Democritus

undividable, 440 B.C., small hard particles, atoms made of a single material formed into different shapes and sizes

Atom

Smallest unit of an element that maintains the properties of that element, made up of protons, neutrons, and electrons

John Dalton

British chemist and teacher, wondered why atoms combined in certain ways, thought elements combine in certain proportions because they are made of single atoms, published theory in 1803, all substances made of atoms, atoms can't be created, divided or destroyed, atoms of the same element are exactly alike, atoms join together to make new substances

J.J. Thomson

British scientist, discovered particles in atom, cathode -ray tube experiment, beam attracted by positively charged plate, found negative electrons, plum pudding model, electrons scattered throughout atom,

Ernest Rutherford

former student of Thomson, shooting gallery, thought particles would go through in one line, some deflected, some bounced straight back, atoms mainly just space, 1911 proposed positive highly dense nucleus with electrons being pushed around, nucleus really small in comparison to atom 

Nucleus

In physical science, an atom's central region, made up of protons and neutrons

Electron cloud

Region around the nucleus of an atom where electrons are likely to be found 

Niels Bohr

 1913 studied atoms react to light, found atoms to move on certain energy levels, no paths between levels, electrons can jump from one path to another, model most used

Erwin Schrodinger and Werner Heisenberg

Electrons don't travel in definite paths, only areas where electrons are likely to be