Ionic vs Covalent Bonds
By the Periodixy Editorial Team · Last reviewed July 10, 2026
Every chemical bond is a tug-of-war over electrons, and the outcome defines the two great bond families. When one atom wins outright and takes electrons from the other, the bond is ionic. When the atoms share, the bond is covalent. That single difference explains why salt shatters but wax bends, why salt water conducts electricity and sugar water doesn't.
Ionic bonds: electrons transferred
An ionic bond forms when a metal hands one or more electrons to a nonmetal. The metal becomes a positive ion (cation), the nonmetal a negative ion (anion), and the opposite charges lock the ions into a rigid crystal lattice. Classic example: sodium gives its lone outer electron to chlorine, producing the Na⁺ and Cl⁻ ions of table salt.
- Partners: a metal + a nonmetal (large electronegativity difference, roughly > 1.7)
- Structure: continuous ion lattice — no individual molecules
- Typical properties: hard, brittle crystals; high melting points; conduct electricity when molten or dissolved
- Examples: NaCl, CaCl₂, MgO, KBr
Covalent bonds: electrons shared
A covalent bond forms when two nonmetals share one or more electron pairs, creating a discrete molecule. Water shares electrons between oxygen and hydrogen; methane shares four pairs around one carbon. Sharing can be equal (nonpolar, as in O₂) or lopsided (polar, as in H₂O) when one atom pulls harder — see electronegativity.
- Partners: nonmetal + nonmetal (small electronegativity difference)
- Structure: individual molecules with definite formulas
- Typical properties: lower melting/boiling points; many are gases or liquids; do not conduct electricity even when dissolved
- Examples: H₂O, CO₂, CH₄, NH₃, O₂
Side-by-side comparison
| Property | Ionic bond | Covalent bond |
|---|---|---|
| Electrons are… | transferred | shared |
| Formed between | metal + nonmetal | nonmetal + nonmetal |
| Particles produced | ions in a lattice | molecules |
| Melting point | high (NaCl: 801 °C) | usually low (H₂O: 0 °C) |
| Conducts electricity? | yes, molten or dissolved | no (acids are the exception) |
| State at room temp | solid crystals | solid, liquid or gas |
| Dissolves in water? | often, into ions | polar ones often; nonpolar rarely |
Worked classification practice
Classify these five compounds
Are MgCl₂, CO₂, NH₃, KBr and CH₄ ionic or covalent?
- MgCl₂: magnesium is a metal, chlorine a nonmetal → ionic.
- CO₂: carbon and oxygen are both nonmetals → covalent.
- NH₃: nitrogen and hydrogen are nonmetals → covalent.
- KBr: potassium (metal) + bromine (nonmetal) → ionic.
- CH₄: carbon and hydrogen → covalent.
Answer: Ionic: MgCl₂, KBr. Covalent: CO₂, NH₃, CH₄.
Why the properties follow from the bonding
Melting an ionic solid means overpowering the attraction between every pair of oppositely charged ions in the lattice — hence the high melting points and brittleness (shift the layers and like charges repel, cracking the crystal). Melting a covalent substance only needs the weak attractions between molecules to loosen; the strong bonds inside each molecule stay intact. And conduction needs mobile charges: dissolved ions carry current, whereas dissolved neutral molecules cannot — which is why the ions guide calls salt water an electrolyte.
Summary
- Ionic = electrons transferred (metal + nonmetal) → ion lattice.
- Covalent = electrons shared (nonmetal + nonmetal) → molecules.
- Ionic compounds: high melting points, brittle, conduct when molten/dissolved.
- Covalent compounds: lower melting points, don't conduct.
- Moderate sharing differences give polar covalent bonds — it's a spectrum.