- Democritus (~450 BC): Proposed that all matter is made of indivisible particles called "atoms".
- John Dalton (1803): Formulated Atomic Theory stating: 1) Elements are made of atoms. 2) Atoms of one element are identical; atoms of different elements differ. 3) Atoms are conserved in reactions. 4) Compounds form in constant ratios.
- Michael Faraday (1839): Discovered a relationship between atomic structure and electricity.
- J. J. Thomson (1896): Discovered electrons using cathode ray tubes; measured electron’s charge-to-mass ratio; proposed the Plum Pudding Model where electrons are embedded in a uniform positive charge.
- Robert Millikan (1909): Measured the exact charge (1.60 × 10⁻¹⁹ C) and mass (9.11 × 10⁻²⁸ g) of an electron using oil droplets.
- Ernest Rutherford (1909): Through the Gold Foil experiment, discovered the nucleus — a small, dense, positively charged center; showed most of the atom is empty space. Later named the proton (1919) and, with Chadwick, discovered the neutron (1932).
Parent Tip: Review the logic above to help your child master the concept of history of the periodic table worksheet.
