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Learn how to read atomic number, mass number, protons, neutrons, and electrons from atomic symbols.

---

## An Atom Identity Card

An atomic symbol is a compact way to write the identity of an atom. Like a student card that carries a name and an ID number, an atomic symbol carries the **element symbol**, **atomic number**, and **mass number**.

The Atomic Structure and Symbolism section from OpenStax explains that the atomic number is the number of protons, while the mass number is the number of protons plus neutrons. The section can be opened through [OpenStax's Atomic Structure and Symbolism](https://openstax.org/books/chemistry-atoms-first-2e/pages/2-3-atomic-structure-and-symbolism).

The notation commonly used in basic chemistry is:

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

Read it like this:

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

Visible text: - is the element symbol
- is the atomic number
- is the mass number

## Read the Numbers by Position

Choose an atom below, then read the positions of the numbers. The upper number is not an exponent. The lower number is not a divisor. They are position labels that tell us which part of the atom is being counted.

Component: AtomSymbolLab
Props:
- title: Atomic Symbol Reader
- description: Change the atom example, then match the upper-left and lower-left numbers
with the particle counts.
- labels: {
chooseAtom: "Choose an atom example",
massNumber: "Mass number",
atomicNumber: "Atomic number",
elementSymbol: "Element symbol",
protonCount: "Protons",
neutronCount: "Neutrons",
electronCount: "Electrons",
neutralAtom: "in a neutral atom",
samples: {
"carbon-12": {
tab: "Carbon",
ariaName: "Carbon-12",
name: $${}^{12}_{6}\mathrm{C}$$,
},
"oxygen-16": {
tab: "Oxygen",
ariaName: "Oxygen-16",
name: $${}^{16}_{8}\mathrm{O}$$,
},
"sodium-23": {
tab: "Sodium",
ariaName: "Sodium-23",
name: $${}^{23}_{11}\mathrm{Na}$$,
},
"chlorine-35": {
tab: "Chlorine",
ariaName: "Chlorine-35",
name: $${}^{35}_{17}\mathrm{Cl}$$,
},
},
}
  Visible text: {
chooseAtom: "Choose an atom example",
massNumber: "Mass number",
atomicNumber: "Atomic number",
elementSymbol: "Element symbol",
protonCount: "Protons",
neutronCount: "Neutrons",
electronCount: "Electrons",
neutralAtom: "in a neutral atom",
samples: {
"carbon-12": {
tab: "Carbon",
ariaName: "Carbon-12",
name: ,
},
"oxygen-16": {
tab: "Oxygen",
ariaName: "Oxygen-16",
name: ,
},
"sodium-23": {
tab: "Sodium",
ariaName: "Sodium-23",
name: ,
},
"chlorine-35": {
tab: "Chlorine",
ariaName: "Chlorine-35",
name: ,
},
},
}

## Three Core Counts

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

Visible text: The atomic number is written as . This number equals the number of protons in the nucleus.

```math
Z = p^+
```

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

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

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

Visible text: The mass number is written as . This number counts protons and neutrons in the nucleus.

```math
A = p^+ + n^0
```

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

```math
n^0 = A - Z
```

## Carbon Example

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

Visible text: For a carbon atom with mass number , the symbol is written:

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

From that symbol, we read:

```math
\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.

Visible text: If that atom is neutral, it also has electrons.

## Do Not Confuse It with Average 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.

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

NIST's column notes explain that the relative atomic mass of an element is derived from averaging the masses of the element's isotopes. The explanation can be opened through [NIST's Atomic Weights and Isotopic Compositions](https://www.nist.gov/pml/atomic-weights-and-isotopic-compositions-column-descriptions).

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 | The average mass of isotopes in a natural sample. |

Visible text: | What you read | Common form | Meaning |
| :------------ | :---------- | :------ |
| Mass number | A whole number, such as in | The number of protons plus neutrons in one specific atom. |
| Relative atomic mass | Often decimal, such as about for carbon | The average mass of isotopes in a natural sample. |

This difference matters in three common readings: atomic symbols give proton and neutron counts, ions change electron count, while isotopes change neutron count. Element identity is still locked by proton count.

## Element Symbols Need Exact Capitalization

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 |
| $$\mathrm{CO}$$ | Not one element, but the formula for carbon monoxide |

Visible text: | Writing | How to read it |
| :------ | :------------- |
| | Carbon |
| | Chlorine |
| | Cobalt |
| | Not one element, but the formula for carbon monoxide |

OpenStax gives the example that $$\mathrm{Co}$$ is cobalt, while $$\mathrm{CO}$$ is the compound carbon monoxide. Capitalization is not decoration. One letter can change the chemical meaning.

Visible text: OpenStax gives the example that is cobalt, while is the compound carbon monoxide. Capitalization is not decoration. One letter can change the chemical meaning.

IUPAC's official periodic table lists element symbols and relative atomic masses that are reviewed periodically. The reference can be opened through [IUPAC's Periodic Table of Elements](https://iupac.org/what-we-do/periodic-table-of-elements/).