DAVID POGUE (Technology Guru): Nineteenth-century scientists relied on relative weight to order the elements.

Imagine if you have two containers, one full of red marbles, one full of blue marbles. If both contain the same number of marbles, but the blue container weighs twice as much, you can infer that the blue marbles weigh twice as much as the red marbles, even if you can't see the marbles at all.

Early chemists devised clever ways of calculating the weights of elements, even gases, relative to the lightest one: hydrogen.

So the chemists knew that different elements have different weights. But why, why not just one big line forever?

IGOR DMITRIEV (Director, Mendeleev Museum): Mendeleev decided that he would arrange them by weight, but also by family.

DAVID POGUE: This is one of Mendeleev's charts. You can see hydrogen sticking out, just as it does today. The families he knew are now arranged in columns. This one has the metals—lithium, sodium and potassium—that explode in water. Next door, calcium and magnesium, which also react with water.

This big block in the middle are metals that are safe to handle, like nickel, iron, zinc and gold.

As we go to the right, the elements become less metallic. These columns are headed by boron, carbon and nitrogen. In this neighborhood, some elements conduct electricity, some don't, and some can't make up their minds.

But next door is a more volatile crowd, headed by oxygen and fluorine.

The table gets its shape from the properties of the elements, like relative weight, conductivity and reactivity. It's true today, as it was in Mendeleev's time.

Though his chart displayed only the 63 elements known at the time, his understanding of the family properties was so strong he was able to leave gaps in his chart, bold predictions of elements yet to be discovered. And when they were eventually found, they proved completely consistent with his descriptions.

Mendeleev lived until 1907, long enough to see three gaps filled by the discoveries of scandium, gallium and germanium.

Since his death, dozens of new elements have been discovered, and incredibly, his chart perfectly accommodates all of them.

Print Transcript