What is the Aufbau Principle?

1 Answer
Truong-Son N.
Apr 23, 2018

The Aufbau Principle suggests that we construct the orbital occupation in an atom or molecule based on lower to higher energy.

Granted, this principle is not always followed, but a TYPICAL case would be, for example, #"C"#, with electron configuration #1s^2 2s^2 2p^2#. The orbital diagram would be:

#underbrace(ul(uarr color(white)darr)" "ul(uarr color(white)darr)" "ul(color(white)(uarr darr)))_(2p)#

#" "#

#ul(uarr darr)#
#2s#

#" "#
#" "#
#" "#
#" "#
#" "#
#" "#
#" "#
#" "#
#" "#

#ul(uarr darr)#
#1s#

When we do so, it would be incorrect to say the ground state is #1s^2 2s^1 2p^3#:

#underbrace(ul(uarr color(white)darr)" "ul(uarr color(white)darr)" "ul(uarr color(white)(darr)))_(2p)#

#" "#

#ul(uarr color(white)(darr))#
#2s#

#" "#
#" "#
#" "#
#" "#
#" "#
#" "#
#" "#
#" "#
#" "#

#ul(uarr darr)#
#1s#

This is because the #2p# orbitals are high enough in energy compared to the #2s# that electron repulsion does not promote the electron into a #2p# orbital.

(That would instead be an excited state, about #"4.182 eV"# higher in energy.)

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