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The History of Atomic Theory
DemocritusDemocritus
Greek philosopher 2400 years ago The Atom
Could matter be divided into smaller and smaller pieces forever?
Or was there a limit to the number of times a piece of matter could be divided?
AtomosAtomos
Matter could NOT be divided into smaller and smaller pieces forever
This piece would be indivisible.
“atomos” =“not to be cut.”
AtomosAtomos Atoms= small, hard
particles that were all made of the same material
Different shapes and sizes.
Atoms were infinite in number, always moving and capable of joining together.
AristotleAristotle
Same time periodDenounced the
idea of atomosEarth, Water, Air,
and Fire
AlchemistsAlchemists
Middle Ages Worked on
turning ordinary metals into gold
Derived modern chem
John DaltonJohn Dalton
English chemistEarly 1800’sRevisited the
particle theoryLaid the
groundwork for Modern theory
Father of the Atom
Dalton’s TheoryDalton’s Theory1. All matter small
particles a.k.a atoms
2. Atoms of the same element are exactly alike. Atoms of different elements are different.
3. Atoms cannot be divided, created, nor destroyed
Dalton’s TheoryDalton’s Theory4. Atoms of different
elements can combine to form compounds
5. In chemical reactions, atoms are combined, separated, or rearranged
Known as the “Billiard Ball Model”
J. J. ThomsonJ. J. Thomson
English scientist1897The electronThe first to
believe an atom is made of even smaller particles.
Thomson ModelThomson Model
Atoms were made from a positively charged substance with negatively charged electrons scattered
A.k.a: the “Plum Pudding Model”
Thomson Model: Cathode RayThomson Model: Cathode Ray
Thomson studied the passage of an electric current through a gas.
As the current passed through the gas, it gave off rays of negatively charged particles.
Robert A. MillikanRobert A. Millikan
American physicist 1909 Determined the mass
of an electron relative to the atom
Millikan’s: Oil Drop ExperimentMillikan’s: Oil Drop Experiment
Mist of fine oil droplets are dropped into the chamber
Some of the droplets were negatively charged
No electricity electrons fell through: gravity is only acting force
Millikan’s Oil Drop ExperimentMillikan’s Oil Drop Experiment
Voltage adjusted so down force = up force
Oil drop suspended between plates
The charge on the droplet was calculated
Mass of electron = charge of electron: charge-to-mass ratio of electron
Mass = 1/2000 of the Hydrogen atom
Ernest B. RutherfordErnest B. Rutherford
English physicist English physicist 19081908Gold Foil Gold Foil
ExperimentExperiment
Gold Foil Experiment: ResultsGold Foil Experiment: Results Most of the positively
charged “bullets” passed right through the gold atoms in the sheet of gold foil without changing course at all.
Some of the positively charged “bullets,” however, did bounce away from the gold sheet as if they had hit something solid.
What the Gold Foil Experiment ProvesWhat the Gold Foil Experiment Proves
Niels BohrNiels Bohr
Danish scientist1913Solar System Model
Bohr ModelBohr Model According to Bohr’s atomic
model, electrons move in definite orbits around the nucleus, much like planets circle the sun.
These orbits are located at certain distances from the nucleus based on relative energies.
1932
Neutron: There was unaccounted mass so there had to be another subatomic particle
Isotopes: Atoms of the same element (identical protons) but different numbers of neutrons and therefore different masses
James ChadwickJames Chadwick
Electron Cloud Model
The Electron Cloud ModelThe Electron Cloud Model Heisenburg Uncertainty Principle: cannot
know speed and location of electron at same time
Electrons in organized orbitals Electrons travel so fast that they seem to
form a “cloud” around the nucleus Location of electron depends on its energy Light spectrum
Electron Cloud:Electron Cloud: Electrons: low NRG are closest to nucleus Electrons: high NRG are further from nucleus
Historical Models Historical Models
Indivisible Electron Nucleus Orbit Electron Cloud
Greek X
Dalton X
Thomson X
Rutherford X X
Bohr X X X
Wave X X X