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Electrovalent bonding is taught in the note for Chemical Bonding. Read it below and look for the section on this topic.

Chemical Bonding

11 min read

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.

Ionic bond in sodium chlorideA sodium atom with one outer electron gives that electron to a chlorine atom with seven outer electrons. Sodium becomes a positive ion with a full outer shell of 8 and chlorine becomes a negative ion with a full outer shell of 8. The opposite charges attract and hold the ions together.NaClelectron transferredNa atom 2,8,1Cl atom 2,8,7Na+Cl−Na⁺ ion 2,8Cl⁻ ion 2,8,8⇄electrostatic attraction
One electron moves from sodium to chlorine.
Two electrons move from magnesium to oxygen, so the ions are Mg²⁺ and O²⁻.

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.

Covalent bondsDot and cross diagrams of three molecules. In hydrogen two atoms share one pair of electrons. In water the oxygen atom shares one pair with each of two hydrogen atoms. In carbon dioxide the carbon atom shares two pairs with each oxygen atom, making two double bonds.HHHydrogen H₂one shared pairOHHWater H₂Otwo single bondsOCOCarbon dioxide CO₂two double bonds
Each atom now has eight outer electrons.
A triple bond makes the nitrogen molecule very stable.

Properties of ionic and covalent compounds

PropertyIonic compounds (e.g. NaCl)Simple covalent compounds (e.g. water, CO₂)
StructureGiant lattice of ionsSeparate small molecules
Melting and boiling pointsHighLow
State at room temperatureSolidsGases, liquids or soft solids
Conduct electricityNot when solid; yes when molten or dissolvedNo
Solubility in waterOften solubleOften not soluble (some dissolve)
Try itFormula of an ionic compound

Choose the charge on the metal ion and the non-metal ion. The ions combine in the ratio that makes the total charge zero.

Check yourself Which type of bond forms between a metal and a non-metal?
Check yourself How many electrons are shared in the double bond of an oxygen molecule O₂?
Check yourself Why does solid sodium chloride not conduct electricity?

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.
Covalent bonding