Notes #2: History of the Atom (Parts 1&2) WCHS Chemistry

*** All models fall short of reality! *** WHAT IS A MODEL? Model = an attempt to understand or explain things Usually things that are really big or really small *** All models fall short of reality! *** Over time, models become more COMPLEX! Why?? BECAUSE WE GET MORE INFORMATION!! BIG IDEA!!!

Credited for the first belief that atoms existed (atomic theory) DEMOCRITUS (460 - 370 B.C.) Credited for the first belief that atoms existed (atomic theory) But… NO PROOF!

Didn’t believe in atoms Set us back 2,000 years :( ARISTOTLE (384 - 322 B.C.) Didn’t believe in atoms Set us back 2,000 years :(

Evidence of Atoms

ANTOINE LAVOISIER (1770’s) Law of Conservation of Mass Mass before = Mass after SO2 + H2O → H2SO3 5 grams = 5 grams

Law of Definite Proportions JOSEPH PROUST (1797) Law of Definite Proportions Each chemical compound has a fixed and unchangeable ratio of its elements Ex: Water Water always has 2 hydrogen to every 1 oxygen H2O2 = completely different compound!

Dalton’s Atomic Theory JOHN DALTON (1803) Dalton’s Atomic Theory All matter is composed of atoms, which are indivisible and indestructible All atoms of the same element are identical All atoms of different elements are different Compounds are composed of 2 or more different elements in a definite ratio

Law of Conservation of Mass = #1 (Lavoisier) JOHN DALTON (1803) cont... Theory was easily accepted because he explained what people already knew…. Just put it all together! Law of Conservation of Mass = #1 (Lavoisier) Law of Definite Proportions = #4 (Proust)

Dalton’s atomic model: JOHN DALTON (1803) Dalton’s atomic model: A marble → No parts

Cathode Ray Tube experiment J.J. THOMSON (1897) Electron discovery Cathode Ray Tube experiment Stream of particles aimed between 2 charged plates Particles deflected away from negatively charged place, so they must also have a negative charge (= electrons!)

Pudding → positively charged medium PLUM PUDDING MODEL Plums → electrons Pudding → positively charged medium

RUTHERFORD’S “GOLD FOIL” EXPERIMENT (1909)

RUTHERFORD’S “GOLD FOIL” EXPERIMENT (1909) Conclusions… NUCLEUS Must be really small - only 1 in 20,000 hit it Positively charged (b/c of protons!) - positive particles bounced off Massive (dense) - was able to make particle bounce backwards

Most of the atom is empty space NEW MODEL Most of the atom is empty space