Download Review of the Atom

History of Atomic Theory
Democritus vs. Aristotle
• proposed all matter made
of particles (“atoms”)
• proposed 4 elements:
Earth, Wind, Fire, Water
John Dalton & Atomic Theory
1803
Dalton proposed Atomic Theory:
1. All matter composed of atoms
‫٭‬2. Atoms are indestructible
- can’t be created or destroyed during physical/chemical changes
‫٭‬3. All atoms of an element are identical (same
mass)
4. Atoms of different elements have different masses
5. Compounds are formed by combining atoms of
different elements
Dalton’s Model: billiard ball model
•Solid, indivisible sphere
Thomson
Discovered the electron
Cathode Ray Tubes
Thomson showed that cathode rays were
streams of negatively charged particles
• rays were much smaller than atoms
Thomson’s Model: plum-pudding model
(AKA: “chocolate chip cookie”)

Solid, positive sphere; negative charges
distributed randomly throughout
Thomson’s Plum-Pudding Model
atom’s (+) charge
evenly spread out
while (-) charge
is in bits – like
chocolate chips in
cookies
Rutherford
Rutherford’s Experiment - 1911


Discovered nucleus with his alpha scattering
gold foil experiment
The experiment showed the atom:


Is mostly empty space
Contains a dense core called nucleus


(-) charged particles surround nucleus
Nucleus contains (+) charged particles called
protons
Gold Foil Experiment
Rutherford’s exp’t: animation
Rutherford’s Model:
nuclear model
source
• atom has a dense (+)
core with (-) electrons
orbiting around the
nucleus
Neils Bohr:
•Proposed that electrons
travel in fixed, circular
pathways around nucleus,
held in place by protons (+)
•worked only for hydrogen
Bohr’s Electron Shell Model:
• Electrons travel in distinct,
fixed energy levels around a
(+) nucleus
Ground state: electrons in
lowest possible energy levels
Excited state: one or more
electrons move up to a higher
energy level
Schrodinger’s Model
Orbital Model


Electron’s are in a cloud surrounding the (+)
nucleus
Electrons are located in “probability regions”
or orbitals



These are not circular orbits!
Orbitals differ in size, shape & spacial orientation
Explained bright-line spectra for all elements, not
just hydrogen
The development of
atomic theory
represents the work
of many scientists
over many years