How to Draw Orbital Diagrams (And Not Get Hund's Rule Wrong)
The "Arrow in the Box" Problem
If you're in a general chemistry course, you've probably spent hours drawing little up and down arrows inside squares. On the surface, it looks like a simple game of Sudoku, but these diagrams are the map to an atom's personality. They determine if a substance is magnetic, how it will bond, and why it reacts the way it does.
The biggest mistake students make isn't the order of the subshells (thanks to the Aufbau diagram), but the way they pair up electrons. Let’s break down the two rules that usually cause the most trouble.
1. Hund's Rule: The "Bus Seat" Analogy
Friedrich Hund noticed that electrons are like people boarding a bus. If there are empty "double seats" (orbitals) in a row (subshell), everyone will take their own window seat before anyone starts sitting next to someone else.
The Rule: In a subshell (like $2p$ or $3d$), you must put one electron in each orbital before you start pairing them up. All these single electrons must have the same spin (usually drawn as an "up" arrow).
Example: Nitrogen (Z=7) The configuration is $1s^2 2s^2 2p^3$.
- For the $2p$ subshell, you have 3 orbitals and 3 electrons.
- Correct: One up-arrow in each of the three boxes.
- Wrong: One full box (up/down) and one half-empty box. Nature hates crowding!
2. Pauli Exclusion: The "No Identical Twins" Rule
Wolfgang Pauli discovered that no two electrons can have the same "address" (quantum numbers). In our diagram, this means that if two electrons share a box, they must have opposite spins.
The Rule: One arrow points up, one arrow points down. That's it. If you draw two up-arrows in the same box, you've broken a fundamental law of physics.
Visualizing the Hard Parts
Once you get past the $p$-block and into the $d$-block (Transition Metals), things get messy. For an element like Iron (Fe), the $4s$ fills before the $3d$, but the $3d$ has 5 orbitals to fill.
Instead of drawing this by hand and hoping you didn't miss a box, you can use our Electron Configuration & Orbital Viewer. It handles all 118 elements, including the weird Aufbau exceptions like Chromium and Copper, where electrons jump subshells to find better stability.
Step-by-Step Walkthrough
- Find the Atomic Number (Z): This tells you the total number of electrons in a neutral atom.
- Follow the Energy Ladder: Fill $1s$, then $2s$, then $2p$, and so on. Use the $n+l$ rule or a standard chart.
- Fill Singly First: When you hit a $p, d,$ or $f$ subshell, use Hund's Rule. Fill all boxes with up-arrows first.
- Pair Up: Only when every box in that subshell has an arrow do you go back and add the down-arrows.
Why does this matter?
In the real world, the number of unpaired electrons determines Paramagnetism. If an atom has unpaired electrons (like Iron), it will be attracted to a magnetic field. If every electron is paired (like Neon), it's Diamagnetic and won't care about your magnets.
Ready to see it in action? Try our Free Orbital Diagram Generator to see exactly how any element on the periodic table looks under the hood.
Summary Checklist
- [ ] Are my arrows in the $p, d, f$ subshells staying single as long as possible? (Hund's Rule)
- [ ] Do my pairs have opposite spins? (Pauli Principle)
- [ ] Did I remember that $4s$ fills before $3d$? (Aufbau Principle)
If you're also working on reaction stoichiometry, don't miss our walkthrough on Finding Limiting Reactants.