AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Relative electrical charges of subatomic particles: 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 of an element has a fixed number of protons in its nucleus, called the atomic number, and a neutral atom has exactly the same number of electrons arranged around the nucleus.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Every atom of an element has a fixed number of protons in its nucleus, called the atomic number, and a neutral atom has exactly the same number of electrons arranged around the nucleus. Because each proton has a relative charge of 1⁺ and each electron has a relative charge of 1⁻, equal numbers of the two particles produce equal but opposite total charges. These cancel completely, so the atom has no overall electrical charge. For example, a carbon atom has 6 protons and therefore 6 electrons: the total positive charge is 6⁺ and the total negative charge is 6⁻, giving an overall charge of 0. If the numbers become unequal, as happens when electrons are gained or lost, the particle becomes a charged ion instead.
What earns the marks
- State that in an atom the number of electrons equals the number of protons.
- Link the equal numbers to the equal magnitudes of the proton and electron charges, 1⁺ and 1⁻.
- Explain that the positive and negative charges cancel, so the overall charge of an atom is zero.
- Use the atomic number to deduce the number of electrons in a neutral atom, for example atomic number 17 means 17 electrons.
- Recognise that unequal numbers of protons and electrons produce an ion with an overall charge, such as 11 protons with 10 electrons giving 1⁺.
Common mistakes and how to put them right
Assuming the number of neutrons must also equal the number of protons.
Put it right: Neutrons vary between isotopes and do not affect the overall charge.
Saying atoms have no charged particles at all.
Put it right: Atoms contain charged protons and electrons, but their charges cancel.
Forgetting that the rule applies to atoms only, so a particle with unequal proton and electron counts is an ion and does carry an overall charge.
Exam tips
- Use the atomic number as the starting point: it gives the proton count, and for an atom the electron count is the same.
- Show the cancellation explicitly, for example 8⁺ and 8⁻ give an overall charge of 0, to make your reasoning clear.
- If a question asks why an atom is neutral, refer to equal numbers of protons and electrons rather than simply stating that it has no charge.
Try the lesson
The start of the Relative electrical charges of subatomic particles: 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 helium contains 2 protons in its nucleus. How many electrons does this atom contain?
Listen
Every atom of an element has a fixed number of protons in its nucleus, called the atomic number, and a neutral atom has exactly the same number of electrons arranged around the nucleus. Because each proton has a relative charge of 1⁺ and each electron has a relative charge of 1⁻, equal numbers of the two particles produce equal but opposite total charges. These cancel completely, so the atom has no overall electrical charge. For example, a carbon atom has 6 protons and therefore 6 electrons: the total positive charge is 6⁺ and the total negative charge is 6⁻, giving an overall charge of 0. If the numbers become unequal, as happens when electrons are gained or lost, the particle becomes a charged ion instead. A strong answer earns marks for these points. State that in an atom the number of electrons equals the number of protons. Link the equal numbers to the equal magnitudes of the proton and electron charges, 1⁺ and 1⁻. Explain that the positive and negative charges cancel, so the overall charge of an atom is zero. Use the atomic number to deduce the number of electrons in a neutral atom, for example atomic number 17 means 17 electrons. Exam tip: Use the atomic number as the starting point: it gives the proton count, and for an atom the electron count is the same.
Key words
Say what each word means before you turn the card.
Atomic number
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Electrical neutrality
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Ion
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Relative charge
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See every definition
- Atomic number
- The fixed number of protons in the nucleus of every atom of an element.
- Electrical neutrality
- Atoms have no overall electrical charge because equal numbers of protons and electrons produce equal but opposite total charges that cancel completely.
- Ion
- A particle that becomes charged when electrons are gained or lost, so the numbers of protons and electrons become unequal.
- Relative charge
- Each proton has a relative charge of 1⁺ and each electron has a relative charge of 1⁻.
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