Once upon a time long ago, philosophers thought that the material world, including themselves, was made of just four simple elements. For the Greeks they were earth, air, fire and water, combined in different ways to make all forms of matter. Some added a fifth immaterial, spiritual element, called quintessence or aether.
More than 3,000 years ago, many different metals including copper, iron, gold and silver were known. These were eventually recognized as simple “elements,” different types of atoms. Non-metals such as carbon and sulfur were common, but were not recognized as elements until the 18th century, the Age of Enlightenment. In that scientific revolution, dozens more elements were discovered, including gases like hydrogen and oxygen that combine to make water.
By 1869 about 64 different elements were known, all with different properties. What was that about? Often in science a good strategy is to look for patterns, and to focus on similarities and differences. That’s what 35-year-old Russian chemist Dmitri Mendeleev did. He noted that lithium, sodium, and potassium are chemically similar and put them in one “group,” and copper, silver and gold in another. Then he ordered the known elements in each group according to their atomic weights in a table.
Mendeleev was not the first to try this, but was the most successful.
He said, “I saw in a dream a table where all elements fell into place as required. Awakening, I immediately wrote it down on a piece of paper; only in one place did a correction later seem necessary.”
But there were some empty boxes in his table, and he predicted new elements that should exist to fill them, such as gallium (partner to aluminum) and germanium (partner to silicon). It was a great breakthrough in our understanding of matter.
Seventy years later, by the 1930s, physicists had made another huge simplification. The elements – more than 100 are now known – are all made of only three different subatomic particles: protons and neutrons stuck together in the atom’s nucleus, and electrons in a surrounding cloud. The number of protons determines the box in the table. Even simpler than the ancient Greek’s four elements!
But now it is even more interesting. Protons and neutrons are made of simpler particles called quarks, and now we know of six types, together with six electron-like particles. That makes 12, all with different masses. They fit in a neat 3×4 “periodic table,” but what is behind that?
I wish I knew.