Topic 4 of 4
Atomic numbers, isotopes and ions
Keep the identity of the element separate from its mass and charge.
O-Level 6092 (2026) / SEC G3 K324 (2027)
Atomic numbers, isotopes and ions
Keep the identity of the element separate from its mass and charge.
Proton number Z is the number of protons; it identifies the element. Nucleon number A is protons plus neutrons. In nuclide notation 126C, carbon has Z = 6 and A = 12, hence 6 protons and 6 neutrons. A neutral carbon atom also has 6 electrons.
- Protons = Z
The element identity fixes the proton count.
- Neutrons = A - Z
Only protons and neutrons contribute to nucleon number.
- Electrons: start with Z
Subtract electrons for a positive ion; add electrons for a negative ion.
Isotopes are atoms of the same element with the same proton number but different neutron numbers. Carbon-12 and carbon-14 both have six protons; they have six and eight neutrons respectively. Their neutral atoms have the same electron arrangement, so their chemical behaviour is similar. An ion forms when electrons are lost or gained; its nucleus does not change.
Worked example
Count particles in ions
Find protons, neutrons and electrons in 2412Mg2+ and 3517Cl-.
- Magnesium: protons = 12; neutrons = 24 - 12 = 12. A 2+ charge means two electrons lost: 12 - 2 = 10.
- Chlorine: protons = 17; neutrons = 35 - 17 = 18. A 1- charge means one electron gained: 17 + 1 = 18.
Mg2+: 12 p, 12 n, 10 e. Cl-: 17 p, 18 n, 18 e.