AQA GCSE Combined Science Foundation (8464), Foundation tier · Physics › Atomic structure › Atoms and isotopes
Practise Mass number, atomic number and isotopes. 8 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How to use the atomic number and mass number to find the numbers of protons, neutrons and electrons, what isotopes are, and how atoms become positive ions. Expect questions using nuclide notation such as \({}^{23}_{11}\mathrm{Na}\), tables of particle numbers and the definition of an isotope.
Key facts
Atomic number = number of protons
Mass number = number of protons + number of neutrons
Neutrons = mass number − atomic number
Neutral atom: number of electrons = number of protons
\({}^{A}_{Z}\mathrm{X}\): A = mass number (top), Z = atomic number (bottom)
Isotopes: same number of protons, different number of neutrons
Atom loses outer electron(s) → positive ion
Notes
Atomic number and mass number
diagram
The atomic number is the number of protons in the nucleus. All atoms of the same element have the same number of protons.
The mass number is the total number of protons and neutrons in the nucleus.
Number of neutrons = mass number − atomic number.
Atoms have no overall charge, because the number of electrons equals the number of protons.
In nuclide notation the mass number is at the top left and the atomic number at the bottom left. \({}^{39}_{19}\mathrm{K}\) has 19 protons, 39 − 19 = 20 neutrons and 19 electrons.
Mass number on top, atomic number underneath. Neutrons = mass number − atomic number.
Isotopes
diagram
Isotopes of an element are atoms with the same number of protons but a different number of neutrons.
So isotopes have the same atomic number but different mass numbers. \({}^{12}_{6}\mathrm{C}\) has 6 neutrons and \({}^{14}_{6}\mathrm{C}\) has 8 neutrons.
Isotopes of hydrogen: same number of protons, different numbers of neutrons.
An isotope is often named by its mass number, e.g. carbon-14 or uranium-235.
Some isotopes have unstable nuclei, so they are radioactive (carbon-14 is one).
Ions
If an atom loses one or more outer electrons, it has more protons than electrons, so it becomes a positive ion.
Losing electrons does not change the nucleus, so the atomic number and mass number stay the same. It is still the same element.
Nuclear radiation can knock electrons out of atoms, turning them into ions. This is called ionisation.
How to answer each type of question
Work out the numbers of particles
1 to 3 marksGrade 5
Protons = the bottom number (atomic number).
Neutrons = top number − bottom number.
Electrons = protons for a neutral atom; subtract any electrons lost for a positive ion.
Example. An atom of iron is represented as \({}^{56}_{26}\mathrm{Fe}\). Give the number of (a) protons, (b) neutrons and (c) electrons in this atom.
Show the model answerHide the model answer
(a) 26 (1) (b) 56 − 26 = 30 (1) (c) 26 (1)
Don’t lose marks
Giving the mass number as the number of neutrons. Subtract the atomic number first.
Saying isotopes have different numbers of protons. A different number of protons means a different element.
Saying a positive ion has gained protons or gained electrons. It has lost electrons.
Swapping the top and bottom numbers in nuclide notation.
More tips
Memory tricks
The mass number is never smaller than the atomic number, so it is the bigger number (the two are equal only for hydrogen-1).
The atomic number is the element's ID: change the number of protons and you have a different element.
'Iso' means 'same': isotopes are the same element, in the same place in the periodic table, with different masses.
Quick check: protons + neutrons must add up to the mass number.
Exam technique
An isotope definition needs both parts: same number of protons AND different number of neutrons.
For 'explain why an atom has no overall charge', say there are equal numbers of protons and electrons AND that their charges are equal and opposite.
Write your subtraction for neutrons (e.g. 56 − 26 = 30) so the examiner can follow your method.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
State what atomic number meansThe atomic number is the number of protons in the nucleus.
Grade 4
State what mass number meansThe mass number is the total number of protons and neutrons in the nucleus.
Grade 4
Explain why atoms have no overall chargeAn atom has equal numbers of protons and electrons, and their charges are equal and opposite.
Grade 5
Work out protons, neutrons and electronsProtons = atomic number; neutrons = mass number − atomic number; electrons = protons in a neutral atom.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
An atom of potassium can be represented as \({}^{39}_{19}\mathrm{K}\). What is the atomic number of this atom?
19
Chlorine has two naturally occurring isotopes, \({}^{35}_{17}\mathrm{Cl}\) and \({}^{37}_{17}\mathrm{Cl}\). What are isotopes?
Atoms of the same element (same number of protons) with different numbers of neutrons.
The lithium atom loses one electron. Name the type of particle that is formed.
A positive ion
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
An atom of potassium can be represented as \({}^{39}_{19}\mathrm{K}\).
(a) What is the atomic number of this atom?[1]
(b) How many neutrons are in the nucleus of this atom?[1]
(c) How many electrons does this atom have? Give a reason for your answer.[2]
(d) What name is given to the total number of protons and neutrons in an atom?[1]
Show the answer and mark scheme
(a)Answer: 19
19
(b)Answer: 20
20
(c)Answer: 19, because an atom has equal numbers of protons and electrons.
19
atoms have no overall charge, so the number of electrons equals the number of protons
(d)Answer: Mass number
mass number
Question 2Medium7 marks
A lithium atom can be represented as \({}^{7}_{3}\mathrm{Li}\).
(a) The lithium atom loses one electron. Name the type of particle that is formed.[1]
(b) Give the number of protons, the number of neutrons and the number of electrons in the particle formed.[3]
(c) What is the overall charge of the particle formed? Explain your answer.[2]
(d) Another atom of lithium has a mass number of 6. Describe how the nucleus of this atom is different from the nucleus of \({}^{7}_{3}\mathrm{Li}\).[1]
Show the answer and mark scheme
(a)Answer: A positive ion
(positive) ion
(b)Answer: 3 protons, 4 neutrons, 2 electrons
3 protons
4 neutrons
2 electrons
(c)Answer: +1, because it has 3 protons but only 2 electrons.
+1 / positive
it has one more proton than electrons / 3 positive charges but only 2 negative charges
(d)Answer: It has 3 neutrons instead of 4 (the same 3 protons).
it has one fewer neutron (3 neutrons instead of 4)