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Gaffer Variety:
Elements 2 SN2 004:
By Willie Gaffer:
In our previous study notes, I began a discussion of the elements getting just a bit into a discussion of electrons. I pointed out that the electrons are the important part of the atom in chemistry. These subatomic particles are directly involved in how the elements combine. We can take a rather simplistic view when dealing with electrons. We don’t want to get into theoretical detail or vague concepts in chemistry. We save that for nuclear physics. For our purposes, we can think of the electrons as being arranged in shells around the nucleus.
Let us keep in mind that the number of electrons in an atom is equal to the number of protons or the atomic number. This means oxygen, with an atomic number of 8, will have 8 electrons. That’s cool, but it does not tell the whole story. How these electrons are arranged is of prime importance to us. That will decide how an atom will combine with other atoms. As we noted, the electrons are arranged in shells and there are rules of nature that govern that.
The maximum number of electrons that can occupy a give shell is given by a simple equation. For convention the shells are numbered in order from the innermost shell to the outermost shell.
The maximum number of electrons in shell n = 2n2.
Where n is the shell number.
This means that the first, or innermost shell of an atom cannot have more than 2 electrons. It’s basic arithmetic.
2 x 12 = 2.
How about the second shell?
In that case we get 2 x 22 = 8
It is an exponential function and the numbers get very large very quickly.
Briefly, shell 3 can have no more than 18 electrons; shell 4 can have no more than 32 electrons and so on.
Another important rule is that the outermost shell can have no more than 8 electrons. This gives us an interesting twist on how electrons are arranged as we come to the outermost shell. We can illustrate that with the element rubidium. This element has an electron arrangement of 2, 8, 18, 8, and 1. We might think the fourth shell should have 9 electrons since it is allowed 32, but that would violate the rule of no more than 8 for the outermost shell. This means the last electron must be all by its lonesome in the outermost ring. Poor guy! This outermost shell is called the valence shell. The actual number of electrons in this shell is a periodic property.
Well, what the heck does that mean?
Basically, it has to do with how the elements are arranged in the periodic table. That is governed by a concept called periodic law.
Periodic law states that many of the physical and chemical properties of the elements tend to recur. They recur in a systematic way as the atomic number of the elements increase. For example, the eighth element, oxygen has properties similar to the sixteenth element, sulfur. You will notice, from the periodic table that sulfur and oxygen are in the same column.
The periodic table is arranged in columns and rows based on shared chemical properties. The columns contain elements that are chemically similar, such as oxygen and sulfur in our example. These elements in the columns are called groups or families. The rows are called periods. To see what that means, we can look at the electron shell arrangements of the elements of column 1.
Hydrogen has its only electron in its only shell.
Lithium has 2 and 1 following the formula 2n2.
Sodium has 2, 8, and1.
Rubidium, as we noted above, has 2, 8, 18, 8, and 1.
Cesium has 2, 8, 18, 18, 8, 1.
Francium has 2, 8, 18, 32, 18, 8, and 1.
We can notice that the valence shell remains the same placing all of these elements in the same, first column. What changes is the electron count and the shell count. The shell count increases by one for each row. This is the periodic nature of the elements. This is why helium is in the eight column at the top of the table. It has a period of 1 as does hydrogen, the only other element with a period of 1. Anther thing to notice is how the rule of no more than 8 in the outer shell affects the electron distribution for the last three elements in our list.
I will try to stay, as much as possible with the
The main group columns are still IA through 8A. The transition group is then given numbers 3B through 12B. That’s not entirely unreasonable. But, all of this makes me believe the table should really be a three dimensional entity instead of a two dimension one. I believe, if the table were three dimensional, the transition group would fall into line behind some of the other groups. However we do it, I will often have to clarify what I mean by being specific about the elements. That’s okay.
That is more than enough to chew on for now. I’ll get more into the layout of this table and what the grouping imply next time. In the meantime, the important things to remember are the shell, electron, and valence. That is not so difficult when you realize that most of it is implicit in the table.