Electron Configurations: First Three Rows

Electron Configurations Up To 3rd Period


The electron configurations and orbital diagrams of the first three rows (periods) of the periodic table occupy an important role. The configurations form the basis of chemical behavior.


Periodic Table to Electron Shells


Periods and Electron Shells



periodic table and electron shells
Figure 1: The relationship between the periodic table and electron shells


The rows of the periodic table correlate to the electron shells. The principle quantum number n represents how many concentric rings the atom has for electrons. 


Figure 1 shows the relationship between rows on the periodic table and the concentric electron shells.



Shells and Subshells



first three electron shells and subshells
Figure 2: Electron shells and subshells


Each electron shell has closely spaced energy levels within the shell called subshells. The number of subshells within each shell equals the principle quantum number of the shell: n=1 has one subshell, n=2 has two subshells, and n=3 has three subshells. 









Subshell Labels


Each subshell possesses a label: s for the first subshell, p for the second subshell, and d for the third subshell.



s subshells have one orbital. p subshells have three orbitals. Each orbital allows two electrons if the electrons have opposite spins.



Orbitals and Energy


first three electron shells and subshells
Figure 3: First three sets of orbitals plotted against increasing energy

Figure 3 shows the first three electron shells, subshells, and orbitals. The graph plots principle quantum number versus increasing energy.











Electron Configurations and Orbital Diagrams



First Two Element Electron Configurations



orbital diagram and electron configuration H and He
Figure 4: The electron configuration and orbital diagram for the first two elements

Hydrogen and helium possess only the single n=1 electron shell. The first electron shell has only one subshell, which is labeled s. The first electron shell accepts one electron into its single s orbital.



H‘s correct electron configuration is 1s1. 1 stands for the electron shell, and s specifies the subshell. The superscript 1 shows how many electrons reside in a subshell, Figure 4.


It follows that when helium as two electrons in its orbital, its configuration is 1s2.


Pauli Exclusion Principle



Furthermore, two electrons occupy the same orbital they must have opposite spins: Pauli Exclusion Principle.




Larger Atoms, Higher Orbitals


Aufbau Principle


Once the 1s orbital fills with the first two electrons of He and H, electrons fill the 2nd electron shell. The 2nd electron shell contains two subshells: s and p. The s subshell has one orbital. On the other hand, the p subshell has three orbitals, Figure 5.



Period 2 first 3 element electron configuration
Figure 5: electron configuration and orbital diagrams of lithium, beryllium, and boron


The 3rd electron fits into the 2s orbital.


But now with the 4th electron in Be, you must choose between placing the fourth electron into an empty 2p orbital , or place it  into the 2s orbital.


Electrons always fill the lower energy levels before filling higher energy orbitals. This is the Aufbau principle.



Hund’s Rule



Furthermore when you add a fourth electron, you must choose where to place the electron: paired in a 2p orbital, or two in adjacent orbitals with parallel spin.




boron and carbon electron config
Figure 6: Boron and carbon illustrate Hund’s rule.

Figure 6 shows what happens when you add a 6th electron for carbon.


Carbon has two unpaired single electrons in the 2p orbitals with parallel spin. 



While p orbitals have more than one orbital of the same energy, they always half-fill each individual orbital before pairing electrons. This is the same for d and f orbitals.




First and Second Period Electron Configurations




electron configuration for 2nd Period
Figure 7: First and second row elements showing core and valence electrons.





Figure 7 shows a complete list of electron configurations and orbital diagrams for the first and second row elements. 


Hydrogen and helium use red. When the 1s level becomes core electrons and the 2s and 2p subshells begin to fill, write the configurations and orbital diagrams in abbreviated form.


Helium has an electron configuration of 1s2.  The core electrons of any element is the same as the next lowest noble gas.


In this case, [He] stands for the core electrons inside the outer valence electrons. Orbital diagrams show just valence electrons.










Third  Period Electron Configurations


Use the same shorthand for the third period just used for the second period, Figure 8. The electron configuration for neon is 1s22s22p6, the same as [Ne]. Using this convenient shorthand for core electrons makes it easier to write. 


Period 3 electron configurations
Figure 8: Third period elements in abbreviated form 

Beyond Third Row Electron Configurations


To consider orbital diagrams and electron configurations for larger atoms, the d and f orbitals come into play. This is complicated by the fact these orbitals do not follow the same order in energy. Heavier atoms are treated elsewhere.