Why atoms bond
Atoms join to make compounds because bonded atoms have lower energy and are more stable. Most atoms become stable when they have a full outer shell, like the noble gases (the octet rule: 8 outer electrons, or 2 for hydrogen and helium). Atoms can reach a full shell by transferring electrons (ionic bonding) or by sharing them (covalent bonding).
Electrovalent (ionic) bonding
A metal atom loses its outer electrons to become a positive ion (cation). A non-metal atom gains them to become a negative ion (anion). The opposite charges attract strongly: this is the ionic bond. Ionic compounds are solids made of a regular lattice of ions.
Covalent bonding
Two non-metal atoms can each fill their outer shell by sharing pairs of electrons. A shared pair is one covalent bond. Sharing one pair makes a single bond, two pairs a double bond and three pairs a triple bond. The product is a molecule.
Properties of ionic and covalent compounds
| Property | Ionic compounds (e.g. NaCl) | Simple covalent compounds (e.g. water, CO₂) |
|---|---|---|
| Structure | Giant lattice of ions | Separate small molecules |
| Melting and boiling points | High | Low |
| State at room temperature | Solids | Gases, liquids or soft solids |
| Conduct electricity | Not when solid; yes when molten or dissolved | No |
| Solubility in water | Often soluble | Often not soluble (some dissolve) |
Choose the charge on the metal ion and the non-metal ion. The ions combine in the ratio that makes the total charge zero.
Key terms
- Ion
- An atom or group of atoms with an electric charge.
- Ionic bond
- The electrostatic attraction between oppositely charged ions.
- Covalent bond
- A pair of electrons shared between two atoms.
- Octet rule
- Atoms tend to gain, lose or share electrons to have eight outer electrons.
Summary
- Atoms bond to get full outer shells.
- Ionic bond: metal gives electrons to non-metal; strong attraction between ions.
- Covalent bond: atoms share electron pairs and form molecules.
- Ionic compounds have high melting points and conduct only when molten or dissolved; simple covalent compounds have low melting points and do not conduct.