Chemistry & Physics: History of the Atom (Dalton, Thomson, Rutherford, and Bohr Models)

John Dalton proposed his Atomic Theory in 1803, suggesting that everything is composed of indivisible, solid atoms. Each element has its own unique type of atom with a characteristic weight, explaining how elements combine in simple whole-number ratios.

In 1897, J.J. Thomson discovered the electron through cathode ray experiments, revealing that atoms were not indivisible. His 'plum pudding' model proposed a positively charged bulk with negatively charged electrons embedded within it, like raisins in a cake.

Ernest Rutherford's 1909 gold foil experiment showed that alpha particles mostly passed through, but some were violently deflected. This led him to propose that atoms are mostly empty space with a small, dense, positively charged nucleus containing most of the atom's mass.

Niels Bohr, in 1913, addressed the instability of Rutherford's model by suggesting electrons orbit the nucleus in specific, fixed energy levels, similar to planets orbiting the sun. Electrons can jump between these levels by gaining or losing discrete 'quanta' of energy, determining an atom's chemical properties through its valence electrons.