Let's apply what we've learned about electron filling rules, and apply it to make box diagrams, and from there, electron configurations. For our first example, let's do the box diagram for chlorine, which has 17 electrons. Using the filling diagram to the left, we know that we start with the 1s orbital, which can take two electrons - one up and one down. Next is the 2s orbital, which can take two electrons. Then the 2p orbital, where - applying Hund's rule - we do three spinning up, then three spinning down. The 3s orbital can take two electrons. And then we have five electrons left for the 3p orbital. So again, we apply Hund's rule, and we do three with spin up, and then our final two electrons we pair, with spin down. So this is the box diagram for chlorine. Now let's use this box diagram to create an electron configuration for chlorine.

All you do to create an electron configuration is take the information from each row of your box diagram and write it in a shorthand. So in our bottom row, in the 1s orbital, we have two electrons, and we write that as “1s2.” Moving upward through the diagram, the 2s orbital has two electrons, so that's 2s2. Then 2p6, 3s2, and 3p5. And this is the electron configuration for chlorine: 1s2 2s2 2p6 3s2 3p5.

Now let's go through the same process for the element bromine, which has 35 electrons. In the same order, we start with 1s, which can take two electrons, one up one down. Then the 2s orbital, then the 2p orbital, which can take six electrons, then the 3s orbital, then 3p, then 4s, then 3d, which can take 10 electrons, because there are five orientations of the d orbital. And then, finally, we have five electrons left for our 4p orbital. Again, we're going to apply Hund's rule, which says we do three with spin up, and then our final two we pair, with spin down. So this is the box diagram for bromine. Now let's use this box diagram to create an electron configuration for bromine.

To do this, go through the exact same process as with chlorine, starting at the bottom, and writing each row in that prescribed shorthand. Doing that, we get an electron configuration of 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5. As you can see, though, with larger atoms, even this shorthand becomes kind of cumbersome. So a further shorthand that chemists use is they'll count backwards in the periodic table to the last noble gas, the group 18 elements. With the example of bromine, the last noble gas is argon, which has an atomic number of 18. We know that the first 18 electrons in bromine filled the exact same way as argon did. And that's these 18 electrons. So the shorthand that chemists use is they'll take all those numbers there, and just replace them with the symbol for that element - in this case, argon. So the shortened electron configuration for bromine is argon 4s2 3d10 4p5.

Now we're going to connect all this back to the periodic table. And to do that, let's look at the two elements we just created electrons configurations for: chlorine and bromine, each there shown in their shortened form. Notice that with both chlorine and bromine, they end with a p5, meaning it's a p orbital with five electrons in it. These are the electrons that engage in chemical reactions, so these electrons heavily determine how this element behaves. So these two elements will behave similarly, which is why they are in the same group of the periodic table. And indeed, every single element in group 17 will end with p5, and that's why they all behave similarly.

If you were to look at the electron configurations for ever single element on the periodic table, you would see an incredible pattern emerge. And that's that groups 1 and 2 (also including helium), their highest energy orbital is an s orbital. That's why we call those the s block elements. Groups 13 through 18 (not including helium), their highest energy orbital is a p orbital, so we call those the p block elements. Groups 3 through 12, their highest energy orbital is a d orbital, and so we call those the d block elements. And then the lanthanides and actinides, their highest energy orbital is an f orbital, so we call those the f block elements.