Covalent Bonding and Molecular Structures

Definition of Covalent Bond:
A covalent bond is formed between atoms when they share electrons with one another.

Participating Atoms:
Covalent bonds are formed specifically by non-metal atoms.

Bond Strength and Mechanism:
Atoms share or transfer electrons to establish strong chemical bonds.

Types of Covalent Bonds:

• Single Bond:
  This occurs when each atom involved in the bond shares $1$ pair of electrons.

• Double Bond:
  This occurs when each atom involved in the bond shares $2$ pairs of electrons.

Nature of Covalent Structures

Formation Diversity:
When atoms engage in covalent bonding, they can form several different types of structures.

Structural Determinants:
The specific structure that is formed depends on the total number of atoms involved and the specific manner in which they are bonded together.

Primary Classifications:
There are $3$ main types of covalent structure:

1. Giant covalent structures.

2. Small molecules.

3. Large molecules.

Giant Covalent Structures

Structure and Bonding Characteristics:

• These structures consist of billions of atoms that are all bonded together.

• The atoms are held in place by strong covalent bonds throughout the entire structure.

Representative Example:
Diamond is a primary example of a giant covalent structure.

Physical Properties:

• Melting and Boiling Points:
  They possess very high melting and boiling points. This is because a massive amount of energy is required to break the numerous strong covalent bonds that hold the structure together.

• State of Matter:
  They are typically solid at room temperature.

Small Molecules

Structure and Bonding Characteristics:

• Each molecule in this category contains only a few atoms.

• Intramolecular Bonds:
  There are strong covalent bonds between the atoms within each individual molecule.

• Intermolecular Forces:
  The separate molecules are held together by weak intermolecular forces.

Representative Example:
Water ($H_2O$) is a classic example of a small molecule.

Physical Properties:

• Melting and Boiling Points:
  These substances have low melting and boiling points. The reasoning is that only a small amount of energy is required to overcome the weak intermolecular forces that hold the molecules to one another.

Large Molecules and Polymers

Structure and Bonding Characteristics:

• These are composed of many identical molecules joined together via strong covalent bonds to form long chains.

• Intermolecular Forces:
  The chains are held together by intermolecular forces. These forces are stronger than those found in small molecules, although they are not as strong as the bonds in giant covalent structures.

Chemical Notation:
Large molecules are often represented with identical bonding on the left and right sides. The variable $n$ is used to represent a large number of repeating units within the chain.

Representative Example:
Polymers are the primary example of large molecules.

Physical Properties:

• Melting and Boiling Points:
  Large molecules have higher melting and boiling points compared to small molecules because their intermolecular forces are stronger, thus requiring more energy to break.

• Comparison to Giant Structures:
  While higher than small molecules, their melting and boiling points are still low when compared to giant covalent structures.

• State of Matter:
  They are normally solid at room temperature.

Electrical Conductivity of Covalent Structures

General Conductivity Rule:
The vast majority of covalent structures do not conduct electricity.

Scientific Reasoning:
They fail to conduct electricity because they do not possess delocalised electrons or ions, which are necessary to carry an electrical charge.