# How do I know which term symbols are acceptable for an #np^2# electron configuration? I found all of them, but which ones should I keep?

##### 1 Answer

I think if you connect orbital configurations with each term symbol, you would have a clearer way to understand term symbols. All of them represent an electron configuration, and so you can relate them back to Hund's Rule and the Pauli Exclusion Principle.

As a note, we call

**DISCLAIMER:** *LONG ANSWER!*

We can start from the **known configuration** of carbon:

#1s^2 2s^2 2p^2#

Now, recall that **term symbols** are defined as:

#\mathbf(""^(2S + 1) L_J)# ,

where:

#2S + 1# is the spin multiplicity, and#S# is the**total spin**due to adding up the#m_s# values for each electron in the same subshell, i.e.#S = sum m_s# .#L# is the**total orbital angular momentum**, or#L = sum m_l# , due to adding up the#m_l# values for each orbital. Each electron in an orbital contributes one value of#m_l# , so if an#m_l = +1# orbital is fully occupied, then you have#L = 2# . Make sure you know the maximum possible#L# before using it to calculate#J# .#J# is the**total angular momentum**, or#J = {|L - S|, |L - S + 1|, . . . , 0, . . . , |L + S - 1|, |L + S|}# . So, an example is#S = 1# and#L_max = 1# . Then#J = {L - S, L - S + 1, L + S} = {0, 1, 2}# .

**THE** **CONFIGURATIONS**

When we consider

#uarr uarr# ,#darr darr# ,#uarr darr# ,#darr uarr#

- The
**first**and**second**are not allowed according to the Pauli Exclusion Principle, which requires the#m_s# for each electron to be opposite. - The
**fourth**is redundant, since electrons are*indistinguishable*(you can't tell whether one electron in particular is spin-up or if it's the other one). So we didn't need to consider that one, actually, but I wrote it out to show that it was something to think about.

For the **third**

#S = sum m_s = -1/2 + 1/2 = 0# , so#2S + 1 = 1# .#L = sum m_l = 0# .- Since
#L = 0# , and#0 <= S < 1# ,#J# only has one value, and#J = 0 pm 0 = 0# .

Therefore, with

**EXAMINING ALL** **CONFIGURATIONS**

This is quite a bit harder, mainly because of the number of possibilities. Work it out, and you'd get:

Utilizing

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

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

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

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

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

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

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

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

Utilizing

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

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

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

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

Utilizing paired electrons nonredundantly, and

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

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

#ul(color(white)(uarr darr))# #ul(color(white)(uarr darr))# #ul(uarr darr)# (You could also have

#darr uarr# configurations for the paired electrons, but again, those are redundant.)

That gives a total of **microstates**, counting 'unacceptable' configurations. There is also a mathematical way to do this, but it should be in your book. I think this visual way can help you make connections.

The ** allowed** ones follow the Pauli Exclusion Principle, and Hund's rule asks for all electrons to be parallel spin when possible, meaning when one tries to

**maximize total spin**.

Therefore,

**EXAMINING ALLOWED** **CONFIGURATIONS**

Now, considering each *nonredundant*, *allowed* configuration, we can determine their term symbols. We expect

#ul(uarr color(white)(darr))# #ul(uarr color(white)(darr))# #ul(color(white)(uarr darr))# (#L = -1# )

#ul(uarr color(white)(darr))# #ul(color(white)(uarr darr))# #ul(uarr color(white)(darr))# (#L = 0# )

#ul(color(white)(uarr darr))# #ul(uarr color(white)(darr))# #ul(uarr color(white)(darr))# (#L = 1# )

- With
#m_l = +1# and#0# orbitals occupied, we would have the maximum possible#L = sum m_l = +1 + 0 = 1# , corresponding with a#P# term. - With two parallel electrons,
#S = sum m_s = 1/2 + 1/2 = 1# , so the spin multiplicity is#2S + 1 = 3# . - With
#S > 0# , we can have**more than one**#J# value. For#J# , we have three possible values. One way to do this is:#L + S = color(red)(2)# ,#L + S - 1 = color(red)(1)# , and#L - S = color(red)(0)# .

Therefore, the **second, third, and fourth** term symbols here are *nearly-degenerate* (very close together in energy) and *nonredundant* configurations.

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

- Finally, for this one, two electrons in the
#m_l = +1# orbital would generate the maximum possible#L = sum m_l = 1 + 1 = 2# . That corresponds with a#D# term, just like how#l = 2# is a#d# orbital (but this is for a#p# orbital configuration!!). - The electrons are paired, so
#S = sum m_s = 1/2 + (-1/2) = 0# .

Therefore, **fifth** term symbol is

So, to summarize our answer, all five allowed configurations correspond with the term symbols

(They are pronounced "singlet S zero", "triplet P zero", etc.)

If you wish, you can identify the electron configurations that are physically represented by each term symbol that is not allowed, and you should find that those have some reason related to Hund's Rule or the Pauli Exclusion Principle.