Before the periodic law, chemistry was just a confusing list of random facts about different substances. Scientists struggled to find any connection between how different metals or gases behaved. Dmitri Mendeleev changed everything when he noticed that elemental properties repeat at regular intervals.
He originally organized his table by atomic weight, which caused a few elements to sit in the wrong spots. Later, Henry Moseley fixed this by sorting the elements by their atomic number instead. The atomic number counts the exact number of protons inside the nucleus of an atom.
This arrangement works perfectly because the number of protons dictates the number of electrons. The electrons then arrange themselves into specific shells around the nucleus. Because these electron shells fill up in a repeating pattern, the chemical traits also repeat.
Elements in the same vertical column end up with the same number of outer electrons. This shared electron layout makes them react with other chemicals in nearly identical ways. The periodic law lets us treat the table like a powerful prediction tool.
A common student mistake is confusing the periodic law with the periodic trends themselves. The law is the underlying rule, while the trends are the specific results we observe.