Nuclide Notation

Summary: The standard symbolism for atoms and ions, written as ᴬ𝓏X, showing the mass number (top), atomic number (bottom), and any charge, from which proton, neutron, and electron counts can be deduced. Tags: igcse chemistry Created: 2026-07-18

Nuclide notation is the internationally agreed way of writing the symbol for a specific atom or Ion so that its complete subatomic composition can be read at a glance. The chemical symbol of the Element is written with two numbers placed in front of it: the mass number (nucleon number) as a superscript at the top, and the atomic number (proton number) as a subscript at the bottom — for example, carbon-12 is written as ¹²₆C and the chloride ion as ³⁵₁₇Cl⁻. Because the Atomic Number (Proton Number) fixes the identity of the element and the Mass Number (Nucleon Number) gives the total count of particles in the nucleus, the number of neutrons is simply the difference between the two. For ions, a charge written at the top right shows how many electrons have been lost or gained relative to the neutral atom, linking directly to Ions and Ionic Bonds. The notation is especially important for distinguishing between isotopes of the same element, such as ³⁵₁₇Cl and ³⁷₁₇Cl, which share an atomic number but differ in mass number. A secure grasp of this notation underpins all of Atomic Structure and later mole calculations in Relative Masses and Moles.

Anatomy of the Symbol

A nuclide symbol has up to three pieces of information attached to the element symbol X:

${}_Z^A{\text{X}}^c$

The mass number is always the larger of the two left-hand numbers (or equal, in the case of ¹₁H).

Working Out Proton, Neutron and Electron Numbers

For any nuclide symbol:

• Protons = atomic number Z
• Neutrons = mass number − atomic number = A − Z
• Electrons (neutral atom) = atomic number Z, because the positive and negative charges balance
• Electrons (ion) = Z − charge
  • A positive ion (cation) has lost electrons, so subtract the charge: Na⁺ has 11 − 1 = 10 electrons
  • A negative ion (anion) has gained electrons, so add them: Cl⁻ has 17 + 1 = 18 electrons

Note that forming an ion changes only the electron count — the nucleus, and therefore the mass number and atomic number, are unchanged. The electrons gained or lost come from the outermost Electron Shell (see Valence Electron).

Table of Example Nuclides

Notice that Na⁺, Al³⁺, and the anions Cl⁻ and S²⁻ all reach the electron count of a nearby noble gas — the driving idea behind Ions and Ionic Bonds and the stability of the Group 0 Noble Gases.

Isotopes in Nuclide Notation

Nuclide notation makes isotopes easy to compare: they have the same atomic number but different mass numbers. For example, ³⁵₁₇Cl and ³⁷₁₇Cl both contain 17 protons and 17 electrons, but 18 and 20 neutrons respectively. The shorthand names “chlorine-35” and “chlorine-37” quote only the mass number, since the atomic number is fixed by the element’s name. The relative abundances of these isotopes are what give chlorine its non-whole-number Relative Atomic Mass of 35.5.

Sources

• Cambridge IGCSE Chemistry 0620 Syllabus (2023-2025) — Section 2.3 Core 2, Cambridge Assessment International Education

Common Misconceptions