Richard Feynman, one of the great physicists of 20th century considers the discovery that “Matter is made up of atoms” to be a very significant one. According to the atomic hypothesis, all things are made of atoms – little particles that move around in perpetual motion,
attracting each other when they are a little distance apart, but repelling upon being squeezed into one another.

The scientific ‘Atomic Theory’ is usually credited to John Dalton. Dalton suggested in his findings that an atom is the smallest unit of matter, which cannot be divided any further. He proposed the atomic theory to explain the laws of definite and multiple proportions obeyed by elements when they combine into compounds.

Dalton’s Atomic Theory

The first law says that any given compound has, a fixed proportion by mass of its constituents. The second law says that when two elements form more than one compound, for a fixed mass of one element, the masses of the other elements are in the ratio of small integers. Dalton suggested, about 200 years ago, that the smallest constituents of an element are atoms. Atoms of one element are identical but differ from those of other elements. A small number of atoms of each element combine to form a molecule of the compound. 

Gay Lussac’s law, also given in the early 19th century, states: When gases combine chemically to yield another gas, their volumes are in the ratios of small integers. Avogadro’s law (or hypothesis) says: Equal volumes of all gases at equal temperature and pressure have the same number of molecules. Avogadro’s law, when combined with Dalton’s theory explains Gay Lussac’s law. Since the elements are often in the form of molecules, Dalton’s atomic theory can also be referred to as the molecular theory of matter. 

From many observations, in recent times we now know that molecules (made up of one or more atoms) constitute matter. Electron microscopes and scanning tunnelling microscopes enable us to even see them. The size of an atom is about an angstrom ( \(\left.10^{-10} \mathrm{~m}\right)\). In solids, which are tightly packed, atoms are spaced about a few angstroms 2 Å apart. In liquids, the separation between atoms is also about the same. In liquids, the atoms are not as rigidly fixed as in solids and can move around. This enables a liquid to flow. In gases, the interatomic distances are in tens of angstroms. The average distance a molecule can travel without colliding is called the mean free path. The mean free path, in gases, is of the order of thousands of angstroms. The atoms are much freer in gases and can travel long distances without colliding. If they are not enclosed, gases disperse away. In solids and liquids, the closeness makes the interatomic force important. The force has a long-range attraction and a short-range repulsion. The atoms attract when they are at a few angstroms but repel when they come closer. The gas is full of activity and the equilibrium is a dynamic one. In dynamic equilibrium, molecules collide and change their speeds during the collision. Only the average properties are constant.

We now know that atoms are not indivisible or elementary. They consist of a nucleus and electrons. The nucleus itself is made up of protons and neutrons. The protons and neutrons are again made up of quarks. Even quarks may not be the end of the story. There may be string-like elementary entities. Nature always has surprises for us, but the search for truth is often enjoyable and the discoveries beautiful.