All matter is made up of numerous tiny substances whizzing around. These particles are exceptionally dense; matter is empty space mostly. Matter seems continuous as the particles are so small, and they move very fast.
Even people of ancient times supposed that matter is made of invisible particles. They deduced this from by observing things like rocks, water, and metals. These sub- stances are quite different from each other. But any given material-copper, for instance-is the same not affected by wher it is found. Without performing complicated experiments, physicists felt that substances could have these consistent behaviors if they were made of unique types 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 instance, is there a smallest possible material particle? There were some scientists, they refused to believe the atomic theory, even around the year 1900. At present, practically everyone accepts this theory. It explains the behavior of matter better than any other theory.
Gradually, scientists identified 92 different kinds of fundamental substances in nature, and named them as elements. Afterward, a few more elements were made artificially.
Each element has its unique type of particle, known as its atom. Atoms of different elements are different in nature. The a bit change in an atom can make a difference in the behavior of it. You can live by breathing in oxygen which is pure, but you can't live in pure nitrogen. Oxygen causes metal to corrode, but nitrogen doesn't. Wood will burn furiously in the atmosphere of pure oxygen, but will not ignite even in pure nitrogen. However, both are gases at room temperature and pressure; both are odorless, are colorless, and are about of equal weight. These substances are different as oxygen has 8 protons, while nitrogen has only 7. There are several other examples in nature where a slight change in atomic structure generates a major difference in the manner a substance behaves.