Ionization Energy (4) Electron Configuration and Periodicity

Ionisation energy (4) Electron Configuration and Periodicity

Well here we go again on ionization energy and its implications for the electron configurations of atoms of elements. 

In this post, I want to discuss how complex electron configurations are built up.  “Built up” being the operative term in fact as we shall see.

In my last post, I talked about the way electrons fill energy levels or shells in small atoms like those in the second row of the Periodic Table Lithium to Neon. 

But what happens if we want to write the ground state electron configuration of an atom with a much more complex electron configuration say neptunium?

Now Neptunium has atomic number 93 i.e. 93 electrons: how are they arranged?

There is an easy approach to building up these complex electron configurations.

First we need to relate our shell structure of the atom to the Periodic Table.

Here is a Periodic Table, the Western not Mendeleev’s type: 

What this Periodic Table shows is that each row corresponds to an electron subshell in the atoms of that row.

In other words, this periodic table shows how the ground state subshells fill up with electrons as atoms increase in size. 

This is called the Auf Bau or in English the “build up” principle.

It matches perfectly the arrangement of elements in the Periodic Table.

And we can see how many electrons are held in each type of subshell.

Subshell	Number of electrons
s	2
p	6
d	10
f	14

Here is a Periodic Table in which each row is labelled with the subshell filling along that row. 

Try as I might I cannot find a copy of this on the Internet anywhere so I have put one here on this blog.

The point is this, if you happen to be in your Chemistry exam with a Periodic Table you can use that as a guide to creating complex electron configurations. 

Another way you may have seen of writing the Auf Bau principle is like this:

My reason for choosing this type of example from the Internet is that the subshells are increasing in energy and distance from the nucleus. 

If you follow the arrows, you will write out a complex atom’s electron configuration.

These arrows match the order with which the subshells are found in the Periodic Table.

The subshell order is shown at the bottom of the illustration. 

So back to Neptunium:

Looking on the Periodic Table, all the subshells are full until you reach the 5f where Np is the fourth element along making it 5f⁴ and you’ll notice that it is an actinide (in the subshell starting from Actinium, Ac) so we must include a 6d¹ subshell to complete the configuration.

A final check adding up all the electrons should come to 93!!

Here is Neptunium’s electron configuration (subshells collected in shell order):

1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 5f⁴ 6s² 6p⁶ 6d¹ 7s²

It can also be written in the short form where the Noble gas Radon (Rn) is included:

[Rn] 5f4 6d1 7s2

It seems to me unlikely that you would be asked to construct such a task from memory in your A level or college examination.

A more likely example would be Molybdenum

Do you agree its electron configuration is:

1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d4 5s2

Note how I have adopted the convention of listing the subshells in shell order even though that is not the order with which they fill the ground state since 4s fills before 3d etc…

The other common convention is to quote the Noble gas configuration before completing the structure so that Molybdenum would become:

[Kr] 4d4 5s2

In the end, you are going to have to satisfy yourself that you can work these out for most of the transition metals and other elements up to probably Xenon in the Periodic Table. 

You need to find a friend from your class and challenge each other to write out randomly chosen electron configurations!!