The shape of the periodic table of elements

The periodic table contains all the known elements. But why does it look like that, with large gaps in the top rows? And what are these elements doing down here? This has to do with the electrons of each element, and which electron shells they occupy. Elements with the same number of electrons in their outermost shell - the same number of valence electrons - have similar properties.

Therefore, it’s practical to order the elements according to their number of valence electrons. In what way do we order them? So that the elements with the same number of valence electrons are located in the same column. But doing this is not that simple. Here, take a look: Let’s start with an atom with one electron in its outer shell.

The atom next to it has two valence electrons, and the one after that has three. On the next row, they start the same way: the first atom has one valence electron, the second has two. But now look what happens with the third element on this row: Its atom does have one more electron, but this electron is not in the outermost shell, but the one inside that. The number of valence electrons remains unchanged: two. And we want all atoms in the same column to have the same number of valence electrons.

We need to move this atom on the previous row to here. And create a gap. Let’s add more elements. With each step to the right, the atoms get one more electron, these are added to the shell below the outermost. Until this shell has 18 electrons.

And only after that can the outermost shell get a third electron. Do you see the difference? On this row, we go directly from two valence electrons to three. On this row however, we have to wait ten steps before we can continue with the third valence electron. And in order for the elements with three valence electrons to line up in the same column we have to make room for these elements.

They are transition metals. We find the transition metals on rows four, five, six, and seven in the periodic table. This is why there is a gap on the upper rows. On the bottom two rows, something similar happens. This time, it’s the third outermost shell that gets filled before the outer two.

In the same way as before, the elements above step to the right, as the third outermost shell becomes filled. And after that, the second outermost shell gets filled before the outermost shell finally gets its third valence electron. This makes the gap even bigger, and the table gets stretched out and very wide. To make it fit a page, we usually “cut” these elements from rows six and seven, and show them underneath the others. So there is a reason why the periodic table is shaped the way it is, with all those gaps.

It’s to keep the atoms that have three valence electrons, and the ones with four to eight valence electrons in the same columns.