Isotopes

O Level / IGCSE Chemistry · Atoms, elements and compounds. 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 different atoms of the same element. They have the same number of protons but different numbers of neutrons, so they have the same proton number and different nucleon numbers. Chlorine-35 and chlorine-37 both have 17 protons, but 18 and 20 neutrons.

Isotopes of an element have the same number of electrons and the same electronic configuration, so their chemical properties are the same. Only their masses differ.

In a symbol such as 3517Cl, the top number is the nucleon number and the bottom number is the proton number. Because an element is a mixture of isotopes, its relative atomic mass is an average that takes account of how abundant each isotope is.

Rules to remember

Common mistake

Do not just take the middle of the two isotope masses. Multiply each mass by its percentage abundance first, so the answer lies closer to the more common isotope.

Worked example

Gallium consists of 60% gallium-69 and 40% gallium-71. What is the relative atomic mass of gallium?

  1. Multiply each mass by its abundance: 69 × 60 = 4140 and 71 × 40 = 2840.
  2. Add them: 4140 + 2840 = 6980.
  3. Divide by 100: 6980 ÷ 100 = 69.8.

Answer: Ar of gallium = 69.8 (closer to 69, the more abundant isotope).

Practice questions

Try each one, then open the answer.

1. Which symbol represents a particle with 16 protons, 18 neutrons and 18 electrons?

  1. A
    3418Ar
  2. B
    3416S
  3. C
    3216S2−
  4. D
    3416S2−
Show answer

Answer: D. 16 protons makes it sulfur; mass number = 16 + 18 = 34. With 18 electrons it has two more electrons than protons, so it is the 2− ion.

2. A sample of chlorine is 75% chlorine-35 and 25% chlorine-37. What is its relative atomic mass?

  1. A
    35.0
  2. B
    35.5
  3. C
    36.0
  4. D
    36.5
Show answer

Answer: B. Ar = (75 × 35 + 25 × 37) ÷ 100 = (2625 + 925) ÷ 100 = 35.5.

3. What does the symbol 2311Na+ represent?

  1. A
    a sodium ion with 11 protons, 12 neutrons and 10 electrons
  2. B
    a sodium atom with 12 protons and 11 neutrons
  3. C
    a sodium atom with 11 protons, 12 neutrons and 11 electrons
  4. D
    a sodium ion with 11 protons, 23 neutrons and 12 electrons
Show answer

Answer: A. Mass number 23, proton number 11 gives 12 neutrons. The + charge means one electron lost: 10 electrons.

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