1. Classification into Blocks
The Periodic Table is arranged in order of increasing atomic number. Elements are classified into blocks according to the subshell containing the highest energy valence electron:
- s-block elements: Groups 1 and 2 (valence electrons in \(s\) orbital).
- p-block elements: Groups 13 to 18 (valence electrons in \(p\) orbitals).
- d-block elements: Transition elements between Groups 2 and 13.
- f-block elements: Lanthanoids and actinoids.
2. Period 3 Element Trends (Na to Ar)
Atomic Radius Decrease
Why does atomic radius decrease across Period 3?
From sodium to argon, the number of protons in the nucleus increases by 1 each step (nuclear charge increases from +11 to +18). However, additional electrons enter the same third quantum shell (\(n = 3\)). Shielding from inner electron shells (\(1s^2 2s^2 2p^6\)) remains virtually constant. The increased nuclear attraction pulls the outer electron cloud closer to the nucleus, causing atomic radius to decrease.
Electronegativity Increase
Electronegativity increases across Period 3 because atomic radius decreases and nuclear charge increases. Bonding electrons in covalent bonds are attracted more strongly towards the smaller nucleus.
3. Melting Point Variations Across Period 3
Melting point reflects the type of bonding and crystal structure possessed by the element:
Metallic Elements (Na, Mg, Al)
Melting point increases from Na to Al. Why?
- Ionic charge increases: \(\text{Na}^+ < \text{Mg}^{2+} < \text{Al}^{3+}\).
- Number of delocalised valence electrons contributed per atom increases from 1 to 3.
- Cation radius decreases, bringing delocalised electrons closer to the nucleus.
- Electrostatic attraction between cations and electrons strengthens.
Macromolecular Silicon (Si)
Silicon has the highest melting point in Period 3 (\(1687\text{ K}\)). Silicon forms a giant macromolecular covalent crystal lattice similar to diamond. Melting silicon requires breaking thousands of strong covalent bonds throughout the 3D network, which requires enormous thermal energy.
Simple Molecular Non-Metals (\(\text{P}_4, \text{S}_8, \text{Cl}_2\)) and Argon
These elements exist as discrete molecules or single atoms held together only by weak London dispersion forces:
- Sulfur (\(\text{S}_8\)): Contains 8 atoms per molecule (128 electrons). Its large electron cloud produces strong London dispersion forces, giving it a higher melting point than phosphorus.
- Phosphorus (\(\text{P}_4\)): Contains 4 atoms per molecule (60 electrons), with weaker London dispersion forces than \(\text{S}_8\).
- Chlorine (\(\text{Cl}_2\)): Diatomic molecule (34 electrons), boiling as a gas at room temperature.
- Argon (\(\text{Ar}\)): Exists as monatomic atoms (18 electrons) with the weakest London dispersion forces, exhibiting the lowest melting point in Period 3.
4. Period 3 Properties Matrix
| Element | Symbol | Structure | Bonding | Melting Point (K) | Electrical Conductivity |
|---|---|---|---|---|---|
| Sodium | Na | Giant metallic | Metallic | 371 | Good (1 delocalised e-) |
| Magnesium | Mg | Giant metallic | Metallic | 923 | Good (2 delocalised e-) |
| Aluminium | Al | Giant metallic | Metallic | 933 | Good (3 delocalised e-) |
| Silicon | Si | Giant covalent | Covalent | 1687 | Semiconductor |
| Phosphorus | P4 | Simple molecular | Covalent (London forces) | 317 | None |
| Sulfur | S8 | Simple molecular | Covalent (London forces) | 392 | None |
| Chlorine | Cl2 | Simple molecular | Covalent (London forces) | 172 | None |
| Argon | Ar | Monatomic | London forces only | 84 | None |
5. Practice Questions
Mark Scheme:
- Sulfur exists as \(\text{S}_8\) molecules whereas phosphorus exists as \(\text{P}_4\) molecules. (1 mark)
- \(\text{S}_8\) molecules are larger and possess more electrons than \(\text{P}_4\) molecules (128 electrons vs 60 electrons). (1 mark)
- Therefore, London dispersion forces between \(\text{S}_8\) molecules are stronger and require more thermal energy to overcome. (1 mark)