The Periodic Table organises elements by increasing atomic number and groups them by chemical behaviour. Trends stem from one variable: effective nuclear charge, defined as , where is the shielding constant from inner electrons.
| Property | Across Period () | Down Group () | Dominant Driver |
|---|---|---|---|
| Atomic radius | Decreases | Increases | Charge () vs Distance () |
| Ionisation energy | Increases | Decreases | Tightly bound electrons |
| Electronegativity | Increases | Decreases | Greater bonding attraction |
Common pitfall: Trends feature competing drivers. Always state which driver dominates (charge vs distance/shielding), rather than just the trend.
Blocks and Calculations
The table divides elements into specific blocks based on their valence orbitals:
| Block | Groups | Characteristics |
|---|---|---|
| s-block | 1–2 | Highly reactive metals, low ionisation energy |
| p-block | 13–18 | Diverse: metals, metalloids, non-metals, noble gases |
| d-block | 3–12 | Transition metals with variable oxidation states |
Mole-mass bridge: Use the molar mass from the periodic table to convert grams to moles with the formula .