Development of Atomic Structure How long have people been interested in understanding matter and its structure? Thousands of years Hundreds of years A few years Never Answer: A

Democritus (ca. 450 BC)

Ancient Philosophy Who: Aristotle & Democritus When: More than 2000 years ago Where: Greece What: Aristotle believed in 4 elements: Earth, Air, Fire, and Water. Democritus believed that matter was made of small particles he named “atoms”. Why: Aristotle and Democritus used observation and inference to explain the existence of everything.

Alchemists Who: European Scientists When: 800 – 900 years ago Where: Europe What: Their work developed into what is now modern chemistry. Why: Trying to change ordinary materials into gold.

Alchemic Symbols

John Dalton (1766-1844) Based his work on Lavoisier (Law of Convservation of Mass)

Dalton’s Atomic Theory (1830) 1) All matter is made of atoms. Atoms are indivisible and indestructible. 2) All atoms of a given element are identical in mass and properties. 3) Compounds are formed by a combination of two or more different kinds of whole atoms. 4) A chemical reaction is a rearrangement of atoms.

J.J. Thomson (1856-1940) Discovery of ELECTRON

Thomson’s Cathode Ray Experiment (1897) Ray of electrically charged particles through vacuum tube Ray “bent” towards positive plate

PLUM PUDDING

Ernest Rutherford (1871-1937) Discovery of NUCLEUS

Rutherford’s Gold Foil Experiment (1909) Shot alpha particles (+) through gold foil 98% passed through undeflected 2% deflected back

Therefore… Small, dense, positively charged mass in center of atom deflecting 2% of alpha particles Called NUCLEUS (contains protons and neutrons) Atom is mostly EMPTY SPACE

Subatomic Particles Particle Symbol Location Charge Mass Electron e- Surrounding nucleus -1 Proton p+ Inside nucleus +1 1 Neutron n0