chem demo
Democritus (400 BC)
Contribution: First proposed the idea of the atom.
Theory: Matter cannot be divided forever; smallest piece = atomos (“not to be cut”).
Model: Solid, hard, indivisible particles of different shapes and sizes.
Aristotle & Plato
Contribution: Rejected atomism; believed matter was made of earth, air, fire, and water.
Effect: Their view dominated for ~2000 years.
John Dalton (Early 1800s)
Contribution: Revived the idea of atoms through experiments.
Theory:
All elements are made of atoms.
Atoms are indivisible and indestructible.
Atoms of the same element are identical.
Atoms combine/separate/rearrange in reactions.
Model: Billiard Ball Model — solid, uniform spheres.
J.J. Thomson (1897)
Contribution: Discovered the electron.
Experiment: Cathode ray tube → found negatively charged particles.
Model: Plum Pudding Model — positively charged sphere with electrons scattered inside.
Ernest Rutherford (1908)
Contribution: Discovered the nucleus.
Experiment: Gold Foil Experiment → atoms mostly empty space, dense positive nucleus.
Model: Nuclear Model — electrons orbit a small, dense, positively charged nucleus.
Niels Bohr (1913)
Contribution: Proposed electrons orbit nucleus in fixed energy levels.
Model: Bohr Model — electrons in specific orbits (quantized energy levels); used 2n² formula.
Kirchhoff & Bunsen
Contribution: Developed the spectroscope.
Discovery: Elements emit specific line spectra when energized → evidence for quantized energy levels.
Louis de Broglie
Contribution: Proposed matter waves — particles (like electrons) act as both particles and waves.
Werner Heisenberg
Contribution: Uncertainty Principle — cannot know both position and energy of an electron at once.
Idea: Described electrons by probability (orbitals), not fixed paths.
Erwin Schrödinger
Contribution: Developed wave equation describing electron behavior mathematically.
Model: Quantum Mechanical / Wave Model — electrons exist in 3D orbitals, described by probability.