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3.2 Atoms and Ions

GCSE Chemistry · AQA · Chapter 3: Structure and Bonding

A free GCSE Chemistry lesson for AQA (8462) from OBA School: why atoms gain or lose electrons, the charges on ions, and how to write the formula of an ionic compound. 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

  • Describe the difference between a mixture and a compound, and between ionic and covalent bonding
  • Explain why metal atoms form positive ions and non-metal atoms form negative ions
  • Link the charge on an ion to the group number of the element
  • Write the electronic structure of simple ions
  • Work out the formula of an ionic compound from the charges on its ions, including compound ions

Key words

  • Ion: An atom or group of atoms with an electric charge, because it has lost or gained electrons.
  • Cation: A positive ion. Metals and hydrogen form positive ions.
  • Anion: A negative ion. Non-metals form negative ions.
  • Ionic compound: A compound made of positive and negative ions, usually a metal combined with a non-metal.
  • Noble gas structure: A full outer shell of electrons, like the Group 0 elements. It is very stable.
  • Compound ion: An ion made of more than one atom, such as sulfate, SO₄²−.

Elements, mixtures and compounds

A compound contains two or more elements chemically combined in a fixed ratio. In a mixture, the substances are only mingled together, so they keep their own properties and are usually easy to separate (for example, salt and sand). A compound has different properties from the elements it was made from, and it is hard to get the elements back.

Atoms of the noble gases (Group 0) already have a full outer shell, so they are very unreactive. Other atoms react to get a full outer shell, in one of two ways:

  • ionic; What the electrons do electrons are transferred from one atom to another, making ions; Usually between a metal and a non-metal
  • covalent; What the electrons do pairs of electrons are shared between atoms; Usually between non-metals only

Why atoms form ions

Atoms have equal numbers of protons and electrons, so they have no overall charge. When a metal reacts with a non-metal, electrons are transferred from the metal atoms to the non-metal atoms. Both end up with a full outer shell: the stable electronic structure of a noble gas (Group 0).

  • Metal atoms lose electrons from their outer shell → positive ions.
  • Non-metal atoms gain electrons into their outer shell → negative ions.
  • The number of protons never changes, so the element stays the same.

Key fact: Charge on an ion = number of protons − number of electrons. Electrons have a charge of −1 and protons +1.

The charge on an ion and the periodic table

For the main groups, the group number tells you how many electrons are in the outer shell. Atoms lose or gain the fewest electrons they can to reach a full outer shell.

  • 1; Outer electrons 1; Change lose 1; Ion charge 1+; Examples Li⁺, Na⁺, K⁺
  • 2; Outer electrons 2; Change lose 2; Ion charge 2+; Examples Mg²⁺, Ca²⁺
  • 3; Outer electrons 3; Change lose 3; Ion charge 3+; Examples Al³⁺
  • 4; Outer electrons 4; Change usually share instead; Ion charge none; Examples C and Si form covalent bonds
  • 5; Outer electrons 5; Change gain 3; Ion charge 3−; Examples N³−
  • 6; Outer electrons 6; Change gain 2; Ion charge 2−; Examples O²−, S²−
  • 7; Outer electrons 7; Change gain 1; Ion charge 1−; Examples F−, Cl−, Br−
  • 0; Outer electrons 8 (He: 2); Change none; Ion charge no ions; Examples already a full outer shell

Worked example: a Group 1 metal reacting with a Group 7 element

Potassium (2,8,8,1) reacts with bromine. Each K atom transfers its one outer electron to a Br atom.
K → K⁺ (2,8,8, like argon) and Br → Br− (full outer shell, like krypton).
Balanced equation: 2K(s) + Br₂(l) → 2KBr(s)

Answer: potassium bromide, KBr, made of K⁺ and Br− ions

Worked example: the electronic structure of an ion

Oxygen is 2,6 (8 protons, 8 electrons). It is in Group 6, so it gains 2 electrons.
The oxide ion has 8 protons and 10 electrons: charge = 8 − 10 = −2.

Answer: O²− has the structure 2,8 (the same as neon)

Naming: a metal ion keeps the element’s name (sodium ion). A simple negative ion ends in -ide: chloride, oxide, sulfide, bromide, nitride. Transition metals can form ions with different charges, shown by a Roman numeral, e.g. iron(II) is Fe²⁺ and iron(III) is Fe³⁺.

Working out the formula of an ionic compound

An ionic compound has no overall charge. The total positive charge must equal the total negative charge. The formula shows the simplest ratio of ions.

Worked example: aluminium oxide

Ions: Al³⁺ and O²−.
Lowest common multiple of 3 and 2 is 6: 2 × (+3) = +6 and 3 × (−2) = −6.
So 2 aluminium ions balance 3 oxide ions.

Answer: Al₂O₃

Exam tip: Quick check: add up the charges in your formula. For Al₂O₃: 2(+3) + 3(−2) = 0 ✓

Compound ions

Some ions contain several atoms bonded together. Learn these five:

  • hydroxide; Formula OH−; Name sulfate; Formula SO₄²−
  • nitrate; Formula NO₃−; Name carbonate; Formula CO₃²−
  • ammonium; Formula NH₄⁺; Name ; Formula

If you need more than one compound ion, put it in brackets and write the number after the bracket.

Worked example: calcium nitrate

Ions: Ca²⁺ and NO₃−. You need two nitrate ions to balance one calcium ion.

Answer: Ca(NO₃)₂, which contains 1 Ca, 2 N and 6 O atoms

Common mistakes

  • Saying a metal atom “gains a positive charge” or gains protons. It loses electrons; the number of protons stays the same.
  • Writing the charge in front of the symbol (+Na) or as Na¹⁺. Write Na⁺, Mg²⁺, O²−.
  • Leaving out brackets: CaNO₃₂ is wrong; write Ca(NO₃)₂.
  • Calling the negative ion by the element’s name: it is magnesium oxide, not magnesium oxygen.
  • Thinking an ion is a different element. Na⁺ is still sodium because it still has 11 protons.

Revision questions

Try these, then check your answers below each one.

Question 1. A magnesium atom (2,8,2) reacts to form an ion. Which ion forms? (Choose from: Mg⁶⁻, Mg⁺, Mg²⁺, Mg²⁻.)

Answer: Mg²⁺

Working: Magnesium loses its 2 outer electrons to get the structure 2,8. It then has 12 protons and 10 electrons, so the ion is Mg²⁺.

Question 2. How many electrons are in an oxide ion, O²⁻? (Oxygen is element 8.)

Answer: 10

Working: 8 + 2 = 10 electrons (structure 2,8).

Question 3. Write the formula of calcium chloride.

Answer: CaCl2

Working: One Ca²⁺ needs two Cl⁻ ions to balance the charge, so the formula is CaCl₂.

Related lessons

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This lesson is section 3.2 (AGCCH0302) 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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