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Atoms. 400 b.c. Greeks. Greeks philosophers ponder the nature of matter: what is it made of? Democritus: basic particle of matter = “atom” which means “ indivisble ”. Envisions these to be “ hard spheres” Aristotle: does not believe in atoms. Dalton’s Atomic Theory 1803.
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400 b.c. Greeks • Greeks philosophers ponder the nature of matter: what is it made of? • Democritus: basic particle of matter = “atom” which means “indivisble”. Envisions these to be “hard spheres” • Aristotle: does not believe in atoms
Dalton’s Atomic Theory1803 1) All matter is composed of extremely small particles called atoms
Dalton’s Atomic Theory1803 • Atoms of one element are all alike (size, mass)
Dalton’s Atomic Theory1803 3) Atoms can’t be created, destroyed or subdivided
Dalton’s Atomic Theory1803 • Atoms of different elements combine in simple whole number ratios to form compounds
Dalton’s Atomic Theory1803 5) In a chemical reaction, atoms are combined, separated, or rearranged
1897 J.J. Thompson Cathode Ray Experiments The electron is discovered…. “PLUM PUDDING” model
Rutherford’s Gold Foil Experiment1909 Ernst Rutherford bombards a piece of gold foil with positively charged alpha particles (the nucleus of a helium atom)
Some alpha particles bounce back
Rutherford’s Gold Foil Experiment Observations: • Most of the alpha particles passed straight through the gold foil as if it wasn’t even there! • The nucleus of the atom is small, dense, and positively charged
Rutherford’s Gold Foil Experiment Interpretation (conclusions): • Atoms are mostly empty space and therefore MATTER is mostly empty space • Atoms have a small, dense, positively charged nucleus
+ + + This alpha particle passes through the atom’s empty space + + + Alpha particles have + charge two + charges will repel each other
Subatomic Particles(Table O) Proton Electron Neutron
Protons • Located in the atom’s nucleus • Positive charge +1 • Mass = 1 amu (Atomic Mass Unit)
Neutrons • Located in the atom’s nucleus • zero charge (neutral) • Mass = 1 amu (Atomic Mass Unit)
Electrons • Located in the electron cloud outside the nucleus • Negative charge -1 • Mass = 1/1836 amu (negligible)
A model of the atom to scale • If YOU are the nucleus of an atom, your closest electron would be…. • ……..in downtown Rhinebeck!