Electron Configuration & Lewis Dot
Pick any of the 118 elements and get its K L M N shells, full 1s² 2s² 2p⁶ notation, noble-gas shorthand, animated Aufbau diagonal rule, orbital boxes, valency, group, period and Lewis dot structure.
Electron configuration shows how electrons fill orbitals in increasing order of energy, the Aufbau principle: 1s 2s 2p 3s 3p 4s 3d 4p 5s and so on. 4s fills before 3d because its (n + l) value is 4 against 5. Iron, Z = 26, is [Ar] 3d⁶ 4s², with shells 2, 8, 14, 2.
Shell-wise distribution (K L M N)
What Class 9 and 10 ask for. Shell n holds up to 2n² electrons.
Orbital configuration
Class 11 notation, filled in Aufbau order.
Orbital boxes — Pauli & Hund
One box = one orbital. Arrows show the two possible spins.
Everything the configuration tells you
Each value is worked out from the configuration and then checked against the periodic table.
Configuration of the common ion
Electrons leave the outermost shell first — the highest n, then the highest l.
Lewis dot symbol
One dot per valence electron, in the standard order.
The Aufbau diagonal rule
Follow the arrows from top right to bottom left — that is the order electrons fill orbitals.
Why that order? The (n + l) rule
Also called the Madelung rule or Bohr–Bury rule.
- Lower (n + l) fills first — l is 0 for s, 1 for p, 2 for d, 3 for f.
- If two orbitals have the same (n + l), the one with the smaller n fills first.
4s has n + l = 4 + 0 = 4, while 3d has 3 + 2 = 5. So 4s fills before 3d even though its shell number is bigger.
| Order | Orbital | n | l | n + l | Max e⁻ |
|---|
The 20 real exceptions
Half-filled and completely filled subshells are extra stable, so a few elements break the simple order.
| Z | Element | Expected | Actual |
|---|
Lewis dot symbol of the chosen element
Put one dot on each of the four sides of the symbol first; only then start pairing them up.
Lewis structures of common molecules and ions
Tap any structure for the full electron count, the bond type and the shape.
From configuration to group, valency and bonding
Write the configuration correctly once, and the group, period, valency, ion and Lewis structure all fall out of it.
Pick an element
Search by name or symbol, type the atomic number, or tap a tile in the mini periodic table.
Read both notations
Shell-wise (2, 8, 1) for Class 9–10 and orbital-wise (1s² 2s² 2p⁶ 3s¹) for Class 11–12.
Watch the Aufbau path
Animate the diagonal rule to see exactly why 4s fills before 3d.
Draw the bonding
Get the Lewis dot symbol, then compare it with 41 ready-made molecules and ions.
Aufbau principle
Electrons occupy the lowest-energy orbital available first. The order is set by the (n + l) rule:
1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p
Pauli exclusion principle
No two electrons in an atom can have all four quantum numbers the same. In practice: one orbital holds at most 2 electrons, and they must have opposite spins (↑↓).
Maximum electrons: s = 2, p = 6, d = 10, f = 14.
Hund's rule of maximum multiplicity
Within the same subshell, electrons occupy separate orbitals with parallel spins first; pairing starts only after every orbital has one electron.
Nitrogen's 2p³ is therefore ↑ ↑ ↑ — not ↑↓ ↑ — which is why nitrogen has three unpaired electrons and a valency of 3.
Lewis dot structures
Count the valence electrons of every atom, use them as bond pairs (shared) and lone pairs (unshared), and give each atom an octet — or a duplet for hydrogen.
Water: 6 + 1 + 1 = 8 electrons → 2 bond pairs + 2 lone pairs on oxygen.
Group and period from the configuration
Period = highest shell number in use.
Group: s-block = number of outer s electrons; p-block = 12 + number of outer p electrons; d-block = (n−1)d + ns electrons.
Bromine is [Ar] 3d¹⁰ 4s² 4p⁵ → period 4, group 12 + 5 = 17.
Half-filled and filled stability
A subshell that is exactly half full (p³, d⁵, f⁷) or completely full (p⁶, d¹⁰, f¹⁴) has lower energy because of symmetrical distribution and smaller exchange energy.
That is why chromium is 3d⁵ 4s¹ and copper is 3d¹⁰ 4s¹ instead of the "expected" 3d⁴ 4s² and 3d⁹ 4s².
Electron Configuration & Lewis Dot — your questions
What is the Aufbau principle?
The Aufbau principle says electrons occupy the lowest-energy orbital available before filling higher ones. The resulting order is 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p, which you can read off the diagonal arrows. Maximum capacity is s = 2, p = 6, d = 10 and f = 14 electrons.
Why does 4s fill before 3d?
Because of the (n + l) rule, also called the Madelung or Bohr-Bury rule: the orbital with the lower (n + l) value fills first, and if two are equal the one with the smaller n goes first. For 4s, n + l = 4 + 0 = 4, while for 3d it is 3 + 2 = 5, so 4s is lower in energy despite the bigger shell number.
What is Hund's rule of maximum multiplicity?
Within the same subshell, electrons occupy separate orbitals with parallel spins first, and pairing begins only after every orbital holds one electron. Nitrogen's 2p³ is therefore three single arrows pointing the same way, not one pair plus one single. That gives nitrogen three unpaired electrons and explains its valency of 3.
Which elements are exceptions to the Aufbau order?
Twenty elements break the simple filling order, because exactly half-filled and completely filled subshells are extra stable. Chromium is [Ar] 3d⁵ 4s¹ instead of 3d⁴ 4s², and copper is [Ar] 3d¹⁰ 4s¹ instead of 3d⁹ 4s². Chromium, copper, molybdenum, silver and gold are the five that come up most often in exams.
How do you write the electron configuration of an ion?
Electrons leave from the outermost shell first — the highest n, and then the highest l within it. Sodium loses its 3s electron, so Na⁺ is 1s² 2s² 2p⁶, the same as neon. Iron loses its two 4s electrons before any 3d electron, so Fe²⁺ is [Ar] 3d⁶. For anions, simply add the extra electrons to the next vacant orbital.
How do you draw a Lewis dot structure?
Count the valence electrons of every atom, then arrange them as shared bond pairs and unshared lone pairs so that each atom reaches an octet, or a duplet for hydrogen. Water has 6 + 1 + 1 = 8 valence electrons, which become two bond pairs to the hydrogens and two lone pairs on the oxygen.
How do you find the group and period from the electron configuration?
The period is the highest shell number in use. For the group, s-block elements use the number of outer s electrons, p-block elements use 12 plus the number of outer p electrons, and d-block elements add the (n−1)d and ns electrons. Bromine is [Ar] 3d¹⁰ 4s² 4p⁵, so period 4 and group 12 + 5 = 17.