Electron Shells and Orbitals

Electron Shells

Electron shells and orbitals hold electrons in a specific pattern.

In a neutral atom, the number of electrons must equal the number of protons. This makes the number of electrons equal to an element’s atomic number. The electrons reside somewhere away from the nucleus. The positive charge concentrates in a small central nucleus, Figure 1.



electrons out there
Figure 1: Nucleus at center of atom with electrons in indistinct cloud enclosing the nucleus

Electrons On the Outside


The electrons do not crowd together in one single cloud. They have limitations as to how many and where they can fit.





Electrons Fit in Shells



Electrons orbit the nucleus at fixed distances. More electrons occupy an energy level further from the nucleus.





periodic table and electron shells
Figure 2: Each row on periodic table represents a concentric shell further from the nucleus

Each Shell Further Away, Higher Energy



Each period (row) of the periodic table corresponds to another concentric ring further from the nucleus, Figure 2.


Principle Quantum Number



The first three rows of the periodic table represent the first three electron shells. The shells are labeled with their principle quantum number: n = 1, n = 2, n = 3….





electron shells and subshells
Figure 3: First four shells and subshells

Shells and Subshells



Every shell has subshells. The number of subshells equals the quantum number (row of periodic table). Each subshell has a label: s, p, d, f



The shells and subshells grow closer and closer for every advance of concentric circles.







Each type of subshell contains a specific number of electron orbitals.



For n = 1, it has a sinelectron shells, subshells, and orbitalsgle s shell which contains one s orbital. In the next highest shell, n = 2 contains an s and p subshell. The s subshell contains one orbital as before. The p subshell has three orbitals.



The number of shells, subshells, and orbitals in each subshell is summarized in Table 1.

Every electron orbital accommodates no more than two electrons. This makes it possible to assign electrons to a specific electron shell, electron subshell, and electron orbital. 


Electron Distribution




first three electron shells and subshells
Figure 4: Period 1 through Period 3 orbitals plotted as increasing principle quantum number versus relative energy

Electrons can be distributed among orbitals until all the electrons are accounted for. 


Figure 4 shows the first three periods of electron shells, subshells, and orbitals.  It is also sufficient for what will serve you when you study organic chemistry.


For the sake of completeness, Figure 5 includes higher energy orbitals: d and f.

expanded electron orbitals
Figure 5: higher energy orbitals which include d and f orbitals


When you examine the order of orbitals, although 3d orbitals are in a lower shell, they are lower in energy than a 4s orbital. In a similar way, 4f is lower than 5d. The 5d itself lower than 6s





Electron Rules   



Place electrons in orbitals following three rules. These rules allow you to place electrons from the lowest energy to the highest energy orbitals. 



Pauli Exclusion Principle




electron spin pairing
Figure 6: Electrons must pair with opposite spins to occupy the same orbital
How Do Electrons Share an Orbital?


The Pauli exclusion principle is more complex than presented here. The important part here, dictates when two electrons occupy the same orbital, they must have opposite spins.


Whether these spins are ‘up’ or ‘down’ makes no difference. It only matters you write them so they are ‘opposite’.



Figure 6 shows how orbital diagrams show electrons with opposite spin when they share a single atomic orbital. The higher orbital with two electrons with both electrons ‘up’ is stamped as wrong.



Aufbau Principle





Aufbau principle
Figure 7: Lower energy electron energy levels fill before higher electron energy levels.
How Do Electrons Go Into Energy Levels?



Aufbau means ‘build up’ in German.  This means electrons fill orbitals and shells from lower energy to higher energy.




Figure 7 shows two orbitals at different levels. A second electron can either fill the lower level with the electrons paired with opposite spin. 




One alternative would be to have each electron fill each orbital with one electron, one at a lower level and one at a higher level. 



However, the second possibility would fill a higher level before all lower orbitals fill. Conversely, the second possibility with singly occupied orbitals is stamped as wrong.



Hund’s Rule



How Do Electrons Fill Orbitals of the Same Level?





electrons paralleled
Figure 8: Orbitals with the same energy take a single electron before electrons pair in a single orbital.


In contrast to s orbitals, p orbitals and d orbitals have more than one orbital at the same energy level. Figure 8 shows a sample p orbital. Each p orbital possesses three atomic orbitals at the same energy.  After the first electron fills a p orbital, the next electron has two ways it can be placed into the p subshell.


Further, the second electron can either occupy a new orbital in the p subshell (on the left), or it can fill an already occupied p orbital by pairing with an electron already present (on the right).


The second electron fills a vacant orbital at the same energy. The alternative is stamped as wrong.