Electron Configuration and the Periodic Table - part 2

To use the periodic table for electron configuration we need to make a small adjustment and then to break it into four separate pieces.

Let’s start with the rearrangement. The periodic table we are all familiar with looks like this:

The problem is that with respect to electrons, Helium is in the wrong place. We need to move it to the little gap next to Hydrogen. That leaves us with this. 

Now we’re going to split the table into 4 sections, like this. 

Each of those 4 sections, then corresponds to one of the four basic orbital types, like this. 

Now, electron configuration is simply a matter of "reading" the periodic table.

Let's talk about "reading"

If we are going to "read" the periodic table in order to determine electron configuration, we need to agree on how we read. We will be reading the table as if it were English text - that is, from left to right and from top to bottom. Here are the rules we will work with:

• Start at the top left
• Read to the right until you get to the end of the line
• When you reach the end of the line, go back to the left, one row down
• Stop when you reach the element that you area looking for

That probably seems pretty straight-forward, but it is worth putting down.

Let's try several examples:

Hydrogen

We start at the top left. That puts us on row 1. As we begin to read across we find ourselves in the s block (1s). Then we count over to hydrogen (the first element). That gives us 1s¹.

Helium

Again, we start at the top left (row 1, s block) and count over to helium (2nd element). That gives us 1s2.

Lithium

Starting at the top left we count over two elements in the first row (1s2). Having finished that row, we go back to the left but down one row. We are now on row 2, but still going into the s block (2s). Lithium is the first element in that row - 2s1. All together, that gives us 1s2 2s1.

Fluorine

Again starting at the top left, we need to go through the first row (1s2) and then move to the second row, where counting 2 elements finishes the s block (2s2). But the 2nd row is not yet finished. That means that we stay on the 2nd row, but move into the p block, counting our way over to fluorine - the 5th element in the block (2p5). Putting it all together gives the electron configuration 1s2 2s2 2p5.

Chlorine

To reach the element chlorine we need to “read” through the first row (1s2), through the entire second row (2s2 2p6) and then through the s block on the third row (3s2) and across most of the p block (3p5). That gives an electron configuration of 1s2 2s2 2p6 3s2 3p5.

It is worth noting that both fluorine and chlorine end in p5. As a result they share many chemical properties. In fact, all of the elements in that column of the table end with p5 and share chemical properties.

We're not done yet

Chemists, always looking for a more efficient (lazy) way to do things, use a shortcut. In addition there are, of course, some quirks about the periodic table and electron configuration that we need to address.

 

Lastly, we need to think about how ions are formed - both negative ions and positive ions.