AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
In an atom the number of electrons is equal to the number of protons in…
Full specification wording
Mass number, atomic number and isotopes: In an atom the number of electrons is equal to the number of protons in the nucleus. Atoms have no overall electrical charge.
Every atom contains a nucleus of protons and neutrons, with electrons arranged in shells around it.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 1 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Every atom contains a nucleus of protons and neutrons, with electrons arranged in shells around it. A proton carries a 1⁺ charge and an electron carries a 1⁻ charge, so when the numbers are equal the positive and negative charges cancel exactly. For example, carbon has six protons, so a neutral carbon atom also has six electrons, giving 6⁺ and 6⁻ and a net charge of zero. If the numbers differ, the particle is an ion, not an atom: losing an electron leaves a positive ion such as Na⁺, while gaining two electrons gives a negative ion such as O²⁻. The rule applies to atoms only, and the equal numbers explain why atoms are electrically neutral overall.
What earns the marks
- State that a proton has a charge of 1⁺ and an electron has a charge of 1⁻, so equal numbers cancel.
- Explain that the nucleus contains protons and neutrons, and that neutrons have no charge.
- Apply the rule to a named element, for example carbon with six protons and six electrons.
- Distinguish a neutral atom from an ion, where electron number differs from proton number.
- Use the idea of overall charge as the sum of positive and negative charges being zero.
Common mistakes and how to put them right
Saying atoms contain equal numbers of protons and neutrons.
Put it right by stating the equal numbers are protons and electrons, while neutron number can differ.
Treating a sodium ion as an atom.
Put it right by noting Na⁺ has lost one electron and so is charged, not neutral.
Believing electrons contribute mass number.
Put it right by explaining mass number counts protons and neutrons only.
Exam tips
- Quote the charges 1⁺ and 1⁻ when explaining why an atom is neutral.
- Use a specific element with its proton and electron numbers to support your answer.
- Check whether the question says atom or ion before applying the equal-numbers rule.
Try the lesson
The start of the Mass number, atomic number and isotopes: In an atom the number of electrons is equal to the number of protons in the nucleus. Atoms have no overall electrical charge. lesson, made from checked teaching notes for this specification point.
Have a go first
A neutral atom of boron has 5 protons in its nucleus. How many electrons does this atom have?
Listen
Every atom contains a nucleus of protons and neutrons, with electrons arranged in shells around it. A proton carries a 1⁺ charge and an electron carries a 1⁻ charge, so when the numbers are equal the positive and negative charges cancel exactly. For example, carbon has six protons, so a neutral carbon atom also has six electrons, giving 6⁺ and 6⁻ and a net charge of zero. If the numbers differ, the particle is an ion, not an atom: losing an electron leaves a positive ion such as Na⁺, while gaining two electrons gives a negative ion such as O²⁻. The rule applies to atoms only, and the equal numbers explain why atoms are electrically neutral overall. A strong answer earns marks for these points. State that a proton has a charge of 1⁺ and an electron has a charge of 1⁻, so equal numbers cancel. Explain that the nucleus contains protons and neutrons, and that neutrons have no charge. Apply the rule to a named element, for example carbon with six protons and six electrons. Distinguish a neutral atom from an ion, where electron number differs from proton number. A common slip: Saying atoms contain equal numbers of protons and neutrons. Instead, stating the equal numbers are protons and electrons, while neutron number can differ. Exam tip: Quote the charges 1⁺ and 1⁻ when explaining why an atom is neutral.
See it
Key words
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Atomic structure
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Electrical charge
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Protons and electrons
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Neutral atoms and ions
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See every definition
- Atomic structure
- Every atom contains a nucleus of protons and neutrons, with electrons arranged in shells around it.
- Electrical charge
- A proton carries a 1⁺ charge and an electron carries a 1⁻ charge, so equal numbers cancel exactly.
- Protons and electrons
- In an atom the number of electrons is equal to the number of protons in the nucleus.
- Neutral atoms and ions
- Atoms have no overall electrical charge, but if the numbers differ the particle is an ion, not an atom.
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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