Chemical periodicity

The Periodic Table organises elements by increasing atomic number $Z$ and groups them by similar chemical behaviour.

Periodic trends across a period (left $\to right)$:

Effective nuclear charge

$$Z_{\text{eff}} = Z - S$$

where $S$ is the shielding constant from inner electrons.

Down a group — Atomic radius increases (new shell), ionisation energy decreases, and electronegativity decreases.

Key blocks

• s-block (groups 1–2): Highly reactive metals, low ionisation energy.
• p-block (groups 13–18): Diverse — metals, metalloids, non-metals, noble gases.
• d-block (groups 3–12): Transition metals with variable oxidation states.

Mole-mass bridge — The molar mass $M$ on the periodic table converts grams to moles: $n = m / M$.

Tip: Periodic trends are predictable because they all stem from the same underlying variable: $Z_{\text{eff}}$.

Common pitfall: Periodic trends have two competing drivers — nuclear charge and shielding/distance. Across a period, charge wins (atoms shrink); down a group, distance wins (atoms grow). State which driver dominates, not just the trend.