OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Use the conventional representation for nuclei to relate the differences…
Full specification wording
Radioactive emissions: use the conventional representation for nuclei to relate the differences between isotopes (to include: identities, charges and masses)
Isotopes are atoms of the same element whose nuclei hold the same number of protons but different numbers of neutrons.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 3 of 12
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Isotopes are atoms of the same element whose nuclei hold the same number of protons but different numbers of neutrons. The conventional nuclear symbol places the mass number A as a superscript before the element symbol and the atomic number Z as a subscript before it, so a nucleus is written with A above Z to the left of the symbol. For carbon-12 the symbol shows A = 12 and Z = 6; carbon-14 shows A = 14 and Z = 6. Because Z fixes the element, both are carbon, so identity is unchanged. The nucleus charge is +Z times the elementary charge, so both carbon isotopes carry a nuclear charge of +6e; the extra neutrons in carbon-14 add mass but no charge. Mass number counts protons plus neutrons, so the mass difference of two units equals two extra neutrons. Use A − Z to find the neutron number and compare isotopes by reading the superscript and subscript directly.
What earns the marks
- Reads the superscript as the mass number A and the subscript as the atomic number Z in the conventional nuclear symbol.
- Identifies isotopes as nuclei with equal Z but different A, so the element identity stays the same.
- Calculates the neutron number as A − Z and uses it to explain the mass difference between two isotopes.
- States that nuclear charge equals +Z elementary charges, so isotopes of one element share the same charge.
- Distinguishes mass number from relative atomic mass when comparing two isotopes of an element.
Common mistakes and how to put them right
Reverses the superscript and subscript positions.
Put it right: Place the mass number above and the atomic number below, both before the symbol.
Treats a change in neutron number as a change of element.
Put it right: Keep the identity fixed by the proton number Z.
Assumes the heavier isotope carries a larger nuclear charge.
Put it right: Recognise that charge depends only on Z, so it is unchanged.
Counts neutrons as A + Z.
Put it right: Subtract instead, using A − Z.
Exam tips
- Write the symbol with A above Z before answering, then read off protons and neutrons directly.
- Check charge by counting protons only, since neutrons carry no charge.
- Compare two isotopes by stating which number is the same and which differs before drawing a conclusion.
Try the lesson
The start of the Radioactive emissions: use the conventional representation for nuclei to relate the differences between isotopes (to include: identities, charges and masses) lesson, made from checked teaching notes for this specification point.
Have a go first
A nucleus is written as ¹⁴₆C. What does the lower number, 6, represent?
Listen
Isotopes are atoms of the same element whose nuclei hold the same number of protons but different numbers of neutrons. The conventional nuclear symbol places the mass number A as a superscript before the element symbol and the atomic number Z as a subscript before it, so a nucleus is written with A above Z to the left of the symbol. For carbon-12 the symbol shows A = 12 and Z = 6; carbon-14 shows A = 14 and Z = 6. Because Z fixes the element, both are carbon, so identity is unchanged. The nucleus charge is +Z times the elementary charge, so both carbon isotopes carry a nuclear charge of +6e; the extra neutrons in carbon-14 add mass but no charge. Mass number counts protons plus neutrons, so the mass difference of two units equals two extra neutrons. Use A − Z to find the neutron number and compare isotopes by reading the superscript and subscript directly. A strong answer earns marks for these points. Reads the superscript as the mass number A and the subscript as the atomic number Z in the conventional nuclear symbol. Identifies isotopes as nuclei with equal Z but different A, so the element identity stays the same. Calculates the neutron number as A − Z and uses it to explain the mass difference between two isotopes. States that nuclear charge equals +Z elementary charges, so isotopes of one element share the same charge. Exam tip: Write the symbol with A above Z before answering, then read off protons and neutrons directly.
Key words
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Conventional nuclear notation
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Atomic number and mass number
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Neutron number from A − Z
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Nuclear charge and isotope identity
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See every definition
- Conventional nuclear notation
- A way of writing a nucleus with the mass number A as a superscript and the atomic number Z as a subscript, both before the element symbol.
- Atomic number and mass number
- The subscript Z is the atomic number and the superscript A is the mass number, which counts protons plus neutrons.
- Neutron number from A − Z
- The neutron number is found by subtracting the atomic number from the mass number.
- Nuclear charge and isotope identity
- Nuclear charge equals +Z elementary charges, so isotopes of one element share the same charge, and Z fixes the element so identity is unchanged.
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