AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
All atoms of a particular element have the same number of protons. The…
Full specification wording
Mass number, atomic number and isotopes: All atoms of a particular element have the same number of protons. The number of protons in an atom of an element is called its atomic number.
The atomic number defines the element.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 2 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The atomic number defines the element. Every atom of a given element has the same proton count, and that count is the atomic number. For example, all carbon atoms have six protons, so carbon's atomic number is 6; all oxygen atoms have eight protons, so oxygen's atomic number is 8. Changing the proton number changes the element, so an atom with seven protons is nitrogen, not carbon. Atoms of the same element can differ in neutron number, forming isotopes, but the proton number stays fixed. In symbol notation the atomic number is written as the lower number, for example ₆C, and it also equals the number of electrons in a neutral atom.
What earns the marks
- Define atomic number as the number of protons in an atom of an element.
- State that all atoms of the same element have identical proton numbers.
- Use examples such as carbon with atomic number 6 and oxygen with atomic number 8.
- Explain that changing the proton number changes the element.
- Link atomic number to the number of electrons in a neutral atom.
Common mistakes and how to put them right
Confusing atomic number with mass number.
Put it right by stating atomic number counts protons only, while mass number counts protons plus neutrons.
Thinking isotopes of an element have different atomic numbers.
Put it right by noting isotopes differ in neutrons, not protons.
Assuming atomic number equals neutron number.
Put it right by using an example such as carbon-14, which has six protons and eight neutrons.
Exam tips
- Write the definition of atomic number precisely, using the word protons.
- Use a named element and its proton number to illustrate your answer.
- If a symbol is given, read the lower number as the atomic number.
Try the lesson
The start of the Mass number, atomic number and isotopes: All atoms of a particular element have the same number of protons. 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
A neutral atom of fluorine contains 9 protons. State the atomic number of this fluorine atom.
Listen
The atomic number defines the element. Every atom of a given element has the same proton count, and that count is the atomic number. For example, all carbon atoms have six protons, so carbon's atomic number is 6; all oxygen atoms have eight protons, so oxygen's atomic number is 8. Changing the proton number changes the element, so an atom with seven protons is nitrogen, not carbon. Atoms of the same element can differ in neutron number, forming isotopes, but the proton number stays fixed. In symbol notation the atomic number is written as the lower number, for example ₆C, and it also equals the number of electrons in a neutral atom. A strong answer earns marks for these points. Define atomic number as the number of protons in an atom of an element. State that all atoms of the same element have identical proton numbers. Use examples such as carbon with atomic number 6 and oxygen with atomic number 8. Explain that changing the proton number changes the element. A common slip: Confusing atomic number with mass number. Instead, stating atomic number counts protons only, while mass number counts protons plus neutrons. Exam tip: Write the definition of atomic number precisely, using the word protons.
See it
Key words
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Atomic number
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Elements and identity
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Protons
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Isotopes
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See every definition
- Atomic number
- The number of protons in an atom of an element.
- Elements and identity
- All atoms of a particular element have the same number of protons, and changing the proton number changes the element.
- Protons
- All atoms of a particular element have the same number of these, and their number is the atomic number.
- Isotopes
- Atoms of the same element can differ in neutron number, forming these, but the proton number stays fixed.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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