Early Atomic Models
Dalton, Thomson, Bohr,
Rutherford
Democritus
Democritus (460 BC – 370 BC) was
among the 1st to suggest the existence
of atoms.
Democritus believed that
atoms were indivisible and
indestructible.
His approach was not
based on the scientific method
Three laws of chemistry
Law of conservation of mass
(Lavoisier):
 Mass is neither created nor destroyed.
Law of definite proportion (Proust):
 A given compound always contains exactly
the same proportion of elements by mass.
Three laws of chemistry (cont.)
Law of multiple proportions (Dalton):
 When two elements form a series of
compounds, the ratios of the masses of the
second element that combine with 1 gram of
the first element can always be reduced to
small whole numbers.
Dalton’s atomic theory
1808
Each element is made up of tiny
particles called atoms.
Dalton’s atomic theory
The atoms of a given element are
identical; the atoms of different
elements are different in some
fundamental way or ways.
Dalton’s atomic theory
Chemical compounds are formed
when atoms of different elements
combine with each other. A given
compound always has the same
relative numbers and types of atoms.
Dalton’s atomic theory
Chemical reactions involve
reorganization of the atoms—
changes in the way they are bound
together.
The atoms themselves are not
changed in a chemical reaction.
Atom song
http://www.youtube.com/watch?v=vUzTQ
WnwfE&feature=related&safety_mode=true&
persist_safety_mode=1&safe=active
Dalton Model
The atom is indivisible
Thomson Model
JJ Thomson obviously knew about
Dalton’s atomic theory when he
discovered the electron.
Thomson took this information and
developed a model of the atom
Thomson Model
Cathode Ray Experiment
Designed to test emissions from metal
Cathode emits rays
Anode receives rays
Thomson Model
This model became known as the plumpudding model
This model said that the atom was a
diffuse positive charge and the electrons
were randomly scattered throughout the
atom.
Thomson Model
In the picture, the blue region is the diffuse
positive charge and the red spheres
represent the electrons randomly placed in
the atom.
Positively
charged space
Electrons
e
e
e
e
e
Cathode Ray Tube (CRT)
http://www.youtube.com/watch?v=IdTx
GJjA4Jw&list=LPGSFwN1M7s3Q&inde
x=4&feature=plcp&safety_mode=true&
persist_safety_mode=1&safe=active
Rutherford Au Foil Expt
Ernest Rutherford developed an
experiment that would help to reveal the
structure and location of the subatomic
particles in the atom.
This experiment was proposed by
Rutherford and carried out by two of his
graduate students: Geiger and Marsden.
Became known as Rutherford Gold Foil
Expt.
Rutherford
http://www.youtube.com/watch?v=bSEOO
Ms5VNU&safety_mode=true&persist_safe
ty_mode=1&safe=active
Two different models…
Which one is right?
Thomson Model
Au foil experiment should
either allow all of the
alpha particles (He2+) to
pass through untouched
(inconsistent with the
nature of solids—should
be tightly packed
positively charged
spheres) or allow no
alpha particles to pass
through which would be
consistent w/ a solid.
Rutherford Model
Unsure of what to expect.
Need to determine the
internal structure of the
atom. The scattering
pattern of the alpha
particles would help
indicate the atomic
structure.
Rutherford Au Foil Expt Applet
This applet illustrates what they saw.
http://micro.magnet.fsu.edu/electromag/jav
a/rutherford/
Rutherford Atom
All positive charge of atom was in nucleus
Essentially all mass was in the nucleus
Rutherford Model
The Au Foil Expt proved several things about the
structure of the atom:
All of the positive charge of the atom resides in a
very small part of the atom—we call this the
nucleus.
The nucleus occupies a very small part of the
atom’s total volume (only <1% of alpha particles
deflected by Au nuclei).
Electrons must be located outside of the
nucleus.
Most of mass of the atom is in the nucleus.
1)Dalton’s “Billiard ball” model (1808) Atoms are
solid and indivisible.
2)Thompson “Plum pudding” model (1904)
Negative electrons in a positive framework.
3)The Rutherford model (around 1911) Atoms
are mostly empty space. Negative electrons orbit
a positive nucleus.
Niels Bohr
Bohr was aware of
different spectra for
different elements
Hydrogen was
postulated to be the
simplest of all
substances because
it had the lightest
mass.
He concentrated on
hydrogen’s spectrum
Bohr Atom
Bohr postulated that the light that was
seen in the spectrum was due to electron
transitions.
We call this model of the atom the solar
system model of the atom.
Bohr Atom
In order to explain discrete spectra, Bohr found that atoms
obey three basic rules:
Electrons have only certain energies corresponding to
particular distances from nucleus. As long as the
electron is in one of those energy orbits, it will not lose or
absorb any energy. The energy orbits are analogous to
rungs on a ladder: electrons can be only on rungs of the
ladder and not in between rungs.
The orbits closer to the nucleus have lower energy.
Atoms want to be in the lowest possible energy state
called the ground state (all electrons as close to the
nucleus as possible).
Bohr Atom
Nucleus
Energy Levels
Erwin Schrödinger
Added orbitals (s, p, d, f) to Bohr’s energy levels
The orbital paths created an electron cloud that looks
fuzzy, rather that distinct orbits precisely on the surface
of spheres
The orbitals allow the electrons to avoid each other,
which is necessary as they are all negatively charged
He did this through some very rigorous mathematical
description of the electrons in the atom.
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