Ionic Bonding

• A dot-and-cross diagram shows the transfer of electrons when atoms become ions and an ionic bond forms
• Conventions for IGCSE:
  • Use dots for one atom's electrons and crosses for the other atom's electrons
  • Show only the outer-shell electrons (Edexcel does not require inner shells)
  • Place each finished ion inside square brackets with its charge written as a superscript at the top-right, outside the brackets
  • Any transferred electron keeps the symbol of its original atom (so a cross from chlorine onto the sodium ion is still drawn as a cross)

Sodium chloride: Na⁺ and Cl⁻

• Sodium (Group 1) has 1 outer electron and loses it
• Chlorine (Group 7) has 7 outer electrons and gains 1 to make 8
• One electron transfers from Na to Cl
• The Na atom becomes Na⁺ (configuration 2,8); the Cl atom becomes Cl⁻ (configuration 2,8,8)
• Formula: NaCl

Magnesium oxide: Mg²⁺ and O²⁻

• Magnesium (Group 2) has 2 outer electrons and loses both
• Oxygen (Group 6) has 6 outer electrons and gains 2 to make 8
• Two electrons transfer from Mg to O
• The Mg atom becomes Mg²⁺; the O atom becomes O²⁻
• Formula: MgO

What goes in, what stays out

• Only the outer shell of each atom is drawn (inner shells are omitted at IGCSE)
• The transferred electrons still belong to their original atom and so are drawn as their original symbol on the new ion
• Square brackets and charge superscripts go around each finished ion, not around the starting atoms