Eli5: how does a periodic table work?
Chemical elements have similarities between each other. Some guy named Mendeleev got the idea to sort the elements according to their similarities.
We figured out that atoms consist of protons, neutrons and electrons. Atoms of different elements differ from each other by the number of those three components which also cause their properties (weight, look, crystal configuration, the way they react chemically with other elements and so on).
Having the periodic table lets us understand other elements better. Conclusions from known elements enabled us to find (and later until today even make) new elements.
There were a few older systems, but the one we’re using today is sorted according to the number of protons (they cause most of the mass, sorted from least to most/lightest to heaviest), and the number of freely movable electrons (1 to 8, so it’s eight rows). An exemption is the block of lanthanides, but I don’t understand how those work.
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.
The electron in an atom form shells around the nucleus, and the number in each shell is limited – 2 in the first, 8 in the second and third, and so on. You line up elements in rows by how many shells they have, then list them based on the number of electrons in the outer shell.
The amazing thing is, elements with the same number of electrons in the outer shell behave in similar ways. For he dample, the right column (noble gases) all have full outer shells, so they are highly unreactive. So each column has elements with similar properties.
The gaps in the upper rows is because the smaller shells have fewer options for how many electrons to have, and the familiar alignment with more elements to the right is because basing the position on how many spaces are left in the outer shell is best for grouping similar elements. Each row is called a “period”.