The modern periodic law
The modern periodic table is based on two things you have just been studying: the atomic number and the electron configuration.
The modern periodic law
The properties of the elements are a periodic function of their atomic numbers.
What "periodic" means here
It means this: arrange the elements in order of increasing atomic number, and after a certain range of elements, elements with similar properties turn up again.
Not similar-ish. Similar in a way you can predict. And the reason is already in table 3.2 — the outer level fills, empties, and fills again, so the pattern of outer electrons repeats as you count up through the atomic numbers.
The idea in one line
The properties repeat because the electron configuration repeats. Everything in the periodic table follows from that.
Mendeleev arranged his table before anyone knew about electrons at all. That he still got the pattern right is a measure of how strongly it shows in the elements themselves.