Size and mass of atoms - AQA GCSE Chemistry Exam Questions

Practise 25 exam-style questions on Size and mass of atoms for AQA GCSE Chemistry. Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with an AI tutor on hand if you get stuck.

25 questions
1–6 marks each
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20 questions, marked as you go

Sample questions with model answers

Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.

Question 1

2 marks

The radius of a gold atom is 0.144 nm.
1 nm = 1 × 10⁻⁹ m

Give the radius of a gold atom in metres. Give your answer in standard form.

Model answer

0.144 nm = 0.144 × 10⁻⁹ m = 1.44 × 10⁻¹⁰ m

Explanation

Multiply by 10⁻⁹ to convert nanometres to metres. In standard form the first number must be between 1 and 10, so moving the decimal point one place to the right lowers the power by one: 0.144 × 10⁻⁹ = 1.44 × 10⁻¹⁰.

Mark scheme

1 mark
0.144 × 10⁻⁹ (m)
allow 0.000 000 000 144 m for this mark
1 mark
1.44 × 10⁻¹⁰ m
must be in standard form for this mark; correct answer with no working scores 2 marks

Question 2

2 marks

An atom of carbon is represented as ¹⁴₆C.

State what the numbers 14 and 6 tell you about this atom.

Model answer

14 is the mass number, which is the total number of protons and neutrons in the atom. 6 is the atomic number, which is the number of protons. (So this atom has 14 − 6 = 8 neutrons.)

Explanation

In this notation the larger, top number is the mass number and the smaller, bottom number is the atomic number. The atomic number also tells you the number of electrons in a neutral atom.

Mark scheme

1 mark
14 is the mass number: the total number of protons and neutrons
allow 'there are 14 protons and neutrons altogether'
1 mark
6 is the atomic number: the number of protons
allow proton number

Question 3

3 marks

The table shows the atomic numbers and mass numbers of four atoms, W, X, Y and Z.

Give the letters of the two atoms that are isotopes of the same element, and explain your answer. Explain why atoms X and Y are atoms of different elements, even though they have the same mass number.

Table with columns "Atom", "Atomic number", "Mass number". Atom: W; Atomic number: 6; Mass number: 12 | Atom: X; Atomic number: 7; Mass number: 14 | Atom: Y; Atomic number: 6; Mass number: 14 | Atom: Z; Atomic number: 8; Mass number: 18.

Model answer

W and Y are isotopes of the same element because they have the same atomic number (6 protons) but different mass numbers, so different numbers of neutrons (6 and 8). X and Y have different atomic numbers: X has 7 protons and Y has 6 protons, so they are different elements. They have the same mass number only because the total of protons and neutrons happens to be 14 in both.

Explanation

The element is decided by the atomic number only. Atoms with the same atomic number are the same element; the same mass number does not mean the same element.

Mark scheme

1 mark
W and Y
both needed and no others
1 mark
same atomic number / number of protons but different mass numbers / numbers of neutrons
1 mark
X has 7 protons but Y has 6 protons / they have different atomic numbers

Question 4

4 marks

Hydrogen has three isotopes: ¹₁H, ²₁H (deuterium) and ³₁H (tritium).

Explain why all three are atoms of hydrogen. Give the number of neutrons in an atom of each isotope.

Model answer

All three are hydrogen because they all have 1 proton (atomic number 1).
¹₁H: 1 − 1 = 0 neutrons
²₁H: 2 − 1 = 1 neutron
³₁H: 3 − 1 = 2 neutrons

Explanation

Isotopes of an element always have the same number of protons. The most common hydrogen atom is unusual because it has no neutrons at all; its mass number equals its atomic number.

Mark scheme

1 mark
they all have 1 proton / the same atomic number (1)
1 mark
¹₁H has 0 neutrons
allow no neutrons
1 mark
²₁H (deuterium) has 1 neutron
1 mark
³₁H (tritium) has 2 neutrons

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