I wanted to take a few minutes to explain for those interested exactly what a valence shell is and how its used to figure out how things will blend together when you mix up combinations of different atoms.  I’m going to try to keep this post as basic as possible to help you understand, and I’m not going to show fancy images of the way each shell moves.  The fact is, these are all just opinions anyway, as its physically impossible for us to actually see an electron to know how it spins around the nucleus.  What Scientist have done is to prove these paths as closely as they can using mathematical models that seem to work most of the time.

All atoms contain a nucleus that has Neutrons and Protons and then an outer shell that contains electrons. To help keep these things separate we say that a proton has a positive charge of 1 unit and an electron has a negative charge of 1 unit.  Compared to the relative size of the atom, the shells are a pretty long ways away from the nucleus and the electrons spin very fast.  The average atom that you might find in nature is usually going to be balanced electrically, which means that it will have the same number of protons and electrons.  However, each atom can at any time have different variants of electrons and neutrons.  The protons always stay the same as far as the number found in a certain type of atom.

The Valence shell is nothing more than the outer most shell of an atom.  The reason shells are discussed is that after many years of research and experimenting, they seem to be an observable fact.  The simplest atom is that of Hydrogen, which is something we have been trying hard on this site to capture from water.  If you look on the Periodic table you will see that it has a “1″ above it and shows up in the first column on the chart.  This tells you that it has a single electron in its valent shell and helps us determine if it will mix with other elements or if it will want to stay by itself.  Noble gases, the far right column on the chart are atoms that are full of electrons and therefore very stable.

Elements that have valence shells that are not full, are the ones used to mix up other compounds.  Before we get too far I want to describe as simply as possible what shells exist and how they fill.  We’re going to start at the closest shell to the atom and work our way outward. 

Each layer has a number that tells you which shell we’re discussing, however, each number can have multiple letters that go along with it.  Each letter can only have 2 electrons within that shell layer and it has to have an opposite spin to the other.  So if we were to say that the He (Helium) atom has the 1S layer full, then we can assume that one of the electrons has an up spin, and the other has a down spin.  We are not going to get into what all this means here, but just remember they spin opposite directions.

So here is an easy way to remember how many shells exist, and remember, each can contain multiple letters layers and that each layer can contain 2 atoms maximum.  Ready?  Because this is really hard…  Here is how many Shells layers you have 1, 2, 3, 4, 4, 3, 2.  Remember that your Periodic table contains 7 rows from top to bottom (forget the two always exploded out as they are for something else that we’ll explain shortly).  So starting at the top row on your chart, you know that there is only 1 Shell for all elements on that row.  Moving to the second row, there are 2 Shell Layers.  The 3rd row there are 3 and so on.  By the time you get to the 6th row you start reversing and by the 7th and final row you are back to 2.

Next we’re going to get into the actual sub shells that make up each Shell layer.  These layers have names associated with them such as Px, Py, Pz and so on but we won’t worry with that here as its not important for this discussion.  For now we only want you to remember the number of sub layers in each shell.

To remember how many electron sets go with each layer, we’ll only count by odd numbers.  Here I’m going to give you your letters to remember.  S = 1, P = 3, D = 5, F = 7. 

Here is a list of how things fill up from the inner most layer to the outer most layer.

1S,  2S,  2P,  3S,  3P,  3D,  4S, 4P,  4D,  4F, 5S, 5P, 5D, 5F,  6S, 6P, 6D, 7S, 7P

Using these figures and looking above for how many physical shells exist in each layer, we can determine the first shell always has a maximum of 2 electrons in the shell.  Since all S shells only have 1 layer, that means ALL S layers have only 2 electrons maximum.  Looking above at our P shell we see that it has 3 sub-layers so we can multiply 3 * 2 and get 6, which means that all P shells can have a maximum of 6 electrons.  So looking here, our second shell (2) has a 2S and a 2P layers, which means it can hold a total of? You guessed it, 8 electrons (S)1*2 + (P)3 * 2 = 2 + 6 = 8. 

So for example:  I tell you that we have an Oxygen atom and I want to know how many electrons are in the valence shell.  To determine this, we first look at the Periodic table and determine that the Atomic number for Oxygen is 8.  Our next job is to take the information we learned above and figure out how many we have in the outside shell and how many more we’d need to fill the layer completely. 

Looking above, you’ll see that our first layer is a 1S layer.  Writing 2 down on our paper we now have accounted for 2 of the electrons and we have 6 more to go.  So next we write down 2S because it is the next shell layer we need to add up.  Again, All S layers can only have 2 electrons max so we’ll write down 2 more for a total of 4.

The next shell we have is the P shell, and looking above we see that P has 3 layers, each having 2 electrons in them.  This means that since we have 4 more to fill, that we’ll end up in the P shell and we’ll have 1 of the P layers left over with nothing in it. 

So remembering how many layers are in each shell, you can figure out how many electrons fit in each.  For example, we know there are 3 layers (2 electrons each) for the P shell, so multiplying 3 x 2 = 6, which means we can have a total of 6 electrons in that layer.  A 4D layer has 5 layers in the D shell and therefore has 5 x 2 = 10 electrons in that shell layer.

I’ll mention here that things have a certain way of filling up, but we won’t go into great detail as to how this works in all cases.  But to give you an example: when your P shell fills, it fills 1 electron in each shell layer, then goes back and adds a second to each layer.  When you go from a P layer and are about to start filling the D layer, the electrons will jump up and fill the next higher S shell, before finishing the D shell of the previous shell.  However, for most conversations this is not going to hurt you to not know, but if you want to learn exactly how scientists think things fill up, then you can research it a little more and learn it.

One more example:

How many electrons exist in the valence shell of the Al (Aluminum) atom?

Al Atomic Number = 13…  (1S =) 2 + (2S =) 2 + (2P =3layers * 2 electrons each =) 6 + ( 3S =) 2 + (3P = ) 1 for a total of 13 electrons.  So as you can see, we will end up with a single electron in the 3P shell.  So if I were to ask you how many more electrons that atom could have in its valent shell to fill it, your answer should be a total of 8 in that shell minus 1 = 7 electrons.  However, remember that it could also more easily loose 1 electron to eliminate the 3P shell and backup to the 3S shell as its full valence shell.

Valent shells are a very important aspect of atoms because all atoms want to have their outer most shells full which is why things mix in the first place.

I hope I haven’t lost anyone with all the math, but learning how electrons flow around an atom is actually quite simple, at least in theory…  Look for more entries to come on other aspects of Chemistry and how things mix… 

–glenn hancock