Structure and Bonding

• Weak Forces of Attraction

low melting points

Strong Forces of Attraction

have high melting points

Solids

A. Giant Structures (Strong Forces) Giant structures, which include giant ionic (e.g., sodium chloride) or giant covalent/molecular structures (e.g., diamond), are solids characterized by very high melting and boiling points

Simple Molecular Solids (Weak Intermolecular Forces) Simple molecular solids (like iodine or sulfur) have low melting and boiling points.

• Forces: The forces within the molecules (covalent bonds) are strong, but the forces between the molecules, known as weak van der Waals forces (intermolecular forces), are weak.

• Energy Required: Because the forces between molecules are weak, it does not take much energy to overcome them to break down the lattice

Liquids

Forces: Like molecular solids, the forces within liquid molecules are strong, but the weak forces between the molecules allow them to overcome these attractive forces easily. Most substances that are liquid at room temperature have a simple molecular structure with weak attractive forces between the molecules, such as considerable van der Waals forces, significant hydrogen bonding, or dipole-dipole bonding.

• Particle Movement: Particles in a liquid have more kinetic energy than in a solid. They are close together but are more or less randomly arranged, allowing them to slide over each other in a fairly random way

Gases

Forces: The forces between the molecules are very weak. The kinetic theory assumes that, in ideal gases, the particles do not attract each other and have no volume.

• Energy Required: Very little energy is needed to overcome the forces of attraction in a gas.

• Particle Movement: Particles in a gas are far apart and move rapidly and randomly

Covalent bond (definition to include in terms of atomic orbital)

formed by the force of attraction between the nuclei of two neighbouring atoms and a pair of electrons between them/ formed when atomic orbitals overlap

Sigma (σ) Bonds:

orm by end-on (head-to-head) overlap of orbitals along the line connecting the two nuclei.
Can occur between:
• Two s orbitals
• One s and one p orbital
• Two p orbitals (end-on)
Features:
• Electron density is symmetrical about the internuclear axis.
• Every single covalent bond is a sigma bond.
• Formed by sp³ or sp² hybrid orbitals in most molecules.

Pi (π) Bonds: The Side-by-Side Connection

form by sideways overlap of two unhybridized p orbitals.
• Electron density lies above and below the internuclear axis (not symmetrical).
• Found in multiple bonds:
 – A double bond = 1 σ + 1 π
 – A triple bond = 1 σ + 2 π
Example: In ethene (C₂H₄), each carbon forms σ bonds with sp² orbitals, and the leftover p orbitals overlap sideways to form a π bond.

Sigma vs Pi: Strength and Stability

σ bonds > π bonds in strength because σ overlap is direct and greater.
• π bonds are weaker since sideways overlap is less effective.
• The π bond adds rigidity to molecules (e.g., in C=C), preventing rotation.