Here's the whole idea in one breath: line every element up in order of its atomic number, and elements with similar personalities turn up again and again at regular spacings — so you fold the line into a grid where the lookalikes share a column. That grid is the periodic table, and it wasn't designed so much as discovered.
Why Bother Lining Them Up at All?
By the 1800s chemists had found dozens of elements — soft silvery metals, choking yellow gases, the lot — and it was chaos. Some were violently reactive, some did almost nothing, and there was no map. Then a Russian chemist named Dmitri Mendeleev sorted them in order of weight and noticed something uncanny: the properties came back around on a schedule. Every so often, another soft super-reactive metal; a set number of steps later, another stable do-nothing gas. The behaviour was periodic — it repeated. (We now know the true running order is atomic number, the proton count you met in Inside the atom, not weight — but the repeat is real either way.)
Rows and Columns, and Where Things Sit
Once you fold the line into a grid, two directions appear. A row across is a period — the original repeating run, ending each time at a stable gas before the pattern restarts on the next row. A column down is a group — every element in a column shares a personality, because they sit at the same point in each repeat.
There's a rough geography, too. Metals — shiny, good conductors, most of the elements — fill the left and middle. The non-metals — dull, brittle or gaseous, poor conductors — huddle in the top right, with a thin staircase of in-betweeners down the divide. So just where an element sits already tells you a lot about it before you know anything else.
Click a few tiles in the toy, then flip the metals switch. Same elements, same order — you're only colouring in the pattern that was hiding in the list all along. That's the engine of this whole topic; everything else is learning to read it.