Transcript Atomic History - Seneca High School

Atomic Structure
History and Theories
The Greeks
4th century B.C.
Democritus
“Atomists” school of thought
Matter is composed of tiny indivisible
particles called atoms.
John Dalton
Dalton’s Atomic Theory
All elements are composed of
submicroscopic indivisible particles called
atoms.
Atoms of the same element are identical.
Atoms of different elements are different.
Atoms of different elements can physically
mix together or chemically combine.
Chemical reactions occur when atoms are
separated, joined, or rearranged.
J.J. Thomson
English physicist discovered electrons in
1897.
Experimented with flow of electron
current through gases.
Electrodes connected to high energy
electricity source.
Positive side – anode
Negative side- cathode
J.J. Thomson – cont.
Cathode ray – glowing beam, which
travels from the cathode to the anode.
Thomson proposed that the cathode ray
had a stream of small negatively
charged particles.
Plum Pudding Theory – Electrons are
imbedded in a positive sphere.
http://chemmovies.unl.edu/ChemAnime/CATHRAYD/CATHRAYD.html
Robert A. Millikan
American
Calculated by 1916 that the electron
had one unit of negative charge and
mass is 1/1840 the mass of Hydrogen
atom.
Sir James Chadwick
Subatomic particles with no charge –
neutron.
Mass is nearly equal to that of the
proton.
Subatomic Particles
Particle
charge relat.mass mass
Proton(p+) + 1
1amu
1.67x10-24g
Neutron(n0) 0
1amu
1.67x10-24g
Electron(e-) - 1 1/1840
9.11x10-28g
amu
amu = atomic mass unit
Ernest Rutherford
1911 – Gold Foil Experiment
Proposed that almost all of the mass
and all the positive charge are
concentrated in a small region at the
center of the atom called the nucleus.
Nucleus- the central core of the atom,
composed of protons and neutrons.
http://www.shsu.edu/~chm_tgc/sounds/ruther.mov
The Development of Atomic Models
Dalton’s Atomic Model – atoms are
billiard ball spheres.
Thomson’s Model – Electrons are
imbedded in a positive sphere.
Rutherford’s Model – Positive dense tiny
nucleus is surrounded by electrons in
empty space.
Atomic Models
Neils Bohr – 1913,
Danish physicist
Planetary Model
Electrons have a
fixed path.
Current Model – Quantum
Mechanical Model
Erwin Schrodinger – 1926
Quantum Mechanical Model – the modern
description of the electrons in the atoms
comes from the mathematical solution to
the Schrodinger equation.
Electron Cloud
Think of a fly trapped in a jar!