Ionic, Covalent & Metallic Bonding

Ionic bonding

Ionic bonding occurs between a metal and a non-metal. The metal atom loses one or more electrons and the non-metal atom gains them, forming oppositely charged ions. The ions are held together by strong electrostatic attraction.

How it forms

1. The metal atom loses one or more electrons.
2. The non-metal atom gains those electrons.
3. Oppositely charged ions form.
4. Electrostatic attraction holds the ions together in a giant ionic lattice.

Example: sodium chloride

A sodium atom loses one electron to form a ${\text{Na}}^{+}$ ion. A chlorine atom gains one electron to form a ${\text{Cl}}^{-}$ ion. Opposite ions attract in all directions, building a giant ionic lattice of alternating positive and negative ions.

Properties of ionic compounds

• High melting and boiling points, because strong electrostatic forces need a lot of energy to overcome.
• Hard but brittle – a lattice can shatter if a layer of ions is forced to line up with like charges.
• Often soluble in water.
• Do not conduct electricity as solids, because the ions are fixed in the lattice. They do conduct when molten or dissolved, because the ions are free to move and carry charge.

Covalent bonding

Covalent bonding occurs between non-metal atoms. Atoms share one or more pairs of electrons so that each atom gains a full outer shell. The shared electrons are attracted to the nuclei of both atoms.

Examples

• Hydrogen chloride (HCl) – one pair of electrons is shared between hydrogen and chlorine.
• Oxygen (${\text{O}}_2$) – two pairs of electrons are shared, forming a double bond.
• Methane (${\text{CH}}_4$) – carbon shares electrons with four hydrogen atoms, giving four single covalent bonds.

Properties of simple covalent substances

• Generally low melting and boiling points, because the forces between molecules are weak.
• Often gases, liquids or soft solids at room temperature.
• Do not conduct electricity, because there are no free-moving charged particles.
• Usually insoluble in water, although some polar covalent substances (such as sugar) do dissolve.

Metallic bonding

Metallic bonding occurs between metal atoms. Each metal atom loses its outer-shell electrons, which become delocalised – free to move throughout the structure. The positive metal ions are held together by strong attraction to this "sea" of delocalised electrons.

Examples

Iron (Fe), copper (Cu) and aluminium (Al) are all held together by metallic bonding.

Properties of metals

• Generally high melting and boiling points.
• Strong and malleable – layers of positive ions can slide over each other without breaking the bond to the delocalised electrons.
• Good conductors of electricity and heat, because the delocalised electrons are free to move and carry charge or thermal energy.
• Solid at room temperature, except mercury, which is a liquid.
• Shiny (lustrous) appearance.