Atoms bond because they become more stable when their outer electron shells are full, usually with eight electrons (the octet rule). How they get there decides the bond type. Understanding the difference between ionic and covalent bonds explains a lot of everyday chemistry: why salt dissolves in water, why oil does not and why metals conduct electricity.
Ionic bonds: giving and taking electrons
An ionic bond forms when one atom transfers electrons to another. This typically happens between a metal, which loses electrons easily, and a nonmetal, which gains them. The metal becomes a positive ion (cation) and the nonmetal a negative ion (anion), and the opposite charges attract.
Example: sodium has one outer electron and chlorine has seven. Sodium gives its electron to chlorine, forming Na⁺ and Cl⁻. Both now have full outer shells. In solid sodium chloride, the ions stack into a repeating 3D lattice in which each Na⁺ is surrounded by six Cl⁻ and vice versa.
Covalent bonds: sharing electrons
A covalent bond forms when two nonmetals share pairs of electrons. Neither atom is strong enough to take electrons from the other, so they share. Hydrogen gas (H₂) shares one pair, oxygen gas (O₂) shares two pairs (a double bond) and nitrogen gas (N₂) shares three pairs (a triple bond). More shared pairs means a shorter, stronger bond.
If the two atoms attract the shared electrons equally, the bond is nonpolar (as in Cl₂). If one atom pulls harder, the electrons spend more time near it and the bond is polar. In water, oxygen pulls the shared electrons away from hydrogen, giving oxygen a partial negative charge and the hydrogens partial positive charges.
Metallic bonds: a sea of electrons
In metals, outer electrons are not attached to any single atom. They move freely through a lattice of positive metal ions, a model often called a "sea of electrons." This explains why metals conduct electricity and heat, and why they are malleable: the layers of ions can slide without breaking the bonds.
How to predict the bond type
- Metal + nonmetal: ionic (for example NaCl, MgO, CaF₂).
- Nonmetal + nonmetal: covalent (for example H₂O, CO₂, CH₄).
- Metal + metal: metallic (for example copper, brass).
For a more precise answer, compare electronegativities. A difference above about 1.7 is generally ionic, between about 0.4 and 1.7 polar covalent, and below about 0.4 nonpolar covalent. Bonding is really a spectrum, and these cutoffs are guidelines.
How bond type explains properties
- Melting point: ionic compounds have high melting points because every ion is held by strong attractions in all directions. NaCl melts at 801 °C. Many simple covalent substances have low melting points because the forces between molecules are weak, even though the bonds inside each molecule are strong. Water boils at 100 °C and methane at about −162 °C.
- Electrical conductivity: ionic compounds conduct when melted or dissolved, because the ions can move, but not as solids. Most covalent compounds do not conduct. Metals conduct as solids thanks to their free electrons.
- Solubility: "like dissolves like." Ionic and polar substances tend to dissolve in polar water; nonpolar substances such as oil dissolve in nonpolar solvents.
Worked example: writing an ionic formula
What is the formula for aluminum oxide?
- Aluminum is in group 13 and loses three electrons: Al³⁺. Oxygen is in group 16 and gains two: O²⁻.
- Find the lowest common multiple of 3 and 2, which is 6. You need two Al³⁺ (total +6) and three O²⁻ (total −6).
- Formula: Al₂O₃. A shortcut is to "cross over" the charges as subscripts.
Common mistakes
- Saying covalent bonds are weak. The bonds inside molecules are strong; it is the attractions between molecules that are often weak.
- Calling NaCl a molecule. Ionic compounds form lattices, so we write formula units, not molecules.
- Forgetting to reduce ratios, such as writing Mg₂O₂ instead of MgO.
Practice questions
- Is KBr ionic or covalent? What about NH₃?
- Write the formula for calcium chloride.
- Why does salt water conduct electricity but pure water barely does?
- Why can you hammer copper into a sheet but not a salt crystal?
Answers: (1) KBr is ionic (metal + nonmetal); NH₃ is covalent (nonmetals). (2) CaCl₂. (3) Dissolved Na⁺ and Cl⁻ ions carry charge; pure water has very few ions. (4) Metal layers slide while the electron sea keeps them bonded; in a salt crystal, shifting layers lines up like charges, which repel and crack the crystal.
Last reviewed 2026-10-23 by the BrainBoost team.