Bonding and Intermolecular Forces

Chapter 3

Why do bonds form?

The molecule has a lower energy (i.e. is more stable) than its separated atoms

Ionic bonding

Electrostatic forces of attraction between oppositely charged ions

Why do ionic solids form?

Oppositely charged ions are attracted to each other in all directions (non-directional bond)?

Covalent bonding

Electrons shared between atoms of non-metals

• Usually not shared equally

Electronegativity

The ability of an atom in a molecule to attract toward itself

Polar covalent bonds

Unequal sharing of electrons

• electronegativity <=2

Non-polar covalent bonds

Equal sharing of electrons

• electronegativity <=0..4

Lewis structure

A representation of covalently bonded molecules

VSEPR

Observed shapes of molecule arise from each electron group around an atom arranging themselves as far away as possible from other electron groups to minimize repulsion between them

Electron groups

Single bond, multiple (double/triple) bond or lone pair 

Electron group arrangements

• Linear (2 electron groups)

• Trigonal planar (3 electron groups)

• Tetrahedral (4 electron groups)

• Trigonal bipyramidal (5 electron groups)

• Octahedral (6 electron groups)

Dipole moment

A measure of the separation of charge in a molecule arising from the unequal sharing of electrons in polar bonds

Intramolecular forces

Within molecules (covalent bonds)

Intermolecular forces

Between molecules

• weaker than intramplecular forces

Types of intermolecular forces

• ion-dipole

• dipole-dipole

• hydrogen bonds

• London dispersion forces

What are intermolecular forces important in?

State of matter, solubility, boiling point, melting point, etc.

Ion-dipole interactions

• occurs between ions and polar molecules

• strength is highly variable

Dipole-dipole interactions

• Occurs between polar molecules

• Weak bonds

London dispersion forces

• Occurs between all molecules; strength depends on size, polarizability

• Weak bonds

Hydrogen bonds

• Occurs between molecules with O-H, N-H and F-H

• Moderate bonds (strongest intermolecular force)