Eli5: how does a periodic table work?
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The “periods” in the table are just the rows. Different elements share different properties. Two of the main properties are their mass, and the number of protons/electrons they have.
So, we sort them by those two quantities. As we go down a column, called a “group,” they get more massive. As we go along a period, they get more protons/electrons.
Well, the number of protons and electrons are equal. You can have a different number, and these are called ions, but we tend to care more about the neutral forms. What we find is that the electrons form sort of layers, and the important thing is how many electrons are in the outer layer. This sort of tells you the type of atom. So, we put the different numbers of outer electrons from 1 to 8 in different columns, and that’s why we call them groups. Groups 1 and 7 like to react a lot, and group 8 really doesn’t like to react!
As we get down the table, we begin to get some funky things happening with the electrons, so we get other groups popping in like the transition metals and the lanthanides. It needs a bit more work to explain these, and they also make the table look less neater, so we sometimes stick some of these below, a bit like Alaska and Hawaii on a US map.
When we move down the groups through the different periods, the more massive elements will change things like how quickly they react or what temperature they melt or boil at.
The guy who invented the table, Mendeleev, realised he could sort the known elements like this. But there were some holes in the table. Since he trusted the patterns, he was able to figure out some elements that hadn’t been discovered yet, and could make a good guess at their mass and different properties. We’ve now filled it in for all the elements that naturally occur, but are trying to push the bounds of what elements we can force into *very* brief existence.
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