An ionic bond forms when one atom gives electrons to another and the resulting opposite charges attract. A covalent bond forms when two atoms hold on to the same pair of electrons. Transfer or sharing: every other difference between the two grows out of that.
The comparison table
| Ionic bond | Covalent bond | |
|---|---|---|
| How it forms | Complete transfer of electrons | Mutual sharing of electrons |
| Usually between | A metal and a non-metal | Two non-metals |
| Particles produced | Positive and negative ions | Molecules, or a giant network |
| Force holding it together | Electrostatic attraction between ions | Attraction of both nuclei for the shared pair |
| Direction | Non-directional | Directional, which gives molecules a definite shape |
| Melting and boiling points | High | Usually low |
| Conducts electricity | When molten or in water, not as a solid | Generally not |
| Solubility | Often soluble in water | Often soluble in organic solvents |
| Example | NaCl, MgO, CaCl₂ | H₂O, CH₄, CO₂ |
How an ionic bond forms: sodium chloride
Sodium has the electron arrangement 2, 8, 1. Chlorine has 2, 8, 7.
- Sodium loses its one outer electron and becomes Na⁺, with the arrangement 2, 8.
- Chlorine gains that electron and becomes Cl⁻, with the arrangement 2, 8, 8.
Both ions now have a complete outer shell, and the opposite charges pull them together. There is no separate "molecule" of salt. Each Na⁺ is surrounded by Cl⁻ ions and each Cl⁻ by Na⁺ ions in a repeating crystal lattice, which is why the formula NaCl gives a ratio and not a count.
How a covalent bond forms: water
Oxygen has six outer electrons and needs two more. Each hydrogen has one and needs one more. Neither atom is able to pull an electron away from the other completely, so they share.
Oxygen shares one pair with each hydrogen atom. Every shared pair counts toward both atoms, so oxygen reaches eight outer electrons and each hydrogen reaches two. The result is a separate, countable H₂O molecule.
Sharing one pair gives a single bond, as in H–H. Sharing two pairs gives a double bond, as in O=O. Sharing three gives a triple bond, as in N≡N.
Why the properties differ
Melting point. To melt an ionic solid you must loosen a lattice in which every ion is held by all its neighbours, and that takes a great deal of energy. To melt a simple covalent substance you only separate whole molecules from one another; the bonds inside each molecule stay intact. That is why salt stays solid on a hot pan while ice melts in your hand.
Conduction. Molten or dissolved ionic compounds contain ions that are free to move, so they conduct. Most covalent substances have neither free ions nor free electrons.
Brittleness. Hit an ionic crystal and the layers shift until like charges sit next to each other. They repel, and the crystal splits.
For how these two bonds fit alongside metallic bonding, hybridisation and molecular shape, see chemical bonding explained with examples.
The exceptions examiners like
- Diamond and silicon dioxide are covalent but have very high melting points, because the covalent bonds run through the whole crystal as a network. There are no small molecules to separate.
- Graphite is covalent and conducts electricity, because each carbon atom has one electron free to move through its layer.
- Hydrogen chloride is covalent as a gas but ionises in water.
- Bonds are rarely purely one kind. When two different atoms share electrons, the more electronegative atom pulls the pair closer, giving a polar covalent bond. Ionic and non-polar covalent are the two ends of a range, and electronegativity difference tells you roughly where a bond sits on it.
Where marks are lost
- Calling NaCl a molecule. Write "formula unit" for ionic compounds.
- Saying ionic solids conduct. They conduct only when molten or in solution.
- Saying covalent bonds are weak. The bonds inside a molecule are strong. It is the forces between molecules that are weak, and those are what break on melting.
- Electron dot structures with the wrong total. Count the outer electrons you started with and make sure the same number appears in the finished diagram.
Quick check
Classify the bonding in MgO, NH₃ and CaCl₂, and say which would conduct electricity when molten.
MgO and CaCl₂ are ionic, each a metal with a non-metal, and both conduct when molten. NH₃ is covalent and does not.