Formation of Ions

  • An ion is an electrically charged atom or group of atoms formed by the loss or gain of electrons.
  • This loss or gain of electrons takes place to obtain a full outer shell of electrons.
  • The electronic structure of ions of elements in Groups I, II, VI and VII will be the same as that of a noble gas – such as helium, neon and argon.
  • Negative ions are called anions and form when atoms gain electrons, meaning they have more electrons than protons.
  • Positive ions are called cations and form when atoms lose electrons, meaning they have more protons than electrons.
  • All metals lose electrons to other atoms to become positively charged ions.
  • All non-metals gain electrons from other atoms to become negatively charged ions.

Formation of positively charged Sodium ion1

Diagram showing the formation of the sodium ion

Formation-of-negatively-charged-Chloride-ion1, IGCSE & GCSE Chemistry revision notes

Diagram showing the formation of the chloride ion

  • The positive and negative charges are held together by the strong electrostatic forces of attraction between opposite charges.
  • This is what holds ionic compounds together.

Oppositely charged ions attraction due to electrostatic attraction, IGCSE & GCSE Chemistry revision notes

Dot-and-cross diagram of sodium chloride

Representing Ionic Bonds

  • Ionic bonds can be represented diagrammatically using dot-and-cross diagrams.
  • These are a simple and quick way to show the formation of an ionic compound.
  • The electrons from each atom should be represented by using solid dots and crosses.
  • If there are more than two atoms, then hollow circles or other symbols / colours may be used.
  • The large brackets should encompass each atom and the charge should be in superscript and on the right hand side outside the brackets.
  • For larger atoms with more electron shells, only the valence shell needs to be drawn.

Ionic bonding – Sodium Chloride, IGCSE & GCSE Chemistry revision notes

Diagram representing the formation of the ionic bond in sodium chloride

Example: Sodium Chloride

  • Sodium is a Group I metal so will lose one outer electron to another atom to gain a full outer shell of electrons.
  • A positive sodium ion with the charge +1 is formed.
  • Chlorine is a Group VII non-metal so will need to gain one electron to have a full outer shell of electrons.
  • One electron will be transferred from the outer shell of the sodium atom to the outer shell of the chlorine atom forming a positive sodium ion with a +1
  • charge.
  • The chlorine atom will gain an electron to form a negatively charged chloride ion with a charge of -1.
  • The ions are then attracted to one another and held together by electrostatic forces.
  • The formula of the ionic compound is thus NaCl.

Oppositely charged ions attraction due to electrostatic attraction, IGCSE & GCSE Chemistry revision notes

Dot-and-cross diagram of sodium chloride

Example: Magnesium oxide

  • Magnesium is a Group II metal so will lose two outer electrons to another atom to have a full outer shell of electrons.
  • A positive ion with the charge +2 is formed.
  • Oxygen is a Group VI non-metal so will need to gain two electrons to have a full outer shell of electrons.
  • Two electrons will be transferred from the outer shell of the magnesium atom to the outer shell of the oxygen atom.
  • Oxygen atom will gain two electrons to form a negative ion with charge -2.
  • The ions are then attracted to one another and held together by electrostatic forces.
  • The formula of the ionic compound is thus MgO.

Magnesium Oxide dot & cross diagram, IGCSE & GCSE Chemistry revision notes

Dot-and-cross diagram of magnesium oxide

Working out the Formulae

  • You may be asked to give the formula of an ionic compound from a given diagram.
  • If it is a dot-and-cross diagram then just count the number of atoms of each element.
  • This is then equal to the empirical formula of the compound.
  • If it is a 3D lattice structure then look for how many ions are in the lattice.
  • Write them down and balance the charges to find the formula for the compound.

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.