Structures and Bonding: Properties of Substances with Covalent Bonding

Covalent bonding occurs when two non-metal atoms share a pair of electrons. This strong bond often leads to the formation of small molecules or large covalent structures.
Small molecules with covalent bonding include water (H2O), methane (CH4) and carbon dioxide (CO2). These have relatively low melting and boiling points due to the weak intermolecular forces (forces between the molecules) that can be easily overcome.
Larger covalent structures include diamond and graphite (both forms of carbon), and silicon dioxide (silica). These have high melting and boiling points as all atoms are connected by strong covalent bonds.
In covalent substances, each atom contributes at least one electron to the bond. The atom feels as if it has a full outer shell of electrons, achieving a stable electron arrangement.
Oxygen and nitrogen are examples of molecules that form double and triple covalent bonds respectively. This is due to the need for more shared electron pairs to reach a stable electron arrangement.
Covalent compounds do not conduct electricity in any state (solid, liquid or aqueous). This is because there are no free electrons or ions to carry the charge.
The properties of substances can also be attributed to their state. Gases consist of small covalent molecules which move freely and quickly, while large covalent structures are solid due to the strong bonds holding the particles fixed in position.
Most covalently bonded substances are insoluble in water since they cannot form interactions with water molecules. However, there are exceptions, such as alcohols and sugars, which can form intermolecular bonds with water making them soluble.
Polar covalent bonds occur when electrons are shared unevenly, resulting in regions of partial positive and negative charge. A classic example is water, where the oxygen atom holds the shared electrons closer than the hydrogen atoms. This creates a polar molecule with being partially negatively charged near the oxygen and partially positively charged near the hydrogens. This polarity allows water to form hydrogen bonds with other polar molecules and ions, which is important for its many functional roles in life processes.
Non-polar covalent bonds happen when electrons are shared evenly. This results in a neutral molecule where all parts of the molecule have the same charge. An example of this is oxygen gas (O2).