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

Reading the mass number as the atomic number in a nuclide symbol. Defining the atomic number as the total number of protons and neutrons. Using the atomic number to count the electrons in an ion.

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

Remember the smaller number is the atomic number; the larger cannot be smaller than it. Write Z when you need to keep track in a nuclear equation, so the columns stay clear. Do not add a unit to the atomic number; it is a count, not a measurement.

AQA · GCSE · Physics

Course version: 8463 · Version from Sept 2016

The number of protons in an atom of an element is called its atomic number.

Full specification wording

Mass number, atomic number and isotopes: The number of protons in an atom of an element is called its atomic number.

The atomic number, given the symbol Z, is simply a count of the protons in the nucleus.

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

Point 3 of 7
2 key terms

Study notes

From the checked teaching notes for this specification point.

The idea

What earns the marks

The atomic number being the number of protons in the nucleus.
Reading the atomic number as the lower number in the nuclide symbol.
Stating that in a neutral atom the atomic number also gives the number of electrons.
Using the atomic number to name the element from a periodic table.

Common mistakes and how to put them right

Reading the mass number as the atomic number in a nuclide symbol.
Defining the atomic number as the total number of protons and neutrons.
Using the atomic number to count the electrons in an ion.
Assuming the atomic number cannot change during radioactive decay.

Exam tips

Remember the smaller number is the atomic number; the larger cannot be smaller than it.
Write Z when you need to keep track in a nuclear equation, so the columns stay clear.
Do not add a unit to the atomic number; it is a count, not a measurement.

Try the lesson

The start of the Mass number, atomic number and isotopes: The number of protons in an atom of an element is called its atomic number. lesson, made from checked teaching notes for this specification point.

Have a go first

What name is given to the number of protons in an atom of an element?

Tip: press 1 to 4 to choose

Listen

The atomic number, given the symbol Z, is simply a count of the protons in the nucleus. It is written as the lower of the two numbers in the nuclide symbol, so in ¹²₆C the 6 is the atomic number and tells you the nucleus holds six protons. Because a neutral atom has as many electrons as protons, the atomic number also gives the electron count for a neutral atom, although not for an ion. The periodic table is ordered by atomic number, so knowing Z is enough to name the element. Being a count, the atomic number has no unit, and it changes only when the nucleus itself gains or loses protons. A strong answer earns marks for these points. the atomic number being the number of protons in the nucleus. reading the atomic number as the lower number in the nuclide symbol. stating that in a neutral atom the atomic number also gives the number of electrons. using the atomic number to name the element from a periodic table. Exam tip: Remember the smaller number is the atomic number; the larger cannot be smaller than it.

See it

A flowchart with the central node 'Atomic number Z' branching to six points: it is a count of protons in the nucleus; it is the lower of the two numbers in the nuclide symbol; it gives the electron count in a neutral atom; it is used to name the element from the periodic table; it is a count, so it has no unit; and it changes only when the nucleus gains or loses protons. — It shows what the atomic number is and the several things it tells you.

Key words

Say what each word means before you turn the card.

atomic number

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

neutral atoms

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

See every definition

atomic number: The number of protons in an atom of an element, given the symbol Z, and written as the lower of the two numbers in the nuclide symbol.
neutral atoms: Atoms that have as many electrons as protons, so the atomic number also gives the electron count, although not for an ion.

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 flowchart with the central node 'Atomic number Z' branching to six points: it is a count of protons in the nucleus; it is the lower of the two numbers in the nuclide symbol; it gives the electron count in a neutral atom; it is used to name the element from the periodic table; it is a count, so it has no unit; and it changes only when the nucleus gains or loses protons.

1Box 1
2Box 2
3Box 3

0 of 3 labelled

There is more to this lesson

The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.

Carry on with the full lesson

A week of Premium from your first lesson · Then the free plan continues · No card, no charge

Test your understanding of Mass number, atomic number and isotopes: The number of protons in an atom of an element is called its atomic number.

Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.

Take topic quiz