Excited States
Atoms can be excited when they absorb energy. The absorption of energy causes one or more electrons within the atom to "jump" to a higher energy orbital. This will change the electron configuration. There are essentially an infinite number of possible excited states for an atom or ion.
Excited state electron configurations will still abide by the Pauli exclusion principle where no more than two electrons having opposite spins can occupy an orbital.
These higher energy configurations will, however, violate either the aufbau principle or Hund's rule.
Violations of the aufbau principle
Let's take a look at an example that violates the aufbau principle. The electron configuration of an atom is shown below:
How do we know it is an excited state?
The aufbau principle is violated. The electrons do not occupy the lowest energy orbitals possible.
What is the identity of the element?
In a neutral atom, the number of electrons equals the number of protons. So, if we count the total number of electrons, we can identify the element. In the example above, there are 9 electrons and we can identify the element as fluorine (F).
What is the ground state configuration?
The ground state configuration can be determined by placing all electrons into orbitals according to the aufbau principle, Pauli exclusion principle, and Hund's rule.
How do we write an "excited state" electron configuration?
Let's say we're going to write an excited state configuration for silicon (Si). We have 14 electrons to work with. The ground state configuration is . First we need to remove one electron from an orbital. It can be any electron from any orbital! Let's take a 2p electron. Then, we add that electron to a higher energy orbital. Again, any orbital! (Now you can see why there are an infinite number of possibilities! We could even do this for more than one electron!) I'm going to place it in a 5s orbital. The excited state configuration I would write is . There are still 14 electrons making it a neutral silicon atom.
Try it!
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Violations of Hund's rule
Hund's rule violations also result in higher energy electron configurations and therefore are excited states. The figure below (a) shows that if an electron is paired when other degenerate orbitals are empty, this will be of higher energy (increased electron repulsion). In b) the unpaired electrons do not have the same spin and is also an excited state.
Try it!
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