AQA GCSE Chemistry (8462), Higher tier · Atomic structure and the periodic table › Atoms, isotopes and electronic structure
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.
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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