01 · Sub-atomic particles
An atom has two regions: a dense central nucleus and a surrounding cloud of electrons. The nucleus holds protons and neutrons packed tightly together. Electrons occupy shells at increasing distances from the nucleus.
The atomic number (Z) equals the number of protons and uniquely identifies the element. The mass number (A) equals protons + neutrons. Electrons in a neutral atom equal protons.
02 · Electron shells
Electrons do not orbit the nucleus like planets. They exist in shells (energy levels) and occupy specific regions called orbitals. For introductory purposes, picture concentric rings.
Each shell has a maximum capacity given by 2n², where n is the shell number. Shell 1 holds 2, shell 2 holds 8, shell 3 holds up to 18 — though period 3 elements fill only 8 before moving to shell 4.
n = 1
max 2
e.g. He: 2
n = 2
max 8
e.g. Ne: 2,8
n = 3
max 18
e.g. Ar: 2,8,8
n = 4
max 32
e.g. Kr: 2,8,18,8
The electrons in the outermost shell are called valence electrons. They determine how an atom bonds with others. Noble gases have full outer shells and rarely react. See our acids and bases guide to see how electron behaviour drives acid–base reactions.
03 · Isotopes
Isotopes are atoms of the same element — same proton count — but with different numbers of neutrons. They have the same chemical properties but different masses.
Carbon-12
6p · 6n · 6e
Stable. ~98.9 % of all carbon.
Carbon-14
6p · 8n · 6e
Radioactive. Used in radiocarbon dating.
Some isotopes are stable; others are radioactive and decay over time, releasing radiation. Read our isotope glossary entry for a concise definition you can link to from your notes.
04 · Ions
When an atom gains or loses electrons it becomes an ion. Because the proton count stays the same but the electron count changes, the atom acquires a net electric charge.
- Cation (positive) — atom loses electrons. Example: Na → Na⁺ (sodium loses 1 electron).
- Anion (negative) — atom gains electrons. Example: Cl → Cl⁻ (chlorine gains 1 electron).
Ions are central to how salts dissolve, how nerve signals travel, and how acids and bases behave. See our acids and bases guide for the role of H⁺ and OH⁻ ions.