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2.1 Ionic bonding

How metal atoms lose electrons and non-metal atoms gain electrons to form ions with full outer shells, drawing dot and cross diagrams for ionic compounds, and working out the numbers of protons, neutrons and electrons in ions.

AQA Edexcel OCR 2 min read

Forming ions

Key terms

  • Ion: an atom or group of atoms with a positive or negative charge.
  • Ionic bond: the strong electrostatic attraction between oppositely charged ions.

Ionic bonding happens when a metal reacts with a non-metal. Electrons are transferred from the metal atoms to the non-metal atoms:

  • metal atoms lose their outer electrons to form positive ions (cations)
  • non-metal atoms gain electrons to form negative ions (anions)

Both ions end up with a full outer shell, the same electronic structure as a noble gas. The oppositely charged ions attract each other strongly.

The charge on an ion depends on the group
GroupElectrons lost or gainedChargeExample
1lose 11+\mathrm{Na^+}
2lose 22+\mathrm{Mg^{2+}}
6gain 22−\mathrm{O^{2-}}
7gain 11−\mathrm{Cl^-}

Dot and cross diagrams

Dot and cross diagrams: sodium (2.8.1) gives one electron to chlorine (2.8.7) forming Na+ (2.8) and Cl− (2.8.8); magnesium (2.8.2) gives two electrons to oxygen (2.6) forming Mg2+ and O2− (both 2.8)
Dot and cross diagrams: sodium (2.8.1) gives one electron to chlorine (2.8.7) forming Na+ (2.8) and Cl− (2.8.8); magnesium (2.8.2) gives two electrons to oxygen (2.6) forming Mg2+ and O2− (both 2.8)
  • Show the electrons of one atom as dots and the other as crosses, so you can see where each electron came from.
  • Draw each ion in square brackets with its charge at the top right.
  • You can draw just the outer shell of each ion.

Magnesium chloride: a magnesium atom has 2 outer electrons, but a chlorine atom only needs 1. So one magnesium atom gives one electron to each of two chlorine atoms, forming \mathrm{Mg^{2+}} and two \mathrm{Cl^-} ions: MgCl2.

Particles in ions

  • protons = atomic number (this never changes)
  • neutrons = mass number − atomic number
  • electrons = protons − the positive charge, or protons + the negative charge
Particles in some ions
IonProtonsNeutronsElectrons
sodium ion, mass number 23111210
chloride ion, mass number 35171818
magnesium ion, mass number 24121210
oxide ion, mass number 168810

Test yourself on this lesson

6 exam questions · 13 marks · with mark schemes and model answers

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