Structure of Atom

Chemistry · Class 11

Simulation · Chemistry · Class 11

Fill the orbitals, one electron at a time

"Fill the orbitals, one electron at a time" is a free, interactive Class 11 Chemistry simulation for NEET and JEE, built on the NCERT section "Filling of orbitals and electronic configuration" in Structure of Atom. Electrons drop one at a time into box diagrams in aufbau order with Hund and Pauli enforced; Cr and Cu snap to d5s1 / d10s1 with an exchange-energy readout. NEET has asked about this idea 2 times in 10 years of papers. JEE Main asked 46 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).

From the lesson Filling of orbitals and electronic configuration in Structure of Atom. Change the values and watch what happens.

Fill the orbitals, one electron at a timeChemistry · Class 11

What it shows

NCERT §2.6.4; §2.6.5; §2.6.6

Electrons drop one at a time into box diagrams in aufbau order with Hund and Pauli enforced; Cr and Cu snap to d5s1 / d10s1 with an exchange-energy readout.

Controls

3 to try

  • Slideratomic number Z 1–36
  • Sliderion charge −2 to +3
  • Togglestrict aufbau / real configuration

Try this

Build Cr and read off its unpaired electrons (6); then find an ion isoelectronic with Ar that has charge +2 (Ca2+). Goal checks Z and charge.

Common mistake

Cr is [Ar]3d4 4s2 and Cu is [Ar]3d9 4s2; pairing before each degenerate orbital has one electron.

In NEET

10 years of papers

Asked 2 times in 10 years of NEET.

See the past questions: Quantum Numbers, Structure of Atom (chapter)

In JEE Main

42 shifts analysed

46 questions from Structure of Atom across the 42 JEE Main shifts Lumi has analysed (2017–2026).

See the past questions

The idea behind it

NCERT §2.6.4; §2.6.5; §2.6.6

  • Aufbau principle: in the ground state, electrons enter the lowest-energy orbital available. The working order is 1s, 2s, 2p, 3s, 3p, then 4s, 3d, 4p, then 5s, 4d, 5p, then 6s, 4f, 5d, 6p, then 7s. It is a guide, and exceptions occur.
  • Pauli exclusion principle: no two electrons in an atom share all four quantum numbers, so an orbital holds two electrons of opposite spin. Subshells hold 2 (s), 6 (p), 10 (d), 14 (f); a shell holds 2n².
  • Hund's rule of maximum multiplicity: in a set of degenerate orbitals, electrons pair only after each orbital has one, so pairing starts with the 4th electron in p, the 6th in d and the 8th in f.
  • Configurations are written as spdf notation (1s² 2s² 2p⁶ …) or as box diagrams with arrows; the box form shows all four quantum numbers.
  • Examples: H 1s¹, He 1s², Li 1s² 2s¹, C 1s² 2s² 2p², N 1s² 2s² 2p³, Ne 1s² 2s² 2p⁶, Na [Ne] 3s¹, Ar [Ne] 3s² 3p⁶. Electrons of the filled inner shells are core electrons; those in the outermost shell are valence electrons.
  • K and Ca put electrons into 4s before 3d; from Sc to Zn the 3d orbitals fill, then 4p from Ga to Kr.
  • Exceptions: Cr is 3d⁵ 4s¹ and Cu is 3d¹⁰ 4s¹, not 3d⁴ 4s² and 3d⁹ 4s², because half-filled and completely filled subshells (p³, p⁶, d⁵, d¹⁰, f⁷, f¹⁴) are extra stable.
  • That extra stability comes from the symmetrical electron distribution (small mutual shielding, stronger pull of the nucleus), smaller repulsion and the largest exchange energy, which is greatest when a subshell is half or completely filled and is also the basis of Hund's rule.

Questions

5 answers

What does the "Fill the orbitals, one electron at a time" simulation show?
Electrons drop one at a time into box diagrams in aufbau order with Hund and Pauli enforced; Cr and Cu snap to d5s1 / d10s1 with an exchange-energy readout.
What mistake does the "Fill the orbitals, one electron at a time" simulation fix?
A common mistake it clears up: cr is [Ar]3d4 4s2 and Cu is [Ar]3d9 4s2; pairing before each degenerate orbital has one electron.
Has NEET asked about this idea?
Yes. NEET has asked about this idea 2 times in 10 years of papers.
Does JEE Main ask questions from Structure of Atom?
Yes. JEE Main asked 46 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
Is this simulation free?
Yes. It is free and runs in the browser on a phone or a laptop. It belongs to the lesson "Filling of orbitals and electronic configuration" in Structure of Atom (Class 11 Chemistry).

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