OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
(d) proton number; nucleon number; isotopes; notation X Z A for the…
Full specification wording
The nuclear atom: (d) proton number; nucleon number; isotopes; notation X Z A for the representation of nuclei
The proton number Z is the number of protons in a nucleus and defines the element.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 4 of 7
- 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. Isotopes are nuclei of the same element with the same Z but different A, hence different neutron numbers; they have identical chemical properties but different masses and nuclear stability. Nuclear notation places A as a superscript and Z as a subscript before the symbol X, for example ¹²₆C has Z = 6 and A = 12, so it contains 6 protons and 6 neutrons. For ²³⁵₉₂U, Z = 92 and A = 235, giving 143 neutrons.
What earns the marks
- Proton number Z is the number of protons in a nucleus and determines the element.
- Nucleon number A is the total number of protons and neutrons in a nucleus.
- The number of neutrons is calculated as A − Z.
- Isotopes are nuclei with the same proton number Z but different nucleon number A, so they have different neutron numbers.
- In the notation X with A as superscript and Z as subscript, the superscript is the nucleon number and the subscript is the proton number.
- For example, ¹²₆C has 6 protons and 6 neutrons; ²³⁵₉₂U has 92 protons and 143 neutrons.
Common mistakes and how to put them right
Confusing proton number with nucleon number.
Put it right: The subscript is Z (protons) and the superscript is A (nucleons), not the other way round.
Thinking isotopes have different proton numbers.
Put it right: Isotopes of an element all have the same Z but different A.
Calculating neutron number as A + Z instead of A − Z, which gives an incorrect neutron count.
Believing that changing the number of neutrons changes the element.
Put it right: The element is fixed by Z, so adding neutrons only creates an isotope.
Exam tips
- Read the superscript and subscript carefully; write down Z and A before calculating neutrons as A − Z.
- Check whether the question asks for protons, neutrons or nucleons; each has a different value.
- For isotope questions, confirm that Z is unchanged and only A differs.
Try the lesson
The start of the The nuclear atom: (d) proton number; nucleon number; isotopes; notation X Z A for the representation of nuclei lesson, made from checked teaching notes for this specification point.
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. Isotopes are nuclei of the same element with the same Z but different A, hence different neutron numbers; they have identical chemical properties but different masses and nuclear stability. Nuclear notation places A as a superscript and Z as a subscript before the symbol X, for example ¹²₆C has Z = 6 and A = 12, so it contains 6 protons and 6 neutrons. For ²³⁵₉₂U, Z = 92 and A = 235, giving 143 neutrons. A strong answer earns marks for these points. Proton number Z is the number of protons in a nucleus and determines the element. Nucleon number A is the total number of protons and neutrons in a nucleus. The number of neutrons is calculated as A − Z. Isotopes are nuclei with the same proton number Z but different nucleon number A, so they have different neutron numbers. Exam tip: Read the superscript and subscript carefully; write down Z and A before calculating neutrons as A − Z.
See it
Key words
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Proton number
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Nucleon number
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Isotopes
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Nuclear notation
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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 in a nucleus.
- Isotopes
- Nuclei of the same element with the same proton number but different nucleon number, hence different neutron numbers.
- Nuclear notation
- A representation of nuclei that places the nucleon number as a superscript and the proton number as a subscript before the symbol X.
Label the diagram
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