Atomic Theory

    Atomic theory states that all matter is composed of very small units called atoms, which are the basic building blocks of all things around us. The concept of atomic theory began in ancient Greece with philosophers such as Democritus, who proposed that matter cannot be divided indefinitely and would eventually be reduced to the smallest unit of matter. Atomic theory was developed scientifically in the early 1800s by John Dalton, who proposed his theory of atoms. Dalton proposed that atoms are indivisible, that all atoms of a given element are identical, and that compounds are formed when atoms of different elements combine in whole number ratios. Later discoveries proved that atoms are indeed divisible into smaller particles, but Dalton's theory helped to explain chemical reactions and the law of conservation of mass.

As scientific knowledge increased, new discoveries led to improvements in atomic theory. J.J. Thomson discovered the electron, proving that atoms are not indivisible. Ernest Rutherford discovered the nucleus, proving that most of an atom's mass is concentrated in a very small, dense region at the center of the atom. Niels Bohr proposed a model of the atom in which electrons orbit the nucleus in specific energy levels. Modern atomic theory is based on the quantum mechanical model of the atom, which proposes that electrons do not orbit the nucleus in specific paths but in areas of probability called orbitals. This greater understanding of atomic structure allows scientists to explain the behavior of elements, the formation of compounds, and many other things such as electricity, medicine, and electronics.