Learning Objective: to understand the order in which electrons fill orbitals and how the orbitals are arranged in the periodic table.

In your notebooks, sketch the 1s, 2s and 2p orbitals.

Now sketch this graphic.

Now with a ruler, draw in columns for Groups I – VIII. Use the periodic table on the wall or in your book for reference.

There are some real mysteries here. Notice that the 3d orbitals are actually part of row 4 of the periodic table. The reason for this is that the orbitals fill in a very special order.

Copy this chart to your notes:

Conclusion: The 1s fills first. Then the 2s, then the 2p then the 3s, then the 3p. Note the4s fills before the 3d, 4p and 5s.
Next step: How may electrons can an s orbital hold? _____________  Why? ________
How many electrons can a set of p orbital hold? _______________   Why?________

Why do orbitals fill in such a weird order?
The answer has to do with the specific energies of the orbitals – and YES – there is definitely some overlap.

The principle that orbitals fill up according to their energy is called the “Aufbrau Principle”. In fact the lowest energy orbital (1S) is always the first to fill.

Here’s a plot of the orbits and their energy levels….

 

Notice that the 4S orbital has a lower energy than the 3 d orbital – this means that the 4s orbital will fill with electrons before the 3d orbitals start to fill (K, Ca).

The electron configuration of an element is simply a list of which orbitals have electrons in them and how many electrons are in each orbital.

Examples: Hydrogen 1S1

Carbon  1S2, 2S2, 2P2

Class Project:
1] Now write a list of the first 20 elements by symbol (name).
2] For each element, draw a pyramid diagram to figure out the order in which the orbitals fill.
3]Write down the electron configuration for each of these 20 elements.
and write the electron configuration for each element.

Conclusion: You can now write the electron configuration for any element.