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1.7 Ions, Atoms and Isotopes

GCSE Chemistry · AQA · Chapter 1: Atomic Structure

A free GCSE Chemistry lesson for AQA (8462) from OBA School: how ions differ from atoms, what isotopes are, and calculating relative atomic mass from isotope abundances. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Explain how positive and negative ions form from atoms
  • Work out the numbers of protons, neutrons and electrons in ions
  • Explain what isotopes are and why they have the same chemical properties
  • Calculate the relative atomic mass of an element from the abundances of its isotopes

Key words

  • Ion: An atom or group of atoms with an electric charge, because it has lost or gained electrons.
  • Positive ion: Formed when an atom loses electrons: more protons than electrons, e.g. Na⁺.
  • Negative ion: Formed when an atom gains electrons: more electrons than protons, e.g. Cl−.
  • Isotopes: Atoms of the same element (same number of protons) with different numbers of neutrons.
  • Abundance: The percentage of an element’s atoms that are a particular isotope.
  • Relative atomic mass (Ar): The average mass of an element’s atoms, taking into account the abundance of each isotope.

Ions

Atoms have equal numbers of protons and electrons. Atoms can lose or gain electrons to form charged particles called ions. The number of protons never changes, so the element stays the same.

  • Na⁺; Protons 11; Electrons 10; Charge +1; Change lost 1 electron
  • Mg²⁺; Protons 12; Electrons 10; Charge +2; Change lost 2 electrons
  • Cl−; Protons 17; Electrons 18; Charge −1; Change gained 1 electron
  • O²−; Protons 8; Electrons 10; Charge −2; Change gained 2 electrons

Exam tip: electrons in an ion = protons − charge. For O²−: 8 − (−2) = 10 electrons. Neutrons are not affected.

Isotopes

Isotopes are atoms of the same element with the same number of protons but a different number of neutrons. So they have the same atomic number but different mass numbers.

Isotopes of an element have the same number of electrons, arranged in the same way, so they have the same chemical properties. They have different masses, so some physical properties are different: a sample of a heavier isotope has a higher density, and some isotopes have an unstable nucleus with too many neutrons, so they are radioactive. For example, carbon-12 is stable but carbon-14 is radioactive – yet both burn to make carbon dioxide in exactly the same way.

  • chlorine-35, ³⁵Cl; Protons 17; Neutrons 18; Electrons 17
  • chlorine-37, ³⁷Cl; Protons 17; Neutrons 20; Electrons 17
  • carbon-12, ¹²C; Protons 6; Neutrons 6; Electrons 6
  • carbon-14, ¹⁴C; Protons 6; Neutrons 8; Electrons 6

Relative atomic mass

Most elements are a mixture of isotopes. The relative atomic mass, Ar, is the average mass of the atoms, taking into account how common (abundant) each isotope is. That is why Ar values are not always whole numbers.

Ar = (mass number 1 × % abundance 1 + mass number 2 × % abundance 2 + …) ÷ 100

Worked example: chlorine

Chlorine is 75% chlorine-35 and 25% chlorine-37.
Ar = (35 × 75 + 37 × 25) ÷ 100 = (2625 + 925) ÷ 100 = 3550 ÷ 100

Answer: Ar = 35.5

Worked example: three isotopes

Magnesium is 79% magnesium-24, 10% magnesium-25 and 11% magnesium-26.
Ar = (24 × 79 + 25 × 10 + 26 × 11) ÷ 100 = (1896 + 250 + 286) ÷ 100 = 2432 ÷ 100 = 24.32

Answer: 24.3 (1 d.p.)

Exam tip: Check your answer lies between the lightest and heaviest mass numbers, and is closest to the most abundant isotope.

Working backwards (stretch)

If you know Ar and there are two isotopes, call the abundance of one isotope x%. The other is then (100 − x)%.

Worked example: boron

Boron has two isotopes, boron-10 and boron-11. Its Ar is 10.8. Find the abundance of boron-10.
10x + 11(100 − x) = 10.8 × 100
10x + 1100 − 11x = 1080, so x = 1100 − 1080 = 20

Answer: 20% boron-10 (and 80% boron-11)

Common mistakes

  • Changing the number of protons when an ion forms. Only electrons are lost or gained.
  • Thinking a negative ion has lost electrons. Negative means it has gained electrons.
  • Saying isotopes have different chemical properties. They have the same electron arrangement, so they react the same way.
  • Taking a simple average of the mass numbers, e.g. (35 + 37) ÷ 2 = 36 for chlorine. You must weight by abundance.
  • Forgetting to divide by 100 at the end of an Ar calculation.

Revision questions

Try these, then check your answers below each one.

Question 1. What are isotopes? (Choose from: Atoms of the same element with different numbers of neutrons, Atoms of different elements with the same number of neutrons, Atoms with the same mass number but different atomic numbers, Atoms of the same element with different numbers of electrons.)

Answer: Atoms of the same element with different numbers of neutrons

Working: Isotopes have the same number of protons but different numbers of neutrons.

Question 2. Boron is 20% boron-10 and 80% boron-11. Calculate the relative atomic mass of boron.

Answer: 10.8

Working: (200 + 880) ÷ 100 = 10.8.

Question 3. True or false? Isotopes of the same element have different chemical properties.

Answer: False

Working: False. Isotopes have the same electron arrangement, so they react in the same way.

Related lessons

Continue in the full course

This lesson is section 1.7 (AGCCH0107) of our free GCSE Chemistry course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Chemistry course.

 

 

 

 

 

 

 

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