Aufbau Principle — Electron Filling Order Mnemonic
Target: mnemonic for Aufbau principle electron filling order
Why is this hard to memorize?
The Aufbau principle states that electrons fill orbitals in order of increasing energy: 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p → 7s → 5f → 6d → 7p. This order is essential for writing electronic configurations of any element — a skill tested every year in NEET and JEE. The tricky part is remembering where the d and f orbitals fit in (4s fills before 3d, 6s fills before 4f). The diagonal rule diagram makes this visual, but a mnemonic makes it instant.
Classic mnemonics you should know
"Write s, sp, spd, spdf in rows, then read diagonals ↗"
Row 1: 1s. Row 2: 2s 2p. Row 3: 3s 3p 3d. Row 4: 4s 4p 4d 4f. Draw diagonal arrows from bottom-left to top-right: 1s → 2s → 2p,3s → 3p,4s → 3d,4p,5s → and so on. The diagonal pattern gives the filling order.
"1s 2s 2p 3s 3p | 4s 3d 4p | 5s 4d 5p | 6s 4f 5d 6p | 7s 5f 6d 7p"
Group by "blocks" separated by s-orbital starts. Each block starts with an s-orbital: 4s before 3d, 5s before 4d, 6s before 4f. This pattern repeats predictably.
"Lower (n+l) fills first. If (n+l) is same, lower n fills first."
Calculate n+l for each orbital: 1s(1+0=1), 2s(2+0=2), 2p(2+1=3), 3s(3+0=3), 3p(3+1=4), 4s(4+0=4), 3d(3+2=5)... Sort by n+l. For ties (like 3p and 4s, both n+l=4), lower n goes first (3p before 4s).
The complete list
- 1s
- 2s
- 2p
- 3s
- 3p
- 4s
- 3d
- 4p
- 5s
- 4d
- 5p
- 6s
- 4f
- 5d
- 6p
- 7s
- 5f
- 6d
- 7p
Frequently asked questions
What is the easiest way to remember Aufbau filling order?
Draw the diagonal rule diagram: write 1s on top, then 2s 2p, then 3s 3p 3d, then 4s 4p 4d 4f. Draw diagonal arrows from lower-right to upper-left. Follow the arrows and you get the exact filling order. This visual method is the most reliable.
Why does 4s fill before 3d?
The (n+l) value for 4s is 4+0=4 and for 3d is 3+2=5. Since 4s has a lower (n+l) value, it fills first according to the Madelung rule. However, once 3d is partially filled, the 3d orbital actually becomes lower in energy than 4s — this is why Cr and Cu lose 4s electrons first when forming ions.
What are the exceptions to Aufbau principle for NEET?
Key exceptions: Cr is [Ar]3d⁵4s¹ (not 3d⁴4s²) and Cu is [Ar]3d¹⁰4s¹ (not 3d⁹4s²). The extra stability of half-filled (d⁵) and fully-filled (d¹⁰) d-subshells causes one 4s electron to shift to 3d. Similar exceptions occur in Mo, Ag, and other elements.
Explore more Chemistry mnemonics
We have 100+ memory tricks for Chemistry — periodic table groups, reactions, exceptions, and more.
Browse all Chemistry mnemonics