AQA GCSE Combined Science (8464), Higher tier · Chemistry › Atomic structure and the periodic table › A simple model of the atom, symbols, relative atomic mass,
Practise Size and mass of atoms. 12 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 big atoms and nuclei are, where the mass of an atom is, mass number and isotopes, and how to work out the numbers of protons, neutrons and electrons in an atom or ion. Particle-counting questions and standard form calculations come up on almost every paper.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Recall relative masses of subatomic particlesProton 1, neutron 1, electron very small.
4
Define mass numberThe total number of protons and neutrons in an atom.
4
Find protons, neutrons and electrons in atomsProtons = electrons = atomic number; neutrons = mass number − atomic number.
5
Define isotopesAtoms of the same element with different numbers of neutrons: same atomic number, different mass number.
5
Recall the sizes of atoms and nucleiAtom radius about 0.1 nm (1 × 10−10 m); nucleus radius less than 1/10 000 of that (about 1 × 10−14 m).
6
Find protons, neutrons and electrons in ionsProtons and neutrons as in the atom; one electron fewer for each + charge, one more for each − charge.
7
Compare sizes using standard formDivide one size by the other, e.g. 1 × 10−10 ÷ 1 × 10−14 = 10 000, and relate the scale to everyday objects.
Notes
How small atoms are
Atoms are very small: the radius of an atom is about 0.1 nm (1 × 10−10 m). 1 nm = 1 × 10−9 m.
The radius of the nucleus is less than 1/10 000 of the radius of the atom (about 1 × 10−14 m).
To picture the scale: if an atom were as wide as a 100 m sports field, its nucleus would be less than 1 cm across.
Where the mass is
Relative masses: proton 1, neutron 1, electron very small.
Protons and neutrons are both in the nucleus, so almost all the mass of an atom is in the nucleus.
The mass number is the total number of protons and neutrons in an atom.
An atom can be written as \({}^{23}_{11}\text{Na}\): the top number (23) is the mass number and the bottom number (11) is the atomic number.
Counting the particles
Protons = atomic number.
Neutrons = mass number − atomic number. For \({}^{23}_{11}\text{Na}\): 23 − 11 = 12 neutrons.
Electrons in an atom = number of protons.
In an ion, the protons and neutrons are unchanged. A 2+ ion has 2 fewer electrons than protons; a 1− ion has 1 more. For example, Mg2+ (atomic number 12) has 12 protons and 10 electrons.
Isotopes
Isotopes are atoms of the same element with different numbers of neutrons.
Isotopes have the same atomic number but different mass numbers, e.g. \({}^{35}_{17}\text{Cl}\) has 18 neutrons and \({}^{37}_{17}\text{Cl}\) has 20 neutrons.
Isotopes of an element have the same number of electrons, so they react in the same way.
Cheatsheet
Radius of an atom ≈ 0.1 nm = 1 × 10−10 m
Radius of nucleus < 1/10 000 of the atom's radius (≈ 1 × 10−14 m)
1 nm = 1 × 10−9 m
Relative mass: proton 1, neutron 1, electron very small
Almost all the mass of an atom is in the nucleus
Mass number = protons + neutrons
Neutrons = mass number − atomic number
Isotopes: same number of protons, different numbers of neutrons
Ions: a positive ion has fewer electrons than protons (Al3+: 3 fewer); a negative ion has more (O2−: 2 more)
How to answer each type of question
Work out the numbers of protons, neutrons and electrons
3 to 4 marks5
Protons = atomic number (the bottom number).
Neutrons = mass number − atomic number.
Electrons = protons for an atom; adjust for the charge if it is an ion.
Example. An atom of aluminium is represented as \({}^{27}_{13}\text{Al}\). (a) Give the numbers of protons, neutrons and electrons in this atom. (b) Aluminium forms the ion Al3+. Give the number of electrons in this ion.
Say they have the same number of protons (same atomic number), so they are the same element.
Say they have different numbers of neutrons, and give the numbers if you can.
Example. Two atoms are represented as \({}^{12}_{6}\text{C}\) and \({}^{14}_{6}\text{C}\). Explain why these atoms are isotopes of the same element.
Show the model answer
Both atoms have 6 protons / the same atomic number, so they are the same element (1). They have different numbers of neutrons: 6 and 8 (1).
Calculate with sizes in standard form
2 marks7
Write down the division: bigger ÷ smaller.
Use the ×10x button on your calculator, or divide the numbers and subtract the powers of 10.
Give the answer as an ordinary number or in standard form, as asked.
Example. The radius of a gold atom is 1.4 × 10−10 m. The radius of its nucleus is 7.0 × 10−15 m. Calculate how many times larger the radius of the atom is than the radius of the nucleus.
Say protons and neutrons are in the nucleus and each has a relative mass of 1.
Say electrons have a very small mass.
Example. Explain why almost all of the mass of an atom is in its nucleus.
Show the model answer
Protons and neutrons are in the nucleus, and each has a relative mass of 1 (1). The electrons are outside the nucleus and have a very small mass (1).
Shortcuts and memory tricks
In a nuclear symbol the mass number is on top and is never smaller than the atomic number underneath.
Neutrons = 'top minus bottom'.
Dividing powers of 10: subtract the powers. 10−10 ÷ 10−14 = 104.
Positive ion = electrons lost, so fewer electrons; negative ion = electrons gained, so more.
Where marks are lost
Adding the two numbers instead of subtracting to find the number of neutrons.
Changing the number of protons for an ion. Only the number of electrons changes.
Saying isotopes have different numbers of protons or electrons.
Writing 1 × 1010 m for the radius of an atom: the power must be negative, 1 × 10−10 m.
Using a relative atomic mass from the periodic table (e.g. 35.5 for chlorine) as a mass number. Mass numbers are always whole numbers.
Exam technique
Show the subtraction for neutrons (e.g. 27 − 13 = 14) so the examiner can follow your working.
In standard form, write a × 10n with a between 1 and 10, unless the question asks for an ordinary number.
When defining isotopes, include all three ideas: same element, same number of protons, different number of neutrons.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Atoms are extremely small. Where is almost all of the mass of an atom found?
in the nucleus
Magnesium has three isotopes: 24Mg, 25Mg and 26Mg. The atomic number of magnesium is 12. What are isotopes?
Atoms of the same element (same number of protons) with different numbers of neutrons.
The radius of an atom is about 0.1 nm. Give this radius in metres.
1 × 10−10 m
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy6 marks
Atoms are extremely small.
(a) What is the approximate radius of an atom? Tick (✓) one box.[1]
1 × 10−2 m
1 × 10−6 m
1 × 10−10 m
1 × 10−14 m
(b) Where is almost all of the mass of an atom found?[1]
(c) An atom of sodium is represented as 2311Na. Give the number of protons, neutrons and electrons in this atom.[3]
(d) What is meant by the mass number of an atom?[1]
Show the answer and mark scheme
(a)Answer: 1 × 10−10 m
(b)Answer: in the nucleus
(in the) nucleus
(c)Answer: 11 protons, 12 neutrons, 11 electrons
protons: 11
neutrons: 12
electrons: 11
(d)Answer: The total number of protons and neutrons in the atom.
(total) number of protons and neutrons (in the nucleus)
Question 2Medium7 marks
Magnesium has three isotopes: 24Mg, 25Mg and 26Mg. The atomic number of magnesium is 12.
(a) What are isotopes?[2]
(b) Give the numbers of protons, neutrons and electrons in an atom of 25Mg.[2]
(c) A magnesium ion, Mg2+, is formed from an atom of 26Mg. Give the number of electrons and the number of neutrons in this ion.[2]
(d) Suggest why the three isotopes of magnesium have the same chemical reactions.[1]
Show the answer and mark scheme
(a)Answer: Atoms of the same element (same number of protons) with different numbers of neutrons.
atoms of the same element / with the same number of protons
(with) different numbers of neutrons
(b)Answer: 12 protons, 13 neutrons, 12 electrons
13 neutrons
12 protons and 12 electrons
(c)Answer: 10 electrons and 14 neutrons
10 electrons
14 neutrons
(d)Answer: They have the same number of electrons / the same electronic structure.
same number of electrons / same electronic structure / same number of outer electrons
Question 3Hard8 marks
Hydrogen has three isotopes: 1H, 2H and 3H. The atomic number of hydrogen is 1.
(a) Give the number of neutrons in an atom of each isotope.[2]
(b) The mass of an electron is about \(\frac{1}{1840}\) of the mass of a proton. Calculate the percentage of the mass of a 1H atom that is due to its electron. Give your answer to 2 significant figures.[3]
(c) Use your answer to explain why the mass of an electron is usually described as 'very small'.[1]
(d) A molecule of 'heavy water' contains two 2H atoms and one 16O atom. The atomic number of oxygen is 8. Calculate the total number of neutrons in the molecule.[2]
Show the answer and mark scheme
(a)Answer:1H: 0, 2H: 1, 3H: 2
1H: 0 and 2H: 1
3H: 2
(b)Answer: 0.054%
mass of atom = 1 + \(\frac{1}{1840}\) (proton masses) / 1841 electron masses
\(\frac{1}{1841}\) × 100
0.054 (%)
(c)Answer: The electron makes up only a tiny fraction (about 0.05%) of the mass of the atom.
the electron is only a tiny percentage of the mass of the atom / almost all of the mass is in the nucleus