Atoms

All matter is made up of countless tiny particles whizzing around. These particles are extremely dense; matter is mostly empty space. Matter seems continuous because the particles are so small, and they move incredibly fast.

Even people of ancient times suspected that matter is made of invisible particles. They deduced this from observing things like water, rocks, and metals. These substances are much different from each other. But any given material—copper, for example—is the same wherever it is found. Even without doing any complicated experiments, early physicists felt that substances could only have these consistent behaviors if they were made of unique types, or arrangements, of particles. It took centuries before people knew just how this complicated business works. And even today, there are certain things that scientists don’t really know. For example, is there a smallest possible material particle?

There were some scientists who refused to believe the atomic theory, even around the year of 1900. Today, practically everyone accepts the theory. It explains the behavior of matter better than any other scheme.

Eventually, scientists identified 92 different kinds of fundamental substances in nature, and called them elements. Later, a few more elements were artificially made. Each element has its own unique type of particle, known as its atom. Atoms of different elements are always different. The slightest change in an atom can make a tremendous difference in its behavior.

You can live by breathing pure oxygen, but you can’t live off of pure nitrogen. Oxygen will cause metal to corrode, but nitrogen will not. Wood will burn furiously in an atmosphere of pure oxygen, but will not even ignite in pure nitrogen. Yet both are gases at room temperature and pressure; both are colorless, both are odorless, and both are just about of equal weight. These substances are so different because oxygen has eight protons, while nitrogen has only seven.

There are many other examples in nature where a tiny change in atomic structure makes a major difference in the way a substance behaves.