Covalent Bonding

In short: A covalent bond is a pair of electrons shared between two non-metal atoms, so each atom fills its outer shell. Atoms can share one, two or three pairs (single, double and triple bonds), and covalent molecular substances have low melting points and do not conduct.

What is a covalent bond?

A covalent bond is a shared pair of electrons between two non-metal atoms. Both nuclei are attracted to the shared pair, and that holds the atoms together.

A group of atoms held together by covalent bonds is a molecule. Examples: ${\mathrm{H}}_2$, $\mathrm{C}{\mathrm{l}}_2$, ${\mathrm{H}}_2\mathrm{O}$, $\mathrm{C}{\mathrm{O}}_2$, $\mathrm{C}{\mathrm{H}}_4$.

How many bonds can an atom make?

An atom shares enough electrons to fill its outer shell. The number of bonds it usually makes is its valency.

Electron dot diagrams

An electron dot diagram (Lewis diagram) shows only the outer electrons. Use dots for one atom and crosses for the other so you can see where each shared electron came from.

Pairs of outer electrons that are not shared are called lone pairs. Oxygen in water has two lone pairs.

Structural formulas

A structural formula shows each shared pair as a line.

Triple bonds are stronger and shorter than double bonds, which are stronger than single bonds. The triple bond in ${\mathrm{N}}_2$ is so strong that nitrogen gas, 78% of our air, hardly reacts.

Diatomic elements

Seven elements exist as pairs of atoms (${\mathrm{X}}_2$) joined by covalent bonds: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂.

Naming covalent compounds

For two non-metals, use prefixes to show how many of each atom.

• The second element ends in -ide.
• Leave out mono on the first element.

$\mathrm{CO}$ is carbon monoxide. $\mathrm{C}{\mathrm{O}}_2$ is carbon dioxide. ${\mathrm{N}}_2{\mathrm{O}}_4$ is dinitrogen tetroxide. $\mathrm{S}{\mathrm{F}}_6$ is sulfur hexafluoride.

Some molecules keep everyday names: water (${\mathrm{H}}_2\mathrm{O}$), ammonia ($\mathrm{N}{\mathrm{H}}_3$), methane ($\mathrm{C}{\mathrm{H}}_4$).

Naming acids

An acid is a compound that releases hydrogen ions, ${\mathrm{H}}^{+}$, when dissolved in water. Its formula usually starts with H. Acids have their own naming rules.

Binary acids contain hydrogen and one other element. Use hydro-, the root of the element, -ic, then "acid".

Oxyacids contain hydrogen, oxygen and another element (a polyatomic ion). Name them from the ion, with no "hydro":

• Ion ending in -ate becomes -ic acid. "I ate something icky."
• Ion ending in -ite becomes -ous acid.

Properties of covalent molecular substances

The covalent bonds inside each molecule are strong. But the forces between molecules (intermolecular forces) are weak.

• Low melting and boiling points: many are gases or liquids at room temperature. Only the weak forces between molecules need to break.
• Do not conduct electricity: there are no ions or free electrons.
• Often soft as solids, like wax or sugar.

Quick summary

• A covalent bond is a shared pair of electrons between two non-metal atoms.
• Atoms share enough electrons to fill their outer shell. H makes 1 bond, O makes 2, N makes 3, C makes 4.
• Single, double and triple bonds share 1, 2 and 3 pairs.
• Seven diatomic elements: ${\mathrm{H}}_2,{\mathrm{N}}_2,{\mathrm{O}}_2,{\mathrm{F}}_2,\mathrm{C}{\mathrm{l}}_2,\mathrm{B}{\mathrm{r}}_2,{\mathrm{I}}_2$.
• Name covalent compounds with prefixes: carbon dioxide, dinitrogen tetroxide.
• Covalent molecular substances have low melting points and do not conduct.

Quiz