How to Read and Write Isotope Notation (AZX and Element-Mass Forms)

Chemists and physicists use a compact notation to describe a particular atom (a nuclide) exactly: which element, how many nucleons, and sometimes its charge. Once you can read it, a symbol such as ²³⁵₉₂U tells you everything about the nucleus at a glance. This guide shows you how to read and write the two common forms, how to add ion charges, and how the same notation makes nuclear equations easy to balance.

Form 1: the full nuclide symbol, ᴬ_Z X

The full symbol has three parts:

• X: the element symbol
• A (top left, superscript): the mass number, the number of protons + neutrons
• Z (bottom left, subscript): the atomic number, the number of protons

Examples:

Symbol	Element	A	Z	Neutrons (A − Z)
¹²₆C	carbon	12	6	6
¹⁴₆C	carbon	14	6	8
³⁵₁₇Cl	chlorine	35	17	18
²³⁵₉₂U	uranium	235	92	143
¹₁H	hydrogen	1	1	0

See mass number explained and atomic number explained.

How to read it: step by step

1. Look at the symbol to identify the element.
2. The top number is A: total nucleons.
3. The bottom number is Z: protons (it must match the element; carbon is always 6).
4. Neutrons = A − Z.
5. For a neutral atom, electrons = Z.

How to write it: step by step

Write the symbol for an atom with 26 protons and 30 neutrons.

1. Z = 26 → look up the element: iron, Fe.
2. A = 26 + 30 = 56.
3. Write A top left, Z bottom left: ⁵⁶₂₆Fe.

Form 2: element–mass names

Often you’ll see isotopes written with just the name (or symbol) and mass number:

• carbon-14, or C-14, or ¹⁴C
• uranium-235, or U-235, or ²³⁵U
• iodine-131, or I-131, or ¹³¹I

The atomic number is left out because the element already tells you Z. This form is common in medicine, geology and news reports (for example “caesium-137 contamination” or “technetium-99m scans”).

Converting between forms:

Write chlorine-37 as a full nuclide symbol.

1. Chlorine, Cl, has Z = 17 (from the periodic table).
2. A = 37.
3. ³⁷₁₇Cl

Adding a charge for ions

For an ion, the charge goes at the top right:

• ²³₁₁Na⁺: sodium ion, 11 protons, 12 neutrons, 10 electrons
• ¹⁶₈O²⁻: oxide ion, 8 protons, 8 neutrons, 10 electrons
• ⁵⁶₂₆Fe³⁺: iron(III) ion, 26 protons, 30 neutrons, 23 electrons

The charge changes only the number of electrons; A and Z are unchanged. See what is an ion?

The four positions around a symbol

It helps to know what each corner means, so you never mix them up:

Position	Meaning	Example
top left	mass number	²³Na
bottom left	atomic number	₁₁Na
top right	charge	Na**⁺**
bottom right	number of atoms in a formula	O₂, H₂O

So ¹⁶₈O₂ means a molecule of two oxygen-16 atoms, while ¹⁶₈O²⁻ means one oxide ion.

Notation in nuclear equations

The same notation describes particles in nuclear reactions, where A and Z must both balance.

Particle	Symbol	A	Z
alpha particle	⁴₂He or ⁴₂α	4	2
beta-minus particle (electron)	⁰₋₁e or ⁰₋₁β	0	−1
positron	⁰₊₁e	0	+1
neutron	¹₀n	1	0
proton	¹₁p or ¹₁H	1	1
gamma ray	⁰₀γ	0	0

Rule: the total of the top numbers is the same on both sides, and so is the total of the bottom numbers.

Worked example 1: alpha decay

Radium-226 emits an alpha particle. What is formed?

²²⁶₈₈Ra → ⁴₂He + ᴬ_Z X

• Top: 226 = 4 + A → A = 222
• Bottom: 88 = 2 + Z → Z = 86 → radon

²²⁶₈₈Ra → ⁴₂He + ²²²₈₆Rn

Worked example 2: beta-minus decay

Carbon-14 undergoes beta decay.

¹⁴₆C → ⁰₋₁e + ᴬ_Z X

• Top: 14 = 0 + A → A = 14
• Bottom: 6 = −1 + Z → Z = 7 → nitrogen

¹⁴₆C → ⁰₋₁e + ¹⁴₇N

See carbon-14 dating and alpha, beta and gamma radiation.

Worked example 3: nuclear fission

Uranium-235 absorbs a neutron and splits into barium-141, krypton-92 and some neutrons. How many neutrons?

¹₀n + ²³⁵₉₂U → ¹⁴¹₅₆Ba + ⁹²₃₆Kr + x ¹₀n

• Top: 1 + 235 = 236 = 141 + 92 + x → x = 3
• Bottom: 0 + 92 = 92 = 56 + 36 + 0 ✓

See nuclear fission vs fusion.

Special notations you’ll meet

• Metastable states: a letter “m” after the mass number, as in technetium-99m (⁹⁹ᵐTc), means the nucleus is in a longer-lived excited state that releases a gamma ray. It’s widely used in medical imaging.
• Hydrogen isotopes have their own names: ¹H is protium, ²H is deuterium (symbol D), ³H is tritium (symbol T).
• Isotope ratios in geology and food science are written like δ¹³C or ⁸⁷Sr/⁸⁶Sr.

Why this notation is so useful

The nuclide symbol packs a lot of information into very little space, which is why it’s used in textbooks, research papers and nuclear safety documents worldwide. It also makes conservation visible. In any nuclear reaction, nucleons aren’t created or destroyed (so the top numbers balance) and electric charge is conserved (so the bottom numbers balance). Writing particles like the electron as ⁰₋₁e may look odd at first, but it’s simply a way of applying the same two rules to every particle involved.

The notation is also a handy reminder of the difference between chemistry and nuclear physics. In a chemical equation such as 2H₂ + O₂ → 2H₂O, the symbols never change their A or Z; only electrons and bonds are rearranged. In a nuclear equation, the symbols themselves change, because one element becomes another.

Worked example 4: identifying an unknown particle

Complete the equation: ¹⁴₇N + ⁴₂He → ¹⁷₈O + ?

• Top: 14 + 4 = 18 = 17 + A → A = 1
• Bottom: 7 + 2 = 9 = 8 + Z → Z = 1
• A = 1 and Z = 1 is a proton, ¹₁p (or ¹₁H).

This reaction, observed by Ernest Rutherford in 1919, was the first artificial transformation of one element into another.

Common mistakes

1. Swapping A and Z. The mass number is always the larger number (except in ¹₁H, where they’re equal).
2. Writing the charge on the left. Charge goes top right.
3. Using the relative atomic mass (e.g. 35.45 for chlorine) as a mass number. Mass numbers are always whole numbers for a particular isotope.
4. Forgetting that Z must match the element. ¹⁴₇C is impossible: anything with Z = 7 is nitrogen.
5. In nuclear equations, balancing only the top numbers and forgetting the bottom ones.

Practice questions

1. How many neutrons are in ²⁰⁷₈₂Pb?
2. Write the full symbol for strontium-90 (Z = 38).
3. Write the symbol for an ion with 17 protons, 20 neutrons and 18 electrons.
4. Complete: ²³⁸₉₂U → ⁴₂He + ?
5. Complete: ¹³¹₅₃I → ⁰₋₁e + ?

Answers: (1) 207 − 82 = 125; (2) ⁹⁰₃₈Sr; (3) ³⁷₁₇Cl⁻; (4) ²³⁴₉₀Th; (5) ¹³¹₅₄Xe.

Key takeaways

• In ᴬ_Z X, the top-left number is the mass number and the bottom-left number is the atomic number.
• Element–mass names (carbon-14, U-235) give only the mass number, since the element fixes Z.
• Ion charges go top right; subscripts at bottom right count atoms in a formula.
• In nuclear equations, top numbers and bottom numbers must each balance.
• Neutrons = A − Z; electrons in a neutral atom = Z.