16.Covalent Bonding: Formation & Representation

Sharing Electrons: Covalent bonds form when two non-metal atoms share pairs of electrons so that both can achieve a full outer shell.
Why it Happens: Unlike ionic bonding (where electrons are transferred), non-metals both need extra electrons, so sharing is the only way for both to become stable.

Dot and Cross Diagrams:
- Used to show which electrons belong to which atom (using dots for one and crosses for the other).
- The shared electrons are placed in the overlap between the two shells.
Displayed Formula:
- Uses the chemical symbols of atoms joined by lines (e.g., H—H).
- Each line represents one shared pair of electrons (one covalent bond).
- This is best for drawing large, complex molecules like glucose.
3D Models:
- Show how the atoms are arranged in real-life space, though they can be harder to predict or draw.

Ammonia (NH3): Nitrogen needs 3 more electrons, and each of the three Hydrogens needs 1. By sharing one electron from each Hydrogen with the Nitrogen, all atoms become stable.
Water (H2O): Oxygen shares electrons with two Hydrogen atoms.
Methane (CH4): Carbon shares electrons with four Hydrogen atoms.

Simple Molecular Substances: Small molecules (like O2, H2O, NH3) held together by strong covalent bonds. However, the forces between the molecules (intermolecular forces) are very weak and easy to break.
Polymers: Very long chains of repeating units (monomers) joined by covalent bonds, used to make plastics.
Giant Covalent Structures: Huge networks of billions of atoms all joined by covalent bonds (e.g., Diamond, Graphite, Silicon Dioxide). These are extremely strong because every atom is bonded to its neighbors.