This document provides a timeline of the development of atomic theory from Democritus in 400 BCE to James Chadwick in 1932. It outlines the key contributors to atomic theory and their discoveries, including: Democritus' early idea of atoms; Lavoisier distinguishing elements from compounds; Dalton's atomic theory of elements composed of indivisible atoms; discoveries of cathode rays, electrons, and the plum pudding model; and Rutherford's nuclear model showing atoms are mostly empty space with a small, positively charged nucleus surrounded by electrons.

1. Atomic Theory : A Timeline Melissa, Joe, Jake and Ryan

2. Democritus (400 BCE) Greek philosopher Theorist, not scientist Atomic Theory: All elements made of atoms Atoms are indivisible, indestructible

3. Lavoisier (1777) French chemist Provided formula for the conservation of matter in chemical reactions Distinguished between elements and compounds

4. John Dalton (1803) English chemist and schoolteacher Discovered atoms Atomic Theory: All elements composed of tiny, indivisible particles called atoms Atoms of the same element are different from those of any other element Atoms of diff. elements can mix physically or can chemically combine in simple whole-number ratios to form compounds Chemical reactions occur when atoms are combined, separated, or rearranged Atoms of one element are never changed into atoms of another element as a result of a chemical reaction

5. William Crookes (1870) Created “Crooks’ Tube” Demonstrated that cathode rays travel in straight lines and produce phosphorescence and heat when they strike certain materials

6. Eugene Goldstein (1886) German physicist Found rays traveling in the opposite of that of the cathode rays in a cathode ray tube Called these “canal rays” (later renamed protons)

7. J. J. Thompson (1897) Discovered electrons Cathode ray tube experiments Hypothesized that cathode rays are tiny, negatively charged particles moving at high speeds (electrons) Measured the charge to mass ratio of the electron Atomic Theory: Plum pudding model – negatively charged particles contained in a positively charged matrix

8. Robert Millikan (1908) American experimental physicist Determined quantity of charge on an electron Used Thompson’s charge to mass ratio to calculate the mass of the electron Performed “oil-drop” experiments

9. Ernest Rutherford (1911) English physicist and chemist Gold foil experiments Atomic Theory: Atom is mostly empty space Small, positively charged nucleus Electrons move around outside of the nucleus Created nuclear model

10. Neil Bohr Danish physicist Stated electrons moved around the nucleus in successive, large orbits Also presented Bohr Atomic Model Atoms absorb or admit radiation only when electrons abruptly jump between allowed, or stationary, states

11. James Chadwick (1932) English physicist Discovered neutrons