The modern periodic table arranges elements in order of increasing atomic number (Z), reflecting Moseley's work. Elements in the same group (vertical column) share the same number of valence electrons and similar chemical properties. Elements in the same period (horizontal row) have the same number of occupied electron shells.
The Block System
| Block | Subshell Filling | Groups | Examples |
|---|---|---|---|
| s-block | s-orbital | 1–2 | Na, Mg, Ca |
| p-block | p-orbitals | 13–18 | C, N, O, F, Ne |
| d-block | d-orbitals | 3–12 | Fe, Cu, Zn |
| f-block | f-orbitals | Lanthanides/Actinides | Ce, U |
🔑 Key Insight
The periodic table is essentially a map of electron configuration. An element's position tells you its highest occupied subshell. E.g. Period 3, Group 16 = 3p⁴ = Sulfur.
⚠️ Examiner Trap – Explaining Trends
Stating "radius decreases because it is further right on the table" is just restating the observation. You must explain the forces: increasing Zeff (more protons, same shielding) pulls the electron cloud tighter. A trend is an observation, not an explanation.