What mistakes do students make with Mass number, atomic number and isotopes?

Explaining a positive charge by the transfer of protons or of positive charge, which scores zero. Writing about positive electrons, which also scores zero. Saying an atom that gains an electron becomes positively charged.

What are the exam tips for Mass number, atomic number and isotopes in Physics AQA GCSE?

Start the answer with the electrons that moved, then state the resulting imbalance, then the charge. Say which object gained the electrons as well as which lost them; both are usually needed. Use the word outer, because it explains why those electrons in particular are the ones removed.

AQA · GCSE · Physics

Course version: 8463 · Version from Sept 2016

Atoms turn into positive ions if they lose one or more outer electron(s).

Full specification wording

Mass number, atomic number and isotopes: Atoms turn into positive ions if they lose one or more outer electron(s).

Outer electrons are held least tightly, so they are the ones that can be pulled away, by rubbing two surfaces together or by ionising radiation.

Checked against the AQA 8463 specification (opens in a new tab) on 22 September 2026.

Point 6 of 7
2 key terms

Study notes

From the checked teaching notes for this specification point.

The idea

What earns the marks

An electron being removed from the outer energy level of the atom.
Stating that there are now more protons than electrons in the particle.
Stating that the ion has an overall positive charge, of +1 for each electron lost.
Stating that only electrons are transferred, never protons.

Common mistakes and how to put them right

Explaining a positive charge by the transfer of protons or of positive charge, which scores zero.
Writing about positive electrons, which also scores zero.
Saying an atom that gains an electron becomes positively charged.
Writing more protons than electrons without saying that electrons were lost, which is ignored unqualified.

Exam tips

Start the answer with the electrons that moved, then state the resulting imbalance, then the charge.
Say which object gained the electrons as well as which lost them; both are usually needed.
Use the word outer, because it explains why those electrons in particular are the ones removed.

Try the lesson

The start of the Mass number, atomic number and isotopes: Atoms turn into positive ions if they lose one or more outer electron(s). lesson, made from checked teaching notes for this specification point.

Have a go first

A neutral sodium atom loses one outer electron. What type of particle is formed?

Tip: press 1 to 4 to choose

Listen

Outer electrons are held least tightly, so they are the ones that can be pulled away, by rubbing two surfaces together or by ionising radiation. When an atom loses one, the protons in the nucleus are unchanged but there is now one fewer electron, so positive charge outweighs negative and the particle is a positive ion with a charge of +1. Lose two and the charge is +2. Protons never move between objects, which is why a mark scheme awards nothing for any answer that mentions transferring positive charge, and nothing for the phrase positive electrons. When a cloth is rubbed on a plastic rod, the cloth becomes positive because it has lost electrons to the rod. A strong answer earns marks for these points. an electron being removed from the outer energy level of the atom. stating that there are now more protons than electrons in the particle. stating that the ion has an overall positive charge, of +1 for each electron lost. stating that only electrons are transferred, never protons. Exam tip: Start the answer with the electrons that moved, then state the resulting imbalance, then the charge.

See it

A top-down flowchart. The first box says outer electrons are held least tightly and points to a box saying electrons are pulled away by rubbing or ionising radiation. That box points to a box saying the atom loses one or more outer electrons. This splits into two boxes: protons in the nucleus unchanged, and one fewer electron. Both of these point to a box saying positive charge outweighs negative, which points to a box saying positive ion, +1 for each electron lost. That points to a final box saying lose two electrons gives charge +2. The rubbing-or-radiation box also points sideways to a box saying only electrons are transferred, never protons. — It shows how losing outer electrons leaves the protons unchanged and produces a positive ion whose charge rises by +1 for each electron lost.

Key words

Say what each word means before you turn the card.

positive ions

Try answering aloud or in your head before turning the card.

outer electrons

Try answering aloud or in your head before turning the card.

See every definition

positive ions: Particles formed when atoms lose one or more outer electrons, so that positive charge outweighs negative.
outer electrons: The electrons held least tightly, so they are the ones that can be pulled away by rubbing two surfaces together or by ionising radiation.

Label the diagram

Some boxes have lost their names. Pick the right name for each number.

The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. The first box says outer electrons are held least tightly and points to a box saying electrons are pulled away by rubbing or ionising radiation. That box points to a box saying the atom loses one or more outer electrons. This splits into two boxes: protons in the nucleus unchanged, and one fewer electron. Both of these point to a box saying positive charge outweighs negative, which points to a box saying positive ion, +1 for each electron lost. That points to a final box saying lose two electrons gives charge +2. The rubbing-or-radiation box also points sideways to a box saying only electrons are transferred, never protons.

1Box 1
2Box 2
3Box 3

0 of 3 labelled

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