Chemical Bonding Principles: Octet Rule and Molecular Structures

Chemical Bonding

Chemical bonding is the union between atoms to form a stable grouping.

The Octet Rule

Noble gases possess very stable atoms that do not typically combine with other atoms, as they do not alter their electronic structure. Most noble gases have 8 electrons in their outermost shell, with the exception of helium, which has 2. It follows that the electronic arrangement characteristic of a noble gas is particularly stable; this arrangement is known as the octet.

Molecular Structures

Molecules consist of a defined, small number of atoms:

• Diatomic: Molecules containing two atoms.
• Triatomic: Molecules containing three atoms, and so on.

They are classified as:

• Molecules of elements: Formed by identical atoms.
• Molecules of compounds: Formed by different atoms.

Crystal Lattices

Crystal lattices, or crystals, are formed by a variable number of atoms, ions, or molecules, generally arranged in a large three-dimensional structure. They are classified as:

• Crystals of elements: Formed by identical atoms.
• Crystals of compounds: Formed by different atoms.

Ionic Bonding

The reason substances with similar appearances have very different properties lies in the nature of the bond joining their atoms. These bonds are generally of three types: ionic, covalent, and metallic.

Formation

According to the octet rule, many atoms tend to gain or lose electrons to achieve greater stability:

• Metals: Tend to lose electrons, forming positive ions or cations.
• Nonmetals: Tend to gain electrons, forming negative ions or anions.

When a large number of positive ions interacts with a large number of negative ions, they acquire stability and form an ionic crystal.

Properties of Ionic Compounds

• They are very stable because their crystal structure is difficult to break.
• They are solid at ambient temperature and have high melting points.
• They are hard and difficult to scratch.
• They are soluble in water, which reduces the electric force between ions, causing the network to collapse easily.
• In a solid state, they do not conduct electricity because the ions are fixed; however, they become conductive when melted or dissolved.