The periodic table is organized in neat rows and columns that spell out the hidden structure of atoms and the rules that govern them. Each column represents a class of elements that react similarly. The leftmost column contains hydrogen, lithium, sodium, potassium, and other very reactive chemical elements. Mix them with water and they react violently. On the other hand, the rightmost column consists of helium, neon, argon, and a series of other elements collectively called the noble gasses. They don't interact with other elements, which is how they got their name. The other columns each have a characteristic reactivity.
Periodic table of elements
As one goes from the top row to the bottom row, the atoms of each element get heavier and heavier. This organization -- the columns of similarly-reactive elements and the increase in atomic mass -- are both hugely powerful clues that eventually led to our modern understanding of the atom and, indeed, of chemistry.
It was 1869 when Russian chemist Dmitri Mendeleev first figured it all out. He used to take what were effectively playing cards, one for each of the elements known at the time, and lay them out in different ways, trying to figure out the key patterns. He was hampered by the fact that not all of the elements had been discovered when he did his momentous work. In fact, only 63 were known at the time. Over and over again, he laid out the cards until he came upon the pattern we recognize today.
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