meaning
Mass number	A whole number, such as $12$ in ${}^{12}_{6}\mathrm{C}$	The number of protons plus neutrons in one specific atom.
Relative atomic mass	Often decimal, such as about $12.01$ for carbon	An isotope-composition-weighted mass for the material being represented.

When reading an atomic symbol, track three facts: atomic symbols give proton and neutron counts, ions change the electron count, and isotopes change the neutron count. The proton count still determines the element's identity.

Capitalization of Element Symbols

Element symbols usually have one or two letters. The first letter is always uppercase, and the second letter, if present, is lowercase.

Writing	How to read it
$\mathrm{C}$	Carbon
$\mathrm{Cl}$	Chlorine
$\mathrm{Co}$	Cobalt
$\mathrm{CO}$	Formula for carbon monoxide, a compound made of carbon and oxygen

Look closely at the last two rows. In $\mathrm{Co}$ , the lowercase $\mathrm{o}$ joins the uppercase $\mathrm{C}$ to form one element symbol: cobalt. If the second letter becomes uppercase, $\mathrm{CO}$ is read as two element symbols, $\mathrm{C}$ and $\mathrm{O}$ , in the formula for carbon monoxide. Uppercase and lowercase letters therefore tell you which substance the formula names.

When you want to check the latest element symbols and relative atomic masses, open the IUPAC Periodic Table of Elements.