AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
The number of protons in an atom of an element is its atomic number.
Full specification wording
Relative electrical charges of subatomic particles: The number of protons in an atom of an element is its atomic number.
The atomic number is the defining count of protons in an atom's nucleus.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 2 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The atomic number is the defining count of protons in an atom's nucleus. Because each proton carries a relative charge of 1⁺, the atomic number also equals the positive nuclear charge and, in a neutral atom, the number of electrons. For example, every carbon atom has six protons, so its atomic number is 6; a neutral carbon atom therefore has six electrons arranged around the nucleus. The atomic number identifies the element and fixes its position in the periodic table. It is not the same as mass number, which counts protons plus neutrons. In the nuclear model, protons and neutrons occupy a tiny central nucleus while electrons occupy shells. Reading a periodic table entry, the smaller whole number is usually the atomic number.
What earns the marks
- State that the atomic number equals the number of protons in the nucleus of an atom.
- Link the proton count to the element's identity and its position in the periodic table.
- Recognise that in a neutral atom the number of electrons equals the atomic number.
- Distinguish atomic number from mass number, which is protons plus neutrons.
- Use a periodic table or symbol notation to obtain the atomic number for a given element.
Common mistakes and how to put them right
Confusing atomic number with mass number.
Put it right by stating that atomic number counts protons only, while mass number counts protons plus neutrons.
Assuming the atomic number equals the number of neutrons.
Put it right by checking the nucleus composition and using mass number minus atomic number for neutrons.
Treating the atomic number as a count of electrons in every atom.
Put it right by noting that electron number equals proton number only in a neutral atom, not in ions.
Exam tips
- When given a symbol with subatomic numbers, identify the lower number as the atomic number before doing any calculation.
- Write a short definition using the words protons and nucleus to secure the key idea.
- If asked to find electrons, first confirm whether the atom is neutral; if it is an ion, adjust for the charge.
Try the lesson
The start of the Relative electrical charges of subatomic particles: The number of protons in an atom of an element is its atomic number. lesson, made from checked teaching notes for this specification point.
Have a go first
An atom of an element contains 11 protons. What is the atomic number of this element?
Listen
The atomic number is the defining count of protons in an atom's nucleus. Because each proton carries a relative charge of 1⁺, the atomic number also equals the positive nuclear charge and, in a neutral atom, the number of electrons. For example, every carbon atom has six protons, so its atomic number is 6; a neutral carbon atom therefore has six electrons arranged around the nucleus. The atomic number identifies the element and fixes its position in the periodic table. It is not the same as mass number, which counts protons plus neutrons. In the nuclear model, protons and neutrons occupy a tiny central nucleus while electrons occupy shells. Reading a periodic table entry, the smaller whole number is usually the atomic number. A strong answer earns marks for these points. State that the atomic number equals the number of protons in the nucleus of an atom. Link the proton count to the element's identity and its position in the periodic table. Recognise that in a neutral atom the number of electrons equals the atomic number. Distinguish atomic number from mass number, which is protons plus neutrons. A common slip: Confusing atomic number with mass number. Instead, stating that atomic number counts protons only, while mass number counts protons plus neutrons. Exam tip: When given a symbol with subatomic numbers, identify the lower number as the atomic number before doing any calculation.
See it
Key words
Say what each word means before you turn the card.
Atomic number
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Mass number
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Nuclear model
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Neutral atom
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See every definition
- Atomic number
- The number of protons in an atom of an element, which identifies the element and fixes its position in the periodic table.
- Mass number
- A count of protons plus neutrons, which is not the same as the atomic number.
- Nuclear model
- A model in which protons and neutrons occupy a tiny central nucleus while electrons occupy shells.
- Neutral atom
- An atom in which the number of electrons equals the atomic number.
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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