AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Proton number Z, nucleon number A, nuclide notation.
Full specification wording
Constituents of the atom: Proton number Z, nucleon number A, nuclide notation.
The proton number Z is the number of protons in a nucleus and defines the element.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 4 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The proton number Z is the number of protons in a nucleus and defines the element. The nucleon number A is the total number of protons and neutrons, so the neutron number is A − Z. Nuclide notation places A as a superscript and Z as a subscript before the chemical symbol, for example ²³⁵₉₂U or ¹⁴₆C. For an atom, Z also equals the number of electrons, so the atom is electrically neutral. For ions, electrons are lost or gained but Z and A remain unchanged. For example, ²³⁵₉₂U has 92 protons, 143 neutrons and 92 electrons when neutral; ²³⁵₉₂U²⁺ has 90 electrons. When writing a nuclide, remember that the superscript A is greater than or equal to the subscript Z (they are equal only for hydrogen-1, ¹₁H).
What earns the marks
- Z is the number of protons in the nucleus and is the quantity that identifies the element.
- A is the total number of nucleons, so A = Z + N, where N is the number of neutrons.
- The number of neutrons is calculated as A − Z.
- In nuclide notation, A is written as a superscript and Z as a subscript before the symbol.
- For a neutral atom, the number of electrons equals Z; for an ion, the electron number is adjusted by the charge.
- The nuclide notation can be interpreted to state proton, neutron and electron numbers for a given species.
Common mistakes and how to put them right
Confusing proton number with nucleon number.
Put it right: Z counts protons only and A counts protons plus neutrons.
Reversing the positions in nuclide notation.
Put it right: A as superscript and Z as subscript.
Assuming ions have different proton numbers.
Put it right: Z and A are unchanged by ionisation, only the electron number changes.
Exam tips
- When asked to determine the number of neutrons, write down A − Z before substituting values to avoid arithmetic slips.
- Check that the superscript A is greater than or equal to the subscript Z; remember they are only equal for hydrogen-1.
- For ion questions, state the neutral electron number first, then add or subtract electrons according to the charge.
Try the lesson
The start of the Constituents of the atom: Proton number Z, nucleon number A, nuclide notation. lesson, made from checked teaching notes for this specification point.
Have a go first
A nuclide is written as ³⁷₁₇Cl. What is the proton number Z of this nuclide?
Listen
The proton number Z is the number of protons in a nucleus and defines the element. The nucleon number A is the total number of protons and neutrons, so the neutron number is A − Z. Nuclide notation places A as a superscript and Z as a subscript before the chemical symbol, for example ²³⁵₉₂U or ¹⁴₆C. For an atom, Z also equals the number of electrons, so the atom is electrically neutral. For ions, electrons are lost or gained but Z and A remain unchanged. For example, ²³⁵₉₂U has 92 protons, 143 neutrons and 92 electrons when neutral; ²³⁵₉₂U²⁺ has 90 electrons. When writing a nuclide, remember that the superscript A is greater than or equal to the subscript Z (they are equal only for hydrogen-1, ¹₁H). A strong answer earns marks for these points. Z is the number of protons in the nucleus and is the quantity that identifies the element. A is the total number of nucleons, so A = Z + N, where N is the number of neutrons. The number of neutrons is calculated as A − Z. In nuclide notation, A is written as a superscript and Z as a subscript before the symbol. A common slip: Confusing proton number with nucleon number: the error is treating Z as the total number of nucleons. Instead, Z counts protons only and A counts protons plus neutrons. Exam tip: When asked to determine the number of neutrons, write down A − Z before substituting values to avoid arithmetic slips.
See it
Key words
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Proton number
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Nucleon number
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Nuclide notation
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Neutron calculation
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See every definition
- Proton number
- The number of protons in a nucleus, which defines the element.
- Nucleon number
- The total number of protons and neutrons.
- Nuclide notation
- A written as a superscript and Z as a subscript before the chemical symbol.
- Neutron calculation
- The neutron number is A − Z.
Label the diagram
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1Box 1
2Box 2
3Box 3
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