IB Chemistry SLTopic 1 — The Nuclear AtomPaper 1 & 2Core skill~9 min read
Protons, Neutrons & Electrons
Two numbers on the periodic table — the atomic number and the mass number — tell you everything you need to count the particles in any atom or ion. Learn how to read them and a whole family of exam questions becomes quick, reliable marks.
📘 What you need to know
The atomic number (Z) is the number of protons in the nucleus — it defines which element you have.
The mass number (A) is the total number of protons + neutrons (the nucleons).
Number of neutrons = A − Z.
In a neutral atom, protons = electrons.
In an ion, the electron count changes: a positive ion has lost electrons, a negative ion has gained them. Protons never change.
Atomic number and mass number
Every element has two defining numbers, shown around its chemical symbol.
The atomic number (Z) — sometimes called the proton number — is the number of protons in the nucleus. It’s what makes an element that element: lithium always has 3 protons, carbon always has 6.
The mass number (A) is the total number of protons and neutrons, since these are the particles with mass.
In a neutral atom the atomic number also equals the number of electrons — because the positive protons and negative electrons must balance.
Element notation: the mass number (A) sits top-left, the atomic number (Z) bottom-left, beside the chemical symbol.
Finding the neutrons
number of neutrons = mass number (A) − atomic number (Z)
Counting the three particles
Here’s the full toolkit. For any atom or ion:
Protons = atomic number (Z) — always, and never changes for a given element.
Neutrons = mass number − atomic number (A − Z).
Electrons = protons in a neutral atom; adjust for the charge in an ion.
The one that trips people up is electrons in ions. Remember: forming an ion changes electrons, never protons. A 2+ charge means two fewer electrons than protons; a 2− charge means two more.
⚠️ Common mix-up
A positive ion (cation) has lost electrons → fewer electrons than protons.
A negative ion (anion) has gained electrons → more electrons than protons.
The number of protons stays the same — that’s why it’s still the same element.
WORKED EXAMPLE
Find the protons, neutrons and electrons in: (a) a Mg2+ ion (Z = 12, A = 24), (b) a neutral carbon atom (Z = 6, A = 12), (c) an atom of element X with mass number 63 and 34 neutrons.
(a) Mg²⁺Protons = 12. Neutrons = 24 − 12 = 12. Electrons = 12 − 2 (lost) = 10.(b) Carbon atomProtons = 6. Neutrons = 12 − 6 = 6. Electrons = 6 (neutral).(c) Element Xprotons = 63 − 34 = 2929 protons means element X is copper; neutral, so 29 electrons too.Mg²⁺: 12p 12n 10e · C: 6p 6n 6e · X = Cu
💡 Exam tip
Always start from protons = Z, then work out neutrons with A − Z, then adjust electrons for any charge.
If you’re given the neutrons and mass number instead, rearrange: protons = A − neutrons — a quick way to identify an unknown element.
Up next: Isotopes — atoms of the same element with the same protons but different numbers of neutrons, and how we use them to work out relative atomic mass.
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