The Modern Nuclear Atom How did our concept of the atom come about, and how did it develop?

Democritus 460-370 AD Philosopher in Thrace (Greece) “Father of Modern Science”

All matter is made of atoms. Between the atoms is empty space All matter is made of atoms. Between the atoms is empty space. The atoms have the qualities of the material that they make up. Atoms are uncuttable spheres. Greek atomos means ‘indivisible’ or uncuttable.

John Dalton 1766-1844 Developed Greek atomos concept John Dalton 1766-1844 Developed Greek atomos concept. Dalton’s Atomic Theory

They differ only in their masses. #3 is called the Law of Definite Proportions: Atoms combine in whole-number ratios to make molecules: 2 H atoms and 1 O atom make 1 H2O molecule.

#4 is called the Law of Conservation of Mass (or Matter) “In an ordinary chemical reaction, mass is neither lost nor gained, because the atoms simply re-arrange into new molecules. No atoms are ever lost or gained.”

J.J. Thompson 1890’s, invents the cathode ray tube. Discovers that ‘cathode rays’ are beams of charged particles (electrons). Electrons come from inside atoms and have negative charges. If negatively charged particles exist, then they must be balanced by positively-charged particles.

Ernest Rutherford Gold Foil Experiment Discovers that atoms have a tiny nucleus in the center. Nucleus contains the + charges.

James Chadwick Discovers the neutron.