Molecular Covalent Structures

Covalent structures are formed when two or more non-metal atoms share electrons, creating a bond.
The force of attraction between these shared electrons and the nuclei of the atoms involved is called a covalent bond.
These shared electrons tend to be in the outermost shells of the atoms involved, also known as the valence shells.
An atom’s ability to attract shared electrons in a covalent bond is referred to as electronegativity. When two atoms in a bond have a large difference in electronegativity, the bond is polar.
Single, double, or triple covalent bonds can form, depending on the number of electron pairs being shared.
Atoms will share electrons in such a way that they can achieve an octet of electrons in their outer shell, with the exception of hydrogen which is stable with two electrons.

Types of Molecular Structures

Molecular structures exist in three forms: single molecules, giant covalent structures, and polymers.
Single molecules include elements such as hydrogen (H2), oxygen (O2) and nitrogen (N2), and compounds like water (H2O), carbon dioxide (CO2) and methane (CH4).
Giant covalent structures contain many atoms, each connected to adjacent atoms by covalent bonds. Examples include diamond, quartz (silica), and graphite.
Polymers are large molecules made up of smaller, repeating units called monomers.

Covalent molecules typically have low melting and boiling points which are due to their weak intermolecular forces.
Giant covalent structures show exception in this case as their many covalent bonds require a large amount of energy to break, resulting in high melting and boiling points.
Covalent molecules are often poor conductors of heat and electricity as they lack free electrons or ions.
Many covalently bonded molecules are soluble in polar solvents, such as water, but are generally insoluble in non-polar solvents.

Lewis dot structures or electron dot diagrams, are depictions of molecules that show bonding between atoms of a molecule and the lone pairs of electrons in the molecule.
They are a simple way to represent the valence electrons of an atom or ion, and are especially informative for molecules of small to medium size.

When a molecule cannot be accurately represented by a single Lewis structure, multiple structures may be used. These alternative structures are called resonance structures.
The real structure is a hybrid or blend of the resonance structures. This is referred to as a resonance hybrid.
Resonance in molecules tends to stabilize the molecule, making it less reactive.