|Helium atom ground state|
The Ancient Greek pre-Socratic philosopher Democritus of Abdera (460 BC — 370 BC) is the first thinker to try to explain the nature of the universe by noting that everything is composed from constantly moving and unchangeable atoms of different sizes and shapes. He wrote, “The only existing things are atoms and empty space; all else is mere opinion.”
However, Democritus’s atomic theory is not an explanation of the atomic structure of matter in the sense of modern science. His observations were purely philosophical—his aim was to articulate a materialistic doctrine. His atomic theory was not popular in Ancient Greece; only one school, that of Epicurus, adopted it. Aristotle’s theory that everything in the universe is made out of four elements of fire, earth, air, and water proved to be a much more popular doctrine. Democritus’s theory was forgotten by the end of the Ancient Roman Empire, while Aristotle’s theory remained the accepted explanation for everything in the universe for more than 2000 years.
In the 17th century, the concept of atoms was used by Robert Boyle, in his work on chemistry, and by Newton, in his work on optics, but the first “scientific” effort to investigate the existence and nature of atoms was made in the 18th century by the French scientist Antoine Lavoisier, who was trying to find out why things burn. During the course of his experiments, Lavoisier discovered several chemical substances (elements) which cannot be separated into other chemical substances and he also came to know that burning is a chemical reaction in which oxygen from the air gets combined to other elements.
Building on Lavoisier’s work, John Dalton in the early part of the 19th century noted that all matter in the universe is made out of atoms, which are themselves divisible, and that all atoms in every element are identical and every element has different kinds of atoms. In 1897, J J Thompson discovered that the atoms have negatively charged electrons and well as something with a positive charge. He imagined an atom to be like a watermelon in which the positive charge is spread across a large sphere and the small electrons carrying negative charge are embedded in the body of the sphere.
Ernest Rutherford is responsible for the picture that most people have of atoms—in 1911, he proposed for the atom a structure like the Solar System in which the positive charge remains stationary in the middle while the electrons with negative charge rotate around the positive charge. But Rutherford’s idea of the structure of the atom, like Thompson’s idea, is incorrect. The atom is not shaped like a solar system and the electrons are not moving around the positive charge. Modern scientists believe that the electrons and the positive charge in the atoms are stationary.
The study of atoms has been moving towards the domain of science since the early part of the 19th century, but there is still lot of philosophizing going on to explain the intricate nature of atoms. It may take a couple of centuries for the study of atoms to become fully scientific.