Practise Mass number, atomic number and isotopes. 13 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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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)
Question 3Hard4 marks
Natural uranium is a mixture of two isotopes, uranium-235 and uranium-238. The atomic number of uranium is 92. One method of separating the two isotopes turns the uranium into a gas. It relies on molecules containing uranium-235, which have slightly less mass, moving slightly faster than molecules containing uranium-238. The temperature of a gas depends on the average kinetic energy of its molecules.
(a) Give the number of neutrons in a nucleus of uranium-235 and in a nucleus of uranium-238.[2]
(b) Explain why, at the same temperature, molecules containing uranium-235 move faster on average than molecules containing uranium-238.[2]
Show the answer and mark scheme
(a)Answer: uranium-235: 143; uranium-238: 146
uranium-235: 235 − 92 = 143
uranium-238: 238 − 92 = 146
(b)Answer: At the same temperature both kinds of molecule have the same average kinetic energy; since KE = ½mv², the lighter uranium-235 molecules must move faster on average.
at the same temperature, both kinds of molecule have the same average kinetic energy
since kinetic energy = ½mv2, the lighter molecules (containing uranium-235) must have a higher average speed