How the periodic table is arranged
- Periods (rows) tell you the outermost shell. Sodium is in period 3, so its last electron is in the third shell.
- Groups (columns) share the same outer-electron pattern, which is why they behave alike: every group 1 metal reacts with water to give hydrogen.
- Blocks name the subshell being filled: s (groups 1–2), p (13–18), d (3–12) and f (the two rows at the bottom).
Trends to remember for JEE and NEET
| Property | Across a period (left → right) | Down a group |
|---|---|---|
| Atomic radius | Decreases | Increases |
| Ionisation enthalpy | Increases (with exceptions: B < Be, O < N) | Decreases |
| Electronegativity | Increases | Decreases |
| Metallic character | Decreases | Increases |
| Electron gain enthalpy | Becomes more negative (Cl is the most negative, not F) | Becomes less negative |
The configurations shown follow NCERT, including the exceptions JEE likes to ask about: Cr (3d⁵ 4s¹), Cu (3d¹⁰ 4s¹), Pd (4d¹⁰), Ag, Pt and Au. Test yourself with Element Hunt, a daily guess-the-element game.
Questions students ask
How many elements are in the periodic table?
118. The last four, nihonium (113), moscovium (115), tennessine (117) and oganesson (118), were named in 2016.
Why are lanthanoids and actinoids shown separately?
They fill the 4f and 5f subshells. Keeping them in two rows at the bottom stops the table from being 32 columns wide.
Why is the configuration of chromium 3d⁵ 4s¹ and not 3d⁴ 4s²?
A half-filled d subshell is extra stable because of exchange energy and symmetry. Copper is 3d¹⁰ 4s¹ for the same reason, with a full d subshell.
Which is the most electronegative element?
Fluorine, with 4.0 on the Pauling scale.