Chemical bonding — ionic, covalent and metallic

Understanding bonding is essential for explaining the properties of substances. CCEA examiners frequently ask you to link bonding type to physical properties.

Ionic bonding

Ionic bonds form between metals and non-metals. Electrons are transferred from the metal to the non-metal:

• Metal loses electrons → becomes a positive ion (cation): e.g. Na → Na⁺
• Non-metal gains electrons → becomes a negative ion (anion): e.g. Cl → Cl⁻

The oppositely charged ions attract each other electrostatically, forming a giant ionic lattice.

Properties of ionic compounds:

• High melting and boiling points (strong electrostatic forces in the lattice require lots of energy to break)
• Solid at room temperature
• Conduct electricity when molten or dissolved in water (ions free to move), but NOT when solid (ions fixed)
• Often soluble in water (polar water molecules can pull apart the ions)

Example: sodium chloride (NaCl), magnesium oxide (MgO).

Covalent bonding

Covalent bonds form between non-metals. Electrons are shared between atoms to achieve full outer shells.

Simple covalent molecules (small): H₂O, CO₂, CH₄, O₂, N₂, HCl.

• Low melting/boiling points (weak intermolecular forces between molecules — NOT the strong covalent bonds within)
• Do NOT conduct electricity (no free electrons or ions)
• Many are gases or liquids at room temperature

Giant covalent structures (large lattice): diamond C, graphite C, silicon dioxide (SiO₂).

• Extremely high melting points (millions of strong covalent bonds to break)
• Very hard (except graphite)
• Generally do not conduct electricity (exception: graphite — delocalised electrons between layers)

Metallic bonding

Metal atoms lose their outer electrons, forming a sea of delocalised electrons surrounding a lattice of positive metal ions. The electrons are free to move throughout the structure.

Properties of metals:

• Good conductors of electricity (delocalised electrons carry current)
• Good conductors of heat (delocalised electrons and vibration)
• High melting points (strong metallic bonds)
• Malleable and ductile (layers of positive ions can slide over each other — the sea of electrons acts as a lubricant)
• Shiny (surface electrons reflect light)

Dot-and-cross diagrams

CCEA Chemistry requires you to draw dot-and-cross diagrams showing the arrangement of outer electrons:

• For ionic compounds: show the transfer of electrons, with square brackets and charges.
• For covalent molecules: show shared pairs of electrons between atoms.