Edexcel IGCSE Chemistry · Spec 1.14-1.17
Atomic Structure & Isotopes
The structure of the atom, atomic number and mass number, and isotopes.
Chemistry revision video
Atomic Structure & Isotopes
Prefer to watch on YouTube? Open this video on YouTube.
Explained
Inside the atom, and where relative atomic mass comes from
An atom has a tiny nucleus containing protons and neutrons, with electrons moving around it in shells. Almost all the mass is in the nucleus, and almost all the volume is the space the electrons occupy.
The three particles
- Proton: relative mass 1, relative charge plus 1, in the nucleus.
- Neutron: relative mass 1, relative charge 0, in the nucleus.
- Electron: relative mass negligible, often written as 1 over 1840, relative charge minus 1, in shells around the nucleus.
An atom has no overall charge, so the number of electrons equals the number of protons. When that balance is broken you have an ion rather than an atom.
Atomic number and mass number
The atomic number, or proton number, is the number of protons. It is what makes an element that element: every carbon atom has six protons and every oxygen atom has eight. Change it and you have changed the element.
The mass number is the number of protons plus neutrons. So the number of neutrons is the mass number minus the atomic number, and that subtraction is the most frequently used calculation in this topic.
Isotopes
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
Because the chemistry of an atom is decided by its electrons, and the number of electrons follows the number of protons, isotopes of an element have identical chemical properties. Their physical properties differ slightly, because they have different masses.
Chlorine 35 and chlorine 37 both have 17 protons and 17 electrons. One has 18 neutrons and the other has 20.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme asking for the similarity and difference between two isotopes credits the number of protons, or the number of electrons, or the electronic configuration for the similarity, and the number of neutrons for the difference. It then adds a fallback: same atomic number and different mass number scores one mark if no other mark has been awarded. The fuller answer is worth more than the shorthand.
On a table of subatomic particles, the same mark scheme rejects any sign given for the relative mass. Mass has no sign; only charge does.
Another note shows how strictly diagrams are read. Where a question asked for the proton, neutron and electron to be identified, the answers had to be in the right order, and putting the proton and neutron the wrong way round scored one mark out of two rather than none.
For the relative atomic mass calculation the mark scheme shows the working in full and allows an error carried forward, so setting the sum out earns credit even when the arithmetic goes wrong. A later paper adds that the answer should be given to three significant figures.
Calculating relative atomic mass
Relative atomic mass is the weighted mean mass of the atoms of an element, compared with one twelfth of the mass of a carbon 12 atom. Weighted is the important word: the more common isotope counts for more.
Multiply each isotope's mass number by its percentage abundance, add the results, and divide by 100.
Chlorine is 75 per cent chlorine 35 and 25 per cent chlorine 37. So 75 times 35 gives 2625, and 25 times 37 gives 925. Adding those gives 3550, and dividing by 100 gives 35.5.
Check the answer sits between the two mass numbers and nearer the more abundant one. If it does not, something has gone wrong, and 35.5 sitting between 35 and 37 but closer to 35 is exactly what you would expect.
Spec 1.14-1.17
What you need to know
- Describe the particles that make up an atom
- Use atomic number and mass number
- Explain what isotopes are, and work out a relative atomic mass
Active recall
Quick check
Answer each question before opening the answer.
State the relative charge and relative mass of a proton, neutron and electron.
Proton: charge +1, mass 1. Neutron: charge 0, mass 1. Electron: charge −1, mass negligible (~1/1836).
What are isotopes?
Atoms of the same element (same number of protons) with different numbers of neutrons.
Chemistry revision app
Take this topic further in the app
275 guided topics with questions marked as you answer them, section checkpoints and timed practice papers. Fifteen topics are free to try, with no card needed.