3.13 - Covalent Bonding

Covalent bonding

• A covalent bond forms when two non-metal atoms share a pair of electrons.
• The bond is held together by the strong electrostatic attraction between the shared pair of negatively charged electrons and the positively charged nuclei of the atoms.
• Atoms share electrons to gain a full outer shell (a stable noble-gas arrangement).
• Several non-metal elements exist as diatomic molecules (e.g. H₂, N₂, O₂, Cl₂).

Hydrogen (H₂)

• Each hydrogen atom has one outer electron and needs one more for a full outer shell.
• Two hydrogen atoms share one pair of electrons, forming a single covalent bond (H-H).

Representing covalent bonds

• Dot-and-cross diagram - Shows outer-shell electrons, using dots for one atom and crosses for the other, with the shared pair in the overlap between shells.
• Displayed formula - Shows each shared pair of electrons as a single line: H-H.

Example covalent molecules

A lone pair is a pair of outer-shell electrons that isn't shared in a covalent bond.

Single-bond molecules

• Chlorine, Cl₂ - Two chlorine atoms share one pair of electrons (one single bond), leaving each chlorine with three lone pairs.
• Hydrogen chloride, HCl - Hydrogen and chlorine share one pair of electrons (one single bond), leaving chlorine with three lone pairs.
• Water, H₂O - Oxygen shares one pair of electrons with each of two hydrogen atoms (two single bonds), leaving oxygen with two lone pairs.
• Ammonia, NH₃ - Nitrogen shares one pair of electrons with each of three hydrogen atoms (three single bonds), leaving nitrogen with one lone pair.
• Methane, CH₄ - Carbon shares one pair of electrons with each of four hydrogen atoms (four single bonds), leaving no lone pairs.

Double and triple bonds

• Oxygen, O₂ - Two oxygen atoms share two pairs of electrons to form a double covalent bond (O=O), leaving each atom with two lone pairs.
• Nitrogen, N₂ - Two nitrogen atoms share three pairs of electrons to form a triple covalent bond (N≡N), leaving each atom with one lone pair.
• Carbon dioxide, CO₂ - Carbon shares two pairs of electrons with each of two oxygen atoms, forming two double bonds (O=C=O).

Displayed formulas

• A single line represents a single bond, a double line represents a double bond, and a triple line represents a triple bond.
• Displayed formulas show only the bonds, so lone pairs aren't drawn.

Comparing bonding diagrams and deducing formulas

Ways to represent molecules

• Dot-and-cross diagram - Shows where the outer electrons come from and how they're shared, but doesn't show 3D shape or relative atom sizes.
• Displayed formula - Shows how atoms are connected and the types of bonds, but doesn't show 3D shape or which atom each electron came from.
• 3D model (ball-and-stick) - Shows the 3D arrangement of atoms in space, but doesn't show outer-shell electrons.

Deducing a molecular formula

• Count the number of atoms of each element in the diagram or model to write the molecular formula.
• For example, a molecule containing 2 carbon atoms and 6 hydrogen atoms has the formula C₂H₆ (ethane).