AQA GCSE Chemistry · Atomic structure

Atoms, ions and the charge they carry

An atom has equal numbers of protons and electrons, so it has no overall charge. Take electrons away and it becomes a positive ion. Add electrons and it becomes a negative ion. Pick an element and try it.

Chemistry 8462: 4.1.1.4–4.1.1.7, 4.1.2.1, 4.2.1.2 Combined Trilogy 8464: 5.1.1, 5.2.1.2

Choose an element

The first 20 elements, arranged by atomic number
Proton (+1) Neutron (0) Electron (−1) Dashed ring: empty shell
Atom sodium atom
Protons
11
Neutrons
12
Electrons
11
Electronic structure: 2,8,1

Neutrons:
Using the periodic table

Working out protons, neutrons and electrons

This panel follows the element you picked above.

23 Na sodium 11
Top numberRelative atomic mass. Round it to get the mass number of the most common isotope.
Bottom numberAtomic number. This is the number of protons.

The three rules

protons = atomic number neutrons = mass number − atomic number electrons = protons (in an atom) electrons = protons − charge (in an ion)
ParticleRelative chargeRelative massWhere
Proton+11Nucleus
Neutron01Nucleus
Electron−1very smallShells around the nucleus

Isotopes are atoms of the same element with different numbers of neutrons. Chlorine’s relative atomic mass is 35.5 because it is a mixture of chlorine-35 and chlorine-37.

Group number and ion charge

Predicting the charge on an ion

Metals lose electrons to form positive ions. Non-metals gain electrons to form negative ions. Either way, the ion ends up with a full outer shell, like a noble gas.

Group 11+lose 1
Li⁺ Na⁺ K⁺
Group 22+lose 2
Mg²⁺ Ca²⁺
Group 33+lose 3
Al³⁺
Group 4–usually share electrons
Group 53−gain 3
N³⁻
Group 62−gain 2
O²⁻ S²⁻
Group 71−gain 1
F⁻ Cl⁻
Group 0nonefull outer shell already
Unlimited practice

Count the particles

Correct: 0 · Streak: 0

Check understanding

Quick quiz

Exam practice

Exam-style questions

Sodium reacts with chlorine to form sodium chloride. Sodium chloride contains sodium ions, Na⁺.

Describe how a sodium atom becomes a sodium ion, and explain why the ion has a charge of 1+.

[4 marks]
Show mark scheme
  1. The sodium atom loses one electron (1)
  2. from its outer shell, giving the structure 2,8 (same as neon) (1)
  3. The ion still has 11 protons but only 10 electrons (1)
  4. so there is one more positive charge than negative charge (1)

A potassium ion is shown as ³⁹₁₉K⁺.

Give the number of protons, neutrons and electrons in this ion.

[3 marks]
Show mark scheme
  1. Protons: 19 (1)
  2. Neutrons: 39 − 19 = 20 (1)
  3. Electrons: 19 − 1 = 18 (1)

A common wrong answer is 20 electrons, from adding one for the + charge. A positive ion has fewer electrons than protons.