1. Democritus (Ancient Greece):
* Proposed: Matter is made of indivisible particles called "atomos" (meaning "uncuttable").
* This was more of a philosophical idea: He had no experimental evidence.
2. John Dalton (Early 1800s):
* Proposed:
* All matter is composed of atoms.
* Atoms of a given element are identical in size and mass.
* Atoms cannot be created or destroyed in chemical reactions.
* Atoms of different elements combine in simple whole-number ratios to form compounds.
* Evidence: Dalton based his theory on experiments with gases and chemical reactions.
3. J.J. Thomson (Late 1800s):
* Proposed: The atom is a sphere of positive charge with negatively charged electrons embedded in it, like "plum pudding".
* Evidence: Thomson discovered the electron through experiments with cathode rays.
4. Ernest Rutherford (Early 1900s):
* Proposed: The atom has a small, dense, positively charged nucleus at its center, with electrons orbiting around it.
* Evidence: Rutherford's famous gold foil experiment demonstrated that atoms are mostly empty space with a concentrated positive charge in the center.
5. Niels Bohr (1913):
* Proposed: Electrons orbit the nucleus in specific energy levels (shells) and can jump between these levels by absorbing or emitting light.
* Evidence: Bohr's model explained the line spectra of elements, which showed that atoms emit light at specific wavelengths.
6. Quantum Mechanical Model (Developed throughout the 20th Century):
* Proposed: Electrons don't orbit in defined paths like planets, but rather exist in regions of probability called orbitals. This model is more complex and relies heavily on mathematics.
* Evidence: This model explains the behavior of electrons in atoms more accurately than previous models, particularly in complex atoms and molecules.
In summary, atomic theory has evolved over time, with each model refining our understanding of the atom. We now know that the atom is a complex and dynamic system with a tiny, dense nucleus surrounded by electrons in orbitals.
