Development of Atomic Structure How long have people

Development of Atomic Structure How long have people been interested in understanding matter and its structure? A. Thousands of years B. Hundreds of years C. A few years D. 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 • Must be some negative component to matter

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 Electron Proton Neutron Symbol Location Charge Mass e- Surrounding nucleus -1 0 p+ Inside nucleus +1 1 n 0 Inside nucleus 0 1