Polarity

What are molecules?

Molecules are substances composed of two or more atoms joined together.
The forces that hold these atoms together are referred to as intramolecular forces.
Generally, all intramolecular forces use a covalent bond, meaning electrons are shared between the two molecules.

Polar vs. Non-Polar Bonds

• Polar: When electrons favor one of the atoms more than another in a compound.

• Non-Polar: When electrons are shared equally between atoms.

What is electronegativity?

Electronegativity is the tendency of an atom to attract electrons towards itself.

• Strongest electronegativity: Oxygen, Nitrogen, Chlorine

• Weakest electronegativity: Hydrogen, Carbon, Phosphorus

Identifying electronegative compounds

Partial charges are used to identify electronegativity:

• More electronegative atom → partial negative charge (attracting more electrons)

• Less electronegative atom → partial positive charge (losing electrons)

Determining shape polarity

The shape of a molecule is considered.

• Uses the VSEPR (valence-shell electron pair repulsion) method.

• Volumes of space (orbitals) are drawn around the nucleus.

• Electrons move within orbitals, and due to repulsive forces, they move apart, changing the shape of the molecule.

What are intermolecular forces?

Forces that exist between molecules.

• These forces are weaker than intramolecular forces.

• If enough energy is provided, the bonds will break.

Dipole-Dipole Forces

Attractive force between opposite charges of polar molecules.

• Molecules orient themselves to be closer to oppositely charged molecules.

London-Dispersion Forces

Attraction between non-polar molecules.

• Weakest force.

• Shared electrons are constantly moving, causing spontaneous, brief polarity.

• This can induce neighboring molecules to become slightly polar.

Hydrogen Bonds

Type of polar bond.

• Strongest intermolecular bond.

• Only exists if hydrogen is bonded to Nitrogen (N), Oxygen (O), or Fluorine (F).

Hydrophobic vs. Hydrophilic Interactions

• Hydrophobic: Avoids water, non-polar, insoluble.

• Hydrophilic: Likes water, polar, forms hydrogen bonds, soluble.

Ionic Compounds and Water

Ionic compounds are hydrophilic molecules (bonds between metals and non-metals).

• Example: NaCl (Na⁺ and Cl⁻)

• Water is polar: Oxygen is negative, Hydrogen is positive.

• Water can pull apart the ionic compound: Oxygen attracts Sodium.

Special Properties of Water

• Only substance that can exist in solid, liquid, gas states.

• Less dense as solid → ice floats.

• Cohesive: Molecules stick together.

• Adhesive: Can attach to other surfaces.

• High heat capacity: Can absorb lots of energy.

• Universal solvent

Acids

• Taste sour

• Dissociate in water to produce H⁺

• pH < 7

• Blue litmus paper turns red

Bases

• Taste bitter

• Dissociate in water to produce OH⁻

• pH > 7

• Red litmus paper turns blue

Buffers

A buffer is a substance that keeps the pH of a solution within a specific range.

• Resists the effects of acids or bases.

• Without a buffer: pH decreases if an acid is added.

• With a buffer: pH changes minimally or not at all.