Timeline by Joseph Dubiel

Democritus (430BCE)

jd2233 — 2025-02-24 19:33:35 UTC

His approximate time of contribution was about 430 bce The Democritus theory is known as atomism which states the atoms comprise all matter. He was the first to propose that matter is composed of small, indivisible particles called atoms. There's no specific model just the idea that atoms are indestructible and vary in shape and size. There are no experiments, purely philosophical reasoning. His ideas were not widely accepted at the time because there was no experimental proof.

John Dalton(1803)

jd2233 — 2025-02-24 19:42:26 UTC

Contributions - proposed the first scientific atomic theory stated that atoms are invisible identical within an element and combine in fixed ratios to form compounds. Model - A solid sphere model. Based on chemical reactions and the law of multiple proportions. Dalton's experiment with gases showed that compounds always form in whole number ratios supporting the idea that atoms are distinct. Based on his theory on Law Of Conservative Mass and Law of multiple proportions - found that elements combine in fixed mass ratios, supporting the idea that atoms exist. He experimented with gases and noticed they exert pressure due to atomic movement. Other names for his theory are Billiard Ball Model and Solid sphere model

JJ Thomson (1897)

jd2233 — 2025-02-24 19:48:27 UTC

He discovered the electron proving that atoms are divisible and contain smaller and negative charged particles. Plum puddling model electrons are embedded in a positively charged pudding like raisins in a cake. He passed electricity through a vacuum tube and observed that negatively charged particles were deflected by magnetic fields. The deflection of cathode rays proved that atoms have internal structure and contain negative charges.

Ernest Rutherford (1911)

jd2233 — 2025-02-24 19:54:31 UTC

He discovered the nucleus and proposed atoms are mostly empty space with dense positively charged cores. Nuclear model - Small central nucleus with electrons orbiting around. Gold foil experiment he shot alpha particles at gold foil and some bounced back instead of passing through. If atoms were like Thomson's plum pudding model all the alpha particles should have passed through. The unexpected deflections indicated the presence of dense positively charged nucleus.

Henry Moseley (1913)

jd2233 — 2025-02-24 20:01:01 UTC

He discovered that atomic number (not atomic mass) determines an element identity leading to the modern arrangements of the periodic table. X-ray Spectroscopy measures the frequency of x-rays emitted by different elements and found a pattern corresponding to atomic number. Before Moseley elements were arranged by atomic mass which led to inconsistencies. His work showed that atomic number is a fundamental property of atoms leading to created periodic table

Niels Bohr (1913)

jd2233 — 2025-02-24 20:06:38 UTC

He proposed that electrons moved in fixed orbits energy levels around the nucleus and can jump between levels by absorbing or releasing energies. Bohr model electrons orbit the nucleus in fixed paths. Hydrogen Emission Spectrum experiment when hydrogen gas was excited it emitted light at specific wavelengths. Classical physics couldn't explain why electrons didn't spiral into the nucleus. Bohr's model introduced quantized energy levels explaining atomic spectra.

Ernest rutherford (1918)

jd2233 — 2025-02-24 20:11:12 UTC

He discovered the proton refining the nuclear model of the atom. Conducted experiments bombarding nitrogen gas with alpha particles and observed the production of hydrogen. This discovery confirmed that the positive charge in the nucleus was due to protons. Other names are the Rutherford model and proton model.

Schrodinger and Heisenberg (1926- 1927)

jd2233 — 2025-02-24 20:15:07 UTC

Electrons exist in probability clouds. not fixed orbits. reasoning - wave equations and the uncertainty principle. Electron cloud model. Other names are wave mechanical model, electron cloud model and probability model.

James Chadwick (1932)

jd2233 — 2025-02-24 20:20:48 UTC

he discovered the neutron. Reasoning - Bombarded beryllium with alpha particles and detected neutral particles. Impact Explained atomic mass and isotopes. Other names are Chadwicks Neutron Model and Modern Atomic Model (combined with quantum mechanics).