Covalent Bonding
Understand how non-metal atoms share electrons to form molecules
Electron Sharing
Single, Double, and Triple Bonds
Covalent bonding occurs when two non-metal atoms share one or more pairs of electrons. Both atoms are attracted to the shared electrons, holding them together. This sharing allows each atom to achieve a full outer shell of electrons, making them more stable.
A single bond involves sharing one pair of electrons (e.g., H₂, HCl). A double bond involves sharing two pairs (e.g., O₂, CO₂). A triple bond involves sharing three pairs (e.g., N₂). Multiple bonds are shorter and stronger than single bonds.
Covalent compounds can be represented using dot-cross diagrams (showing electron arrangement), structural formulae (showing bonds as lines), or molecular formulae (showing the number of atoms).
Hydrogen
Single bond
Two hydrogen atoms share one pair of electrons, forming a single covalent bond.
Simple molecular compounds (e.g., H₂O, CO₂, CH₄) are made of small molecules. The covalent bonds within molecules are strong, but the forces between molecules (intermolecular forces) are weak. This gives them low melting and boiling points—little energy is needed to separate the molecules. They don't conduct electricity because there are no free charges.
Giant covalent structures (e.g., diamond, graphite, silicon dioxide) have billions of atoms joined by covalent bonds in a continuous network. They have very high melting pointsbecause many strong covalent bonds must be broken. Diamond is extremely hard; graphite has layers that can slide (making it slippery) and conducts electricity due to delocalised electrons between layers.
Covalent Bond
Click to reveal definition
Question:
Explain why water (H₂O) has a much lower boiling point than silicon dioxide (SiO₂), even though both contain covalent bonds.
Solution:
Water (H₂O): Water is a simple molecular compound. It exists as small, discrete molecules. The covalent bonds within each H₂O molecule are strong, but the intermolecular forces between water molecules are relatively weak. When water boils, only these weak intermolecular forces need to be overcome, not the covalent bonds. This requires little energy, so water has a low boiling point (100°C).
Silicon dioxide (SiO₂): Silicon dioxide is a giant covalent structure. Each silicon atom is covalently bonded to four oxygen atoms in a continuous 3D network—there are no separate molecules. When SiO₂ melts or boils, many strong covalent bonds must be broken throughout the structure. This requires a very large amount of energy, giving SiO₂ a very high boiling point (2230°C).
What type of atoms form covalent bonds?