Ionic Bonding

What an ion is

• An ion is a charged atom (or group of atoms) formed when an atom loses or gains electrons
• The number of electrons transferred equals the size of the charge:
  • lose 1 electron → 1+ charge
  • lose 2 electrons → 2+ charge
  • gain 1 electron → 1− charge
  • gain 2 electrons → 2− charge

Cations and anions

• A cation is a positive ion, formed when an atom loses electrons (it now has fewer electrons than protons)
• An anion is a negative ion, formed when an atom gains electrons (it now has more electrons than protons)
• The driving force in every case is to reach a full outer shell — the stable electron arrangement of a noble gas
  • Metals (1–3 outer electrons) lose those few electrons to form cations
  • Non-metals (5–7 outer electrons) gain electrons to form anions
• Group 4 elements rarely form simple ions (losing or gaining 4 electrons costs too much energy)
• Group 0 noble gases already have full outer shells and so do not form ions

Predicting ion charges from group number

Examples of ion formation

• Sodium (Group 1) has 1 outer electron. Losing it gives Na⁺, with configuration 2,8 (the same as neon)
• Magnesium (Group 2) has 2 outer electrons. Losing both gives Mg²⁺, configuration 2,8 (neon)
• Chlorine (Group 7) has 7 outer electrons. Gaining 1 gives Cl⁻, configuration 2,8,8 (argon)
• Oxygen (Group 6) has 6 outer electrons. Gaining 2 gives O²⁻, configuration 2,8 (neon)