We begin in ancient Greece
Democritus (Greek: Δημόκριτος, meaning “chosen of the people”; c. 460 – c. 370 BCE) Ancient Greek pre-Socratic philosopher remembered today for his formulation of an atomic theory of the universe. He proposed that all matter was composed of small indivisible particles called atoms.
Aristotle (330 BCE) disagreed. he asserted that the elements of fire, air, earth, and water were not made of atoms, but were continuous.
John Dalton (1766-1844 CE)
Billiard ball model
Part 1: All matter is made of atoms.
Part 2: All atoms of a given element are identical in mass and properties.
Part 3: Compounds are combinations of two or more different types of atoms.
When Dalton proposed his model electrons and the nucleus were unknown.
J. J. Thomson
Discover of the electron
In 1913 Bohr proposed his quantized shell model of the atom to explain how electrons can have stable orbits around the nucleus. The motion of the electrons in the Rutherford model was unstable because, according to classical mechanics and electromagnetic theory, any charged particle moving on a curved path emits electromagnetic radiation; thus, the electrons would lose energy and spiral into the nucleus.
To remedy the stability problem, Bohr modified the Rutherford model by requiring that the electrons move in orbits of fixed size and energy. The energy of an electron depends on the size of the orbit and is lower for smaller orbits.
Radiation can occur only when the electron jumps from one orbit to another. The atom will be completely stable in the state with the smallest orbit, since there is no orbit of lower energy into which the electron can jump.
Electron Cloud model
Early development of our understanding of the Atom
Historical development of atomic models