How Ionic and Covalent Bonds Form

Two ways to stick atoms together

Table salt (sodium chloride) is a hard, white solid that melts at 801 °C. Methane, the main part of the natural gas that heats many Ontario homes, is a gas that boils at −162 °C. Both are compounds. The big difference in their properties comes from how their atoms are held together.

In this lesson you will compare the two main kinds of chemical bond: ionic bonds, where electrons are transferred, and covalent bonds, where electrons are shared. You will see how ionization energy, electron affinity and electronegativity from the earlier lessons predict which kind of bond forms.

Why do atoms bond at all?

Atoms bond because the bonded arrangement has lower energy than the separate atoms. Lower energy means more stable, just as a ball rolls downhill to a lower, more stable position. The noble gases already have full outer shells and very stable arrangements, which is why they rarely bond. Other atoms can reach a noble-gas arrangement by losing, gaining or sharing electrons. This is the octet rule: atoms tend to bond so that each has eight valence electrons (two for hydrogen, which matches helium).

Forming an ionic bond: electron transfer

An ionic bond forms between a metal with a low ionization energy and a non-metal with a large electron affinity: in practice, two atoms with a large difference in electronegativity.

1. The metal atom loses its valence electron(s) to become a cation. Sodium (2, 8, 1) → Na⁺ (2, 8), the arrangement of neon.
2. The non-metal atom gains those electrons to become an anion. Chlorine (2, 8, 7) → Cl⁻ (2, 8, 8), the arrangement of argon.
3. The oppositely charged ions attract each other. This electrostatic attraction between positive and negative ions is the ionic bond.

Removing an electron from sodium actually costs energy (its ionization energy). The reason the process happens is that huge numbers of ions attract each other and pack into a crystal lattice, which releases much more energy than was spent. In a crystal, every Na⁺ is attracted to all the Cl⁻ ions around it. There are no separate molecules in an ionic compound: the formula NaCl gives the ratio of ions (1:1), called a formula unit.

Forming a covalent bond: electron sharing

When two non-metal atoms meet, both have high ionization energies, so neither will give up an electron. Instead they share a pair of electrons. A covalent bond is the attraction of two nuclei for a shared pair of electrons between them.

• In H₂, each hydrogen atom shares its one electron. The shared pair counts for both atoms, so each has 2 electrons, like helium.
• In Cl₂, each chlorine (7 valence electrons) shares one electron. Each chlorine then has 6 unshared electrons plus the 2 shared ones = 8.
• Atoms can share more than one pair. O₂ has a double bond (2 shared pairs, 4 electrons). N₂ has a triple bond (3 shared pairs, 6 electrons).

Covalently bonded atoms form molecules: separate, neutral groups of atoms with a definite formula (H₂O, CO₂, CH₄). The molecular formula gives the actual number of atoms in one molecule.

A note on metals

Metal atoms bonded to each other (in a copper wire, for example) do neither. Their valence electrons are released into a shared "sea" that moves freely through a lattice of positive metal ions. This metallic bonding explains why metals conduct electricity. You will compare it with the other bond types in Grade 12 Chemistry (SCH4U); in this unit we focus on ionic and covalent bonds.