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We will look a little deeper into how electrons are arranged so that we can explain why the octet rule is something we observe in nature.
The Weird Quantum realm, Again...
The Weird Quantum realm, Again...
Remember that Electrons DO NOT orbit the nucleus in circular paths like planets. Instead, they occupy probability clouds around the nucleus the shapes of which you can see to the right. We call these areas orbitals.
There are 4 different types of orbitals:
s orbitals are spherical. There is only 1 type of s orbital.
p orbitals are symmetrical halves. There are 3 types of p orbitals that are oriented either along the x, y, or z axis.
There are 5 types of d orbitals.
There are 7 types of f orbitals.
Orbital box diagrams
Orbital box diagrams
Drawing the orbital shapes to represent all the electrons in an atom would be too difficult. So chemists use what are known as orbital box diagrams.
Each arrow represents an electron.
each orbital is represented by a box or a line
Each orbital can hold 2 electrons
Orbitals have an energy level and a shape. so a 1s orbital means that the orbital is in the first energy level and has the s shape.
electron configurations
electron configurations
Electron configurations are shorthand for orbital box diagrams. The first number is the energy level (shell) followed by a letter for the orbital shape, then the subscript is the number of electrons in those orbitals. The s orbital only has 1 box, so it can only hold 2 electrons max. there are 3 boxes for p orbitals which hold 6. 5 boxes for d orbitals which can fit 10 electrons, and 7 boxes for f orbitals can hold 14 electrons.
Abbreviated Electron Configurations
Abbreviated Electron Configurations
After writing the electron configurations of several elements, you may notice that elements contain the configuration of all the elements that came before them.
To make things yet easier on themselves, chemists decided that they would use the noble gases as a shorthand. Any element that contained the complete electron configuration of a noble gas could use its elemental symbol to stand in for its electron configuration.
3 rules of electron notation
3 rules of electron notation
The Aufbau Rule
The Aufbau Rule
We must fill lower energy orbitals first. Often times this means filling the lowest energy level's orbitals then the next energy level. However, this is not always the case.
Imagine that you are parking a car in a parking garage. Generally people want to park closest to the garage entrance, so they start by filling in the available spaces in the ground floor. For electrons this space is the 1s orbital. The next available spaces are on the floor above. These are the 2s and 2p orbitals. This continues with the 3s and 3p next.
The next orbital to fill is not the 3d, but instead is the 4s orbital. Why is this the case?
Imagine that you have the choice to park farther away on the 3rd floor or park next to the stairs on the 4th floor. It is easier to park next to the stairs on the 4th. This is why electrons choose the 4s over the 3d orbitals.
Follow the arrows to remember the correct order in which orbitals are filled.
Remembering the pattern of the different "blocks" also helps to remember which orbitals are filled.
Pauli Exclusion Principle
Pauli Exclusion Principle
Two electrons can share one orbital, but they must have opposite spins. We represent this by writing the arrows in opposite directions. One up and the other down.
Hund's Rule
Hund's Rule
Electrons will fill empty orbitals of the same shape first before pairing up. This is also known as the "bus seat rule". Imagine you are riding a bus, and there are already other people sitting on some benches but not others. You would probably choose an empty bench and only sit next to another rider if there were no empty benches.
The 1s and 2s must be filled completely before filling the 2p orbital. However, there are not enough electrons to completely fill the 2p.
How do we arrange electrons in the 2 p?
We should spread out the electrons as much as possible before doubling them up in an orbital.
Practice
Practice
Additional Optional Practice (below)
Additional Optional Practice (below)
orbital box diagram worksheet.docPOGIL Electron Configurations.pdfPage updated
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