the idea
Every electron has an address, and they fill the cheapest seats first.
Electrons sit in orbitals, regions where they're likely to be. Orbitals group into subshells (s, p, d, f), and subshells group into energy levels (n = 1, 2, 3…). Electrons fill from lowest energy up.
n: the floorEnergy level 1, 2, 3… Higher floors are farther from the nucleus.
ℓ: room types (1 room), p (3 rooms), d (5), f (7). Each room holds 2.
mℓ: which roomThe specific orbital within the subshell.
mₛ: which bedSpin up ↑ or down ↓. Two per room, opposite spins (Pauli).
Plant version: Hund's rule is the bus-seat rule. On a bus with empty double seats, strangers each take their own row before anyone sits next to someone. Electrons in p and d orbitals do the same thing: one per orbital, all spinning the same direction, before any of them pair up.
Fill order (diagonal rule): 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d…
4s fills before 3d because it's slightly lower in energy for these atoms. That's why the d-block starts in period 4.
try it
Pick an element
Tap any element. On a phone, swipe the table sideways.
trends
Three trends, one cause: how hard the nucleus pulls on the outer electrons
Atomic radius
↓ down a group: more shells, so bigger.← across a period: bigger toward the left. Moving right adds protons to the same shell, which pulls the electrons in tighter.
Ionization energy
Energy to remove an electron.Up and to the right is highest, because small atoms hold electrons tightly. Noble gases are the max.
Electronegativity
Pull on shared electrons in a bond.Up and to the right, with fluorine (3.98) as the champion. Noble gases are usually left out because they rarely bond.
Effective nuclear charge (Zeff) is the real driver: protons minus the shielding from inner electrons. Across a period, Zeff climbs and the shielding barely changes. Down a group, the new shells put the outer electrons farther away and behind more shielding. Color the table above by each trend and watch it happen.
practice
Write the configuration
practice
Compare the trend
exam traps
Common mistakes
- Filling 3d before 4s. Potassium is [Ar]4s¹, not [Ar]3d¹.
- Forgetting the two famous exceptions: Cr is [Ar]4s¹3d⁵ and Cu is [Ar]4s¹3d¹⁰. A half-full or full d subshell is extra stable.
- Removing electrons from 3d first when making transition-metal ions. They leave from 4s first: Fe²⁺ is [Ar]3d⁶.
- Pairing electrons before every orbital in the subshell has one (Hund's rule). Nitrogen's 2p is ↑ ↑ ↑, not ↑↓ ↑ _.
- Saying atoms get bigger left to right. They get smaller.