Specification Point 1.40:
Draw dot-and-cross diagrams to show the formation of ionic compounds by electron transfer, limited to combinations of elements from Groups 1, 2, 3 and 5, 6, 7 only outer electrons need be shown

Dot-and-Cross Diagrams of Ionic Compounds

Ionic Compounds: Formed when atoms of metals transfer electrons to atoms of non-metals to form compounds made up of ions

Dot-and-Cross Diagrams: Shows the ionic bonds between ions by the loss and gain of electrons

  • Only the outer shell of electrons of each atom is drawn
  • Electrons in one atom will be represented as crosses, whilst electrons in the other atom will be represented as circles, indicating which ion gains or loses electrons
  • The charges of each ion is displayed at the upper right corner of each ion

Example 1:

Sodium Chloride, NaCl, has a typical ionic bond

Dot-and-cross Sodium Chloride, CIE IGCSE Chemistry

The dot-and-cross diagram of Sodium Chloride

Explanation

  • Sodium is a group 1 metal so will lose one outer electron to another atom to have a full outer shell of electrons
  • They will lose an electron to form a positive ion with the charge 1+
  • Chlorine is a group 7 non – metal so will need to gain an 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
  • Chlorine atom will gain an electron to form a negative ion with charge 1-

          Formula of Ionic Compound:    NaCl

Example 2:

Magnesium Oxide, MgO, has a typical ionic bond

Magnesium Oxide, CIE IGCSE Chemistry

The dot-and-cross diagram of Magnesium Oxide

Explanation

  • Magnesium is a group 2 metal so will lose two outer electrons to another atom to have a full outer shell of electrons
  • They will lose two electrons to form a positive ion with the charge 2+
  • Oxygen is a group 6 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-

          Formula of ionic compound:    MgO

Need help?

Aiming for a Level 9?

See if you’ve got what it takes. Test yourself with our topic questions.

Author: Jamie

Jamie got a First class degree in Chemistry from Oxford University before going on to teach chemistry full time as a professional tutor. He’s put together these handy revision notes to match the Edexcel IGCSE Chemistry specification so you can learn exactly what you need to know for your exams.