In the 19th century, many elements had been discovered but they couldn't structure the atom. Instead they just classified them by their atomic weight.
It wasn't until 1863 that a man named Newlands posted his law of octaves. This told him that every 8 elements have similar properties. People didn't believe him because he missed out crucial elements that affected the table.
However, Mendeleev made a better version of the periodic table. He left gaps for unknown elements, therefore every eighth known element had the same properites. When some of these unknown elements were discovered, Mendeleev's predictions were proven correct. This is the basis for today's periodic table.
When scientists found out about atomic structures, they sorted the elements by proton numbers in verticle groups.
Going down a group, the atoms increase in occupied energy levels and they become less attracted by the…