Ionic Compounds
Properties:
- Made up of ions (not atoms or molecules)
- Strong electrostatic forces
- Soluble in water
- High melting and boiling points
- Conduct electricity when molten
Structure:
Ions arranged in regular crystal lattice pattern
Covalent Compounds
Properties:
- Made up of molecules (not ions)
- Low intermolecular forces
- Most are insoluble in water
- Low melting and boiling points
- Usually gases or liquids at room temperature
Structure:
Atoms held by covalent bonds, weak intermolecular forces
Structural Differences Between Ionic and Covalent Compounds
Structure of Ionic Compounds
Ions are arranged in a regular crystal lattice pattern with alternating cations and anions. Strong ionic bonds hold the oppositely charged ions together, forming a giant structure. Example: Sodium chloride's giant crystal lattice.
Effects of Structure on Physical Properties
Strong electrostatic attractions require lots of heat to break, resulting in high melting and boiling points. Ionic compounds do not conduct electricity in solid state due to fixed ions but conduct when molten or in aqueous solution since ions become free to move. Such solutions that conduct electricity are called electrolytes.
Structures of Simple Covalent Compounds
Atoms form molecules held by strong covalent bonds; molecules are attracted to each other by weak intermolecular forces (van der Waals or hydrogen bonds).
Effects of Simple Covalent Structures on Physical Properties
Weak intermolecular forces lead to low melting and boiling points; they melt or vaporize quickly. Covalent compounds do not conduct electricity as they lack ions, making them non-electrolytes.
Distinguishing Ionic and Covalent Solutions Using Conductivity
Materials:
- 2 cells, a bulb (3.8V, 0.3A), 3 connecting wires fitted with crocodile clips
- Iron nails or graphite rods
- 40 ml each of: sodium chloride solution, sugar solution, dilute sulphuric acid, ethanol
- Distilled water and a 100 ml beaker
Procedure:
- Set up the apparatus as shown below.
- Pour 40 ml of sodium chloride solution into the beaker.
- Dip the nails into the solution.
- Observe the bulb and record “light” or “no light” in the table of results.
- Remove the nails from the beaker.
- Rinse the beaker and nails with distilled water.
- Repeat steps 2 to 6 using sugar solution, dilute sulphuric acid, and ethanol.
Table of Results:
| Liquid | Observation |
|---|---|
| Sodium chloride | |
| Sugar solution | |
| Sulphuric acid | |
| Ethanol |
Classification:
When the bulb gives light, the liquid used is ionic. When the bulb does not give light, the liquid used is covalent.
Covalent Bond Types & Intermolecular Forces
Pure Covalent Bond
A covalent bond where the shared pair of electrons are equally donated by the atoms involved. Example: Bonding in ammonia, NH₃, where both hydrogen and nitrogen contribute an electron.
Dative Covalent Bond
A covalent bond where only one atom contributes both electrons. Example: Formation of ammonium ion NH₄⁺ where nitrogen donates a lone pair to H⁺.
Polar and Non-polar Covalent Bonds
Polar Covalent Bond
Electrons are shared unequally due to electronegativity differences. The more electronegative atom gets a partial negative charge (δ⁻), less electronegative gets partial positive (δ⁺). Example: Hydrogen chloride (HCl).
Non-polar Covalent Bond
Electrons are equally shared between atoms with similar electronegativity. Example: H–H molecule, Cl–Cl molecule.
Types of Intermolecular Forces
Hydrogen Bonding
Occurs between molecules containing hydrogen and a more electronegative atom. This attraction is called a hydrogen bond.
Van der Waals Forces
Weak attractive or repulsive forces present between molecules.
Effects of Intermolecular Forces on Physical Properties
- Water’s high boiling point is due to hydrogen bonding.
- Surface tension of water is high because of hydrogen bonds.
- Hydrogen bonds give water its liquid state.
- Van der Waals forces cause higher boiling points in some elements (e.g., halogens).
Conductivity Test
Electrolytes (Conduct Electricity)
- Sodium chloride solution
- Copper (II) sulphate solution
- Hydrochloric acid
- Sulphuric acid
Non-electrolytes (Don't Conduct)
- Pure water
- Sugar solution
- Ethanol
- Molten sulphur