What are isotopes and ions, and how are nuclei represented by symbols?
Isotopes and ions: atomic and mass number notation, what makes isotopes of an element, and how atoms become ions by losing or gaining electrons.
A focused answer to Edexcel GCSE Physics 6.3, 6.4, 6.6 and 6.9, covering atomic (proton) number and mass (nucleon) number notation, the definition of isotopes as atoms with the same protons but different neutrons, and how atoms form positive ions by losing electrons.
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What this dot point is asking
Edexcel statements 6.3, 6.4, 6.6 and 6.9 want you to describe nuclei using atomic (proton) number and mass (nucleon) number and the symbol notation, to recall that isotopes have the same protons but different neutrons, that a neutral atom has equal protons and electrons, and that atoms form positive ions by losing electrons.
Atomic and mass number
From these two numbers you can find every count in a neutral atom: the protons equal the atomic number, the neutrons equal the mass number minus the atomic number, and the electrons equal the protons. This is one of the most frequently tested skills in the topic, so practise reading the numbers off the symbol.
Isotopes
Because they have the same number of protons, isotopes are the same element with the same chemistry, but their nuclei differ in mass. Carbon-12 and carbon-14 are isotopes of carbon: both have 6 protons, but carbon-12 has 6 neutrons and carbon-14 has 8. Some isotopes are stable and some are radioactive (unstable), which is the link to the rest of the radioactivity topic.
Ions
Ions form when the number of electrons no longer matches the number of protons. Losing a negative electron leaves the atom with a net positive charge; gaining one gives a net negative charge. Note that forming an ion changes the electrons only, not the nucleus, so the element (atomic number) stays the same.
How Edexcel examines this
This dot point is examined on both tiers and is a reliable source of marks. The most common question gives a nuclear symbol and asks for the numbers of protons, neutrons and electrons, where the mark scheme rewards using the atomic number for protons, subtracting to find neutrons, and equating electrons to protons for a neutral atom. The isotope definition is examined as a two-mark recall question, and examiners insist on both halves: same number of protons and different number of neutrons; an answer giving only "different mass" is incomplete. Ion questions ask how an atom becomes a positive (or negative) ion, rewarding the loss (or gain) of electrons leaving more protons than electrons (or vice versa). A frequent slip is to imply that forming an ion or an isotope changes the number of protons; protons define the element and are unchanged when forming an ion, while isotopes keep the same protons by definition. Reading the symbol notation accurately is the underlying skill, so drill it.
Try this
Q1. An atom is . State its number of protons and neutrons. [2 marks]
- Cue. 8 protons and neutrons.
Q2. State how an atom becomes a positive ion. [1 mark]
- Cue. By losing one or more electrons.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20203 marksAn atom is represented as carbon-14. Carbon has an atomic number of 6. State the number of protons, neutrons and electrons in a neutral atom of carbon-14.Show worked answer →
The atomic (proton) number is 6, so there are 6 protons (1 mark). The mass (nucleon) number is 14, and neutrons = mass number minus atomic number = neutrons (1 mark). In a neutral atom the number of electrons equals the number of protons, so there are 6 electrons (1 mark). Markers reward using the atomic number for protons, subtracting to find neutrons, and equating electrons to protons for a neutral atom.
Edexcel 20223 marksDefine what is meant by isotopes, and explain how a sodium atom becomes a positive ion (a sodium ion).Show worked answer →
Isotopes are atoms of the same element that have the same number of protons (the same atomic number) but different numbers of neutrons (and so different mass numbers) (2 marks). A sodium atom becomes a positive ion by losing one (outer) electron, so it then has one more proton than electrons and an overall charge of (1 mark). Markers reward the isotope definition (same protons, different neutrons) and the formation of a positive ion by losing electrons, leaving more protons than electrons.
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Sources & how we know this
- Pearson Edexcel GCSE (9-1) Physics (1PH0) specification — Pearson (2016)