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“Order is never observed; it is disorder that attracts attention because it is awkward and intrusive.” 

-Eliphas Levi

What is Electron Configuration?

Electron Configuration explains how electrons naturally fill orbitals in a way that maximizes stability and minimizes repulsive negative charges.

Sure electrons fly around somewhat randomly within (or even occasionally between) orbitals, but there is a method to their madness. Remember the Bohr model? There are quantized energy levels or areas of probability based on energy that define where electrons will hang out.

Let's start with the basics: There are 7 main energy levels that match up with the 7 rows in the Periodic Table. Each level has sublevels (we're going to look at the s, p, d, and f ) and each sublevel has orbitals which each contain 2 electrons.

electron orbitals
  • s-sublevels hold 2 electrons.
    They have just one orbital.
  • p-sublevels hold 6 electrons,
    divided between 3 orbitals
  • d-sublevels hold 10 electrons,
    divided between 5 orbitals
  • s-sublevels hold 14 electrons,
    divided between 7 orbitals
 

 

Lost yet? Try it again with shoes.

  • SHELVES = SUBLEVELS
  • BOXES = ORBITALS
  • SHOES = ELECTRONS

An s-shelf could hold 2 shoes in one box.
(Chemistry speak: An s-sublevel could hold 2 electrons in one orbital)

A d-shelf could hold 10 shoes in 5 boxes.
(Chemistry speak: A d-sublevel could hold 10 electrons in five orbitals)

orbitals using shoes

One shoe box (orbital) can only hold 2 shoes,
but different shelves (sublevels) can hold
1, 3, 5, or 7 boxes.

 

Okay, so we have a structure of shelves (sublevels) and boxes (orbitals) just ready to hold some shoes (electrons). Now we need sa few simple rules for how to place them!

Rule 1: The Aufbau Principle

The Aufbau Principle says that electrons will fill orbitals systematically, starting at the lowest energy level (shelf) possible and working their way up in a predictable order - not unlike how we walk up a staircase starting at the bottom and walking up one step at a time.

The diagram to the right here is a handy way to remember the filling order from 1s through 6d. Just follow the arrows!

 

Rule 2: Hund's Rule

I like to think of Hund's rule as the "everyone wants their own seat on the bus" rule. Electrons have negative charges so when they get close together they want to repel each other. According to Hund's Rule electrons will fill orbitals ONE at a time until they have to pair up for lack of open space to minimize these repellent negative charges.

It's like strangers on a bus filling all the empty seats first before doubling up.

Hund's Rule

Notice a second electron entering an s-orbital has no choice but to pair up immediately (nowhere else to go) but electrons entering p
(d and f) orbitals will stay as far apart as possible until they have to double up.

 

Rule 3: Pauli Exclusion Principle

Yes, I'm back to the shoe analogy but I think it works. Shoes fit better in a box when you place them heel-to-toe. Maximizes your space, minimizes overlap.

Electrons "fit" better in the same orbital when one is oriented "spin up" and the other "spin down" since that minimizes the repellent charges of the electrons. That's why we draw one electron arrow UP and the other arrow DOWN.

Pauli Exclusion Principle

 

Writing Electron Configurations

eDistribution icon

If you have an iPad, check out the FREE app eDistribution.

Your goal here is to inventory and place electrons into orbitals, in order, until you've found all of an atom's electrons suitable homes.

Let's try Carbon. Carbon has 6 electrons to place. Accoring to the Aufbau Principle you start at energy level 1 and go from there.

writing an econfig for carbon

"Noble Gas" Electron Configuration

Once you get much past 20 electrons writing the full electron configuration for an element becomes cumbersome. The good news is there is a short cut!

You can use a shorthand based on the Noble Gas that preceeds (comes before) the element you are mapping.

Noble gas config

 

Here are some other great resources on how to approach electron configuration:

Check out the videos below - Mr. Kent explains orbitals, electron configuration and the Periodic Table and writing electron configurations. 1-2-3 Understanding!

How much do you need to know about Electron Configuration?