Isotopes

AS Level Chemistry · Atomic structure. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have the same proton number and different nucleon numbers.

In the notation xyA, x is the nucleon (mass) number and y is the proton (atomic) number. So 3517Cl and 3717Cl are isotopes: both have 17 protons, but 18 and 20 neutrons.

Chemical reactions depend on electrons. Isotopes have the same number of electrons in the same arrangement, so their chemistry is the same. Their masses differ, so physical properties that depend on mass, such as density, differ.

Rules to remember

Common mistake

Students pick two atoms with the same top number as isotopes. Check the bottom number: isotopes must have the same proton number.

Worked example

Atom X has 35 protons and 44 neutrons. Atom Y has 35 protons and 46 neutrons. Are they isotopes, and how are they written?

  1. Both have 35 protons, so both are the same element, bromine.
  2. The neutron numbers differ (44 and 46), so they are isotopes.
  3. Nucleon numbers: 35 + 44 = 79 and 35 + 46 = 81.

Answer: Yes, they are isotopes: 7935Br and 8135Br, with the same chemical properties.

Practice questions

Try each one, then open the answer.

1. An atom contains 20 protons and 24 neutrons. How is this isotope written?

  1. A
    2420Ca
  2. B
    2044Ca
  3. C
    4424Cr
  4. D
    4420Ca
Show answer

Answer: D. The proton number (20) identifies calcium and is written at the bottom; the nucleon number is 20 + 24 = 44, written at the top.

2. Which statement defines isotopes?

  1. A
    atoms of the same element with the same number of protons but different numbers of neutrons
  2. B
    atoms with the same nucleon number but different proton numbers
  3. C
    atoms of different elements with the same number of neutrons
  4. D
    atoms of the same element with different numbers of electrons
Show answer

Answer: A. Isotopes have the same proton number, so they are the same element, but different numbers of neutrons and so different nucleon numbers. A different number of electrons gives an ion, not an isotope.

3. Which statement about the isotopes 35Cl and 37Cl is correct?

  1. A
    they have different numbers of electrons
  2. B
    37Cl has two more protons than 35Cl
  3. C
    they react with sodium to give different compounds
  4. D
    a sample of 37Cl2 gas is denser than a sample of 35Cl2 gas under the same conditions
Show answer

Answer: D. Isotopes differ only in neutrons, so they have the same electrons and the same chemical reactions. The extra neutrons change physical properties that depend on mass, such as density.

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