Fundamentals of Chemical Bonding and Material Properties

Understanding Chemical Bonding Principles

Chemical bonding is the union between atoms, molecules, or ions to form larger structures. Molecules, in turn, can be joined by intermolecular forces to form multimolecular structures.

The Octet Rule

To form a stable molecule or structure, atoms that bond tend to gain, lose, or share electrons to complete their valence shell with eight electrons.

Types of Chemical Bonds

Ionic Bond

The ionic bond originates as a result of electrostatic forces exerted between oppositely charged ions, thus forming an ionic crystal. This typically occurs between elements with very different electronegativities.

Covalent Bond

The covalent bond between two atoms occurs when these atoms share electrons so that they complete their valence shell, thus fulfilling the Octet Rule. This bond typically forms between elements of high electronegativity.

Metallic Bond

The metallic bond forms between identical atoms of low electronegativity metals.

Intermolecular Forces

Beyond the primary chemical bonds, molecules can interact through weaker intermolecular forces:

• Dipole-Dipole Forces: Established between the positive dipole of one molecule and the negative part of another dipole.
• Dispersion Forces (London Dispersion Forces): Established between molecules due to temporary dipole formation caused by the instant motion of electrons.
• Hydrogen Bond: Established between the hydrogen atom of one molecule and a small, highly electronegative atom (fluorine, oxygen, or nitrogen) in another molecule.

Molecular Geometry

Molecular geometry is based on a simple model denominated Valence Shell Electron Pair Repulsion (VSEPR). Electron pairs in the valence shell of a molecule can be represented by clouds or balloons of negative electric charge. The bonding pairs and lone pairs arrange themselves in space as far as possible from each other, so that the repulsion forces between them are minimal.

Properties of Substances

Properties of Molecular Substances

Molecular substances exhibit distinct characteristics:

• Can exist in all three states of aggregation (liquid, gas, solid).
• Soft and fragile.
• Do not conduct electric current.
• Solubility varies depending on whether they are formed by nonpolar or polar molecules.

Properties of Covalent Solids

Covalent solids, also known as network solids, have unique properties:

• High melting temperature.
• Very hard.
• Fragility.
• Low conductivity (due to low mobility of electrons).
• Insoluble.

Properties and Structure of Ionic Solids

Ionic solids are characterized by:

• Solid state at room temperature.
• High or very high melting temperature, and high boiling point in liquid state.
• High hardness.
• Fragile and brittle.
• Insoluble in organic solvents.
• Do not conduct electricity in the solid state, but do in the liquid state or in solution (due to free ions moving towards electrodes).

Properties of Metals

Metals possess a distinctive set of properties:

• Metallic luster.
• High density.
• High melting temperature and hardness.
• Excellent electrical and thermal conductivity.

Process of Covalent Bond Formation

The formation of a covalent bond can be explained by different models:

• According to the Lewis Model: The covalent bond is formed when the forces of attraction and repulsion are balanced, settling the electron pair bond in the region between the nuclei.
• According to the Electron Clouds Model: The density of negative charge between the two nuclei exerts an attractive force on each of them and keeps them together.

Bond Energy

Bond energy is the energy required to break a chemical bond (it is the same amount of energy that is released when the bond is formed).