The big idea: Every atom is built from three subatomic particles: protons, neutrons and electrons.
- The protons and neutrons sit together in a tiny, dense central nucleus. - The electrons move around the nucleus in shells (energy levels).
Almost all the mass is packed into the nucleus, yet almost all the volume of the atom is the empty space the electrons occupy.
A lithium atom: 3 protons and 4 neutrons in the tiny central nucleus, with 3 electrons (2 + 1) in shells around it. Almost all the mass is in the nucleus; almost all the volume is empty space.
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Mass lives in the nucleus: A proton and a neutron each have a relative mass of about 1. An electron is roughly 1836 times lighter, so its mass is treated as negligible.
That is why the mass of an atom is essentially the mass of its nucleus (protons + neutrons).
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Because the real masses and charges are tiny, chemists compare them using relative values. These three rows are worth memorising — they are tested directly.
| Particle | Where | Relative mass | Relative charge |
|---|---|---|---|
| Proton, p⁺ | nucleus | 1 | +1 |
| Neutron, n⁰ | nucleus | 1 | 0 |
| Electron, e⁻ | shells (around nucleus) | ≈ 1/1836 (negligible) | −1 |
Why a neutral atom has equal p and e: Each proton carries a charge of +1 and each electron −1. In a neutral atom the positives and negatives cancel exactly, so:
number of protons = number of electrons.
Neutrons have no charge, so they change the mass but never the charge.
Deflection in an electric field: Fired between charged plates, the particles separate by their charge-to-mass behaviour:
- the electron (−1, very light) deflects strongly toward the positive plate, - the proton (+1) deflects toward the negative plate (less, as it is far heavier), - the neutron (0) travels straight through, undeflected.
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Two whole numbers describe any atom. The atomic number Z is what makes an element that element; the mass number A counts the heavy particles in the nucleus.
Atomic number, Z
- = number of protons in the nucleus.
- Defines the element (and its place in the periodic table).
- In a neutral atom it also equals the number of electrons.
Mass number, A
- = number of protons + neutrons (the nucleons).
- So neutrons = A − Z.
- Different from Z because neutrons have mass but Z does not count them.
The nuclear (isotope) symbol: An atom is written with the mass number on top and the atomic number on the bottom, before the element symbol:
For example means lithium with Z = 3 protons and A = 7 nucleons, so 7 − 3 = 4 neutrons, and (being neutral) 3 electrons.
Counting for an ION: An ion has gained or lost electrons, so its charge changes the electron count only:
- a positive ion (cation) has lost electrons → electrons = Z − charge. - a negative ion (anion) has gained electrons → electrons = Z + charge.
Protons and neutrons are unchanged — only electrons move when an ion forms.
How this is tested: S1.2 nearly always asks you to deduce particle counts from a nuclear symbol.
- Paper 1A (MCQ): pick the row with the correct protons, neutrons and electrons — often for an ion. - Paper 2: state a nuclear charge, or deduce the full nuclear symbol from given proton/neutron numbers.
The routine never changes: protons = Z, neutrons = A − Z, electrons = Z − (charge).
Don't lose the easy mark: Only the electron count changes for an ion. A common slip is changing the proton or neutron count to balance the charge — never do that. Read the charge, then adjust electrons only.
IB-style question — deduce the particle counts in an ion
An ion is represented by the nuclear symbol . Deduce the number of protons, neutrons and electrons in this ion. [3]
How to score the marks
- Mark 1 — protons. The atomic number is the number of protons, Z = 13, so there are 13 protons.
- Mark 2 — neutrons. Neutrons = mass number − atomic number = A − Z = 27 − 13, so there are 14 neutrons.
- Mark 3 — electrons. The 3+ charge means the ion has lost 3 electrons, so electrons = Z − 3 = 13 − 3 = 10 electrons.
Final answer
13 protons, 14 neutrons and 10 electrons.