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K326 / K328 / 2027
Particles and Atoms

Lesson 3 of 3

Atomic numbers, isotopes and ions

Keep the identity of the element separate from its mass and charge.

O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)

  • I can define proton and nucleon numbers.Syllabus

    Syllabus K326 / K328, 2.2(c). Define proton and nucleon numbers

  • I can read and write nuclide notation.Syllabus

    Syllabus K326 / K328, 2.2(d). Read and write nuclide notation

  • I can recognize isotopes.Syllabus

    Syllabus K326 / K328, 2.2(e). Recognize isotopes

  • I can count particles in atoms and ions.Syllabus

    Syllabus K326 / K328, 2.2(f). Count particles in atoms and ions

Atomic numbers, isotopes and ions

Keep the identity of the element separate from its mass and charge.

Make a guess. You are not marked.

Chlorine-35 and chlorine-37 each react with sodium. How do their reactions compare?

  1. They make different compounds with sodium
  2. Chlorine-37 reacts faster because it is heavier
  3. They react in the same way
Show the answer

They react in the same way

Isotopes have the same number of protons and electrons, and chemical reactions depend on electrons. Only the neutron number differs.

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.

Count the particles
  1. Protons = Z

    The element identity fixes the proton count.

  2. Neutrons = A - Z

    Only protons and neutrons contribute to nucleon number.

  3. 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 exampleCount particles in ions

Find protons, neutrons and electrons in 2412Mg2+ and 3517Cl-.

Try it first. Then show the worked stepsHide the worked steps
  1. Magnesium: protons = 12; neutrons = 24 - 12 = 12. A 2+ charge means two electrons lost: 12 - 2 = 10.
  2. Chlorine: protons = 17; neutrons = 35 - 17 = 18. A 1- charge means one electron gained: 17 + 1 = 18.
Answer

Mg2+: 12 p, 12 n, 10 e. Cl-: 17 p, 18 n, 18 e.

Check your understanding

Two particles both have 8 protons, with 8 and 10 neutrons. One of them has 10 electrons. What can you conclude?

  1. Both are oxygen isotopes, and the particle with 10 electrons is an O2- ion.
  2. They are isotopes, so the one with 10 neutrons must also have 10 electrons.
  3. They are two different elements, because their nucleon numbers are different.
Show the answer

Both are oxygen isotopes, and the particle with 10 electrons is an O2- ion.

The same proton number (8) means the same element, oxygen. Different neutron numbers make them isotopes. Ten electrons against 8 protons gives a 2- charge, which is about electrons, not the nucleus.