Evolution of Atomic Theory: From Democritus to Chadwick

1. 470-360 BC. (?) DEMOCRITUS First to develop an atomic theory- thought all matter was divisible into smaller and smaller particles T H E G R E E K E R A Denied Democritus’ theory- believed his ideas were ridiculous. He believed in the ‘4 Element’ theory of matter 384-322 BC. FOUR ELEMENT THEORY Proposed by Empedocles- the idea that all matter is made up of either earth, water, fire, or air. ARISTOTLE

2. These include • All matter is composed of tiny, indivisible particles, called atoms • All atoms of the same element have identical properties • In chemical reactions atoms are never destroyed- they either join or separate John Dalton brought back Democritus’ theory- he proposed explanations for the known laws that describe the nature of matter. DALTON’S ATOMIC THEORY 1766-1844

3. F U R T H E R D EV E L O P M E N T J.J. Thomson Ernest Rutherford 1856-1940 1871-1937 Experimented with shooting alpha particles (positively charged particles) through thin pieces of gold foil. Most of the particles passed through the foil. However, a percentage deflected. He hypothesized that an atom must have a positively charged nucleus, that is surrounded by empty space. Analysed the results from measuring the mass of particles and their electric charges with a cathode ray tube. He suggested that the cathode rays were sub-atomic particles- he proposed the existence of electrons

4. F U R T H E R D EV E L O P M E N T Hantaro Nagaoka James Chadwick 1865-1950 1891-1971 Proposed the idea that atoms must have neutral particles, as well as positive and negative, called neutrons. A Japanese scientist who drew the atom as a large, positively charged sphere surrounded by a ring of electrons