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History of the Atom
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Atomic Theory: History
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Prominent scientists and their
contributions to atomic theory
Development of current atomic
theory
Show that science is a constantly
changing field
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Demicritius: 500 BC
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Suggested that matter consists of
small indivisible particles called
atoms.
No one believed him.
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Antoine Lavoisier: 1789
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Law of Conservation of Mass:
Mass is not created or destroyed
during a chemical reaction
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1803 - John Dalton
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Dalton's Atomic Theory
1) Matter is composed of small indivisible particles called
atoms
2) An element is composed entirely of one type of atom.
The properties of all atoms of one element are identical
and are different from those of any other element.
3) Atoms of different elements can physically mix together
or chemically combine in simple whole number ratios to
form compounds.
4) Chemical reactions occur when atoms are separated,
joined or rearranged. Atoms of one element, however, are
never changed into atoms of another element as a result
of a chemical reaction.
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Dalton’s Theory
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Joseph Proust: 1806
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Law of Definite Proportions: A
chemical compound always
contains the same mass ratio of
elements
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Dmitri Mendeleev: 1869
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Published first Periodic table
Arranged elements by atomic
mass and allowed gaps where
necessary
Remarkable due to fact the he
was not located close to scientific
research
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Goldstein: 1886
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Discovered positive particles
using a cathode ray tube
Particles are called protons
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J J Thomson: 1897
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Cathode Ray Experiment
Beam of electrons that flows from the cathode (-) to
the anode (+).
Discovered the electron in 1897.
Discovered that most of the mass of an atom was positively
charged
Measured the Charge to Mass ratio ( e/m) of the electron.
Nobel Prize in Physics in 1906 for his discovery of the
electron
Proposed “Plum Pudding Model” of the atom
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Ernest Rutherford: 1898
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Identified two types of radiation emitted
from Uranium.
He calls them alpha and beta.
In 1900, Paul Villard finds a third type
which he calls gamma.
During this same year, it is discovered
that beta radiation is composed of
electrons, and gamma radiation is very
short wavelength electro-magnetic rays.
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Types of Radiation
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Terms
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Radioactivity - the process by which
materials give off unseen rays
Radiation - the penetrating rays and
particles emitted by a radioactive source
Nuclear Stability: As the neutron/proton ratio
deviates from 1:1, the nucleus becomes
more unstable
Half-life: time required for one-half of the
nuclei of the radioisotope to decay to a new
element
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Marie Curie: 1902
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With her husband discovered
radioactivity while studying radium
and polonium
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Ernest Rutherford: 1909
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Showed what happens to an
element during radioactive decay.
Constructed the first nuclear model
of the atom, a cornerstone of
present-day physics.
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R A Milliken: 1909
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Oil drop experiment
Measured the charge and mass of
an electron
Nobel prize in 1923
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Oil Drop Experiment
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Ernest Rutherford: 1911
Gold Foil Experiment
Theory:
 Shoot a stream of alpha ( + charged ) particles at a very
thin layer of gold foil and they should easily pass through
with only minor deflection.
Results:
 Most particles passed straight through with no deflection
 Some were sharply deflected back from the gold foil
 Rutherford said, " It was almost as incredible as if you
fired a 15 inch shell at a piece of tissue paper and it came
back and hit you.“
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Gold Foil Experiment:
Conclusions
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Most of the mass and all of the positive charge of the
atom resides in a small region called the nucleus.
The magnitude of the positive charge is different for
different atoms.
The charge of an atom is neutral. There are as many
electrons outside the nucleus as there are protons in the
nucleus.
Electrons stay outside the nucleus unless acted upon by
some force.
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Frederick Soddy: 1913
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Formulated the idea of isotopes
Isotopes are atoms of the same
element with different masses due
to a different number of neutrons
Note that the neutron has yet to be
discovered
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Rutherford: 1919
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Atoms can be transformed into
elements by tampering with the
nucleus
Demonstrated that the nucleus of
an atom could be breached
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Neils Bohr: 1922
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Observed “line Spectra” and noticed that
atoms do not emit energy continuously,
but in precise quantities.
Suggested that electrons are located in
energy levels which keeps them from
falling into the nucleus
Predicted that specific energy levels hold
only a certain # of electrons
Proposed the model of electrons orbiting
the nucleus of the atom similar to how the
planets orbit the sun
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Bohr Model
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Erwin Schrodinger:
1925
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Schrödinger’s equation is used
to describe the state of
electrons in the atom
Develops Quantum Mechanical
Model of the atom
Quantum Mechanical Model of
the atom describes the location
of electrons defined by a 3 D
mathematical probability region29
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James Chadwick: 1932 
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Using alpha particles, discovered
a neutral atomic particle with a
mass close to that of the proton.
This is what we know as a neutron.
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Parts Of Atom
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Particle
Symbol Charge
Mass
Electron
Proton
Neutron
ep+
nO
1/1840
1
1
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11+
0
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Otto Hahn and Fritz Strassmann:
1938
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Discovered nuclear fission while
working with uranium
They would bombard it with
neutrons and the nucleus would
split into 2 parts
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Fission Reaction
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Fission vs. Fusion
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Fission is splitting of the nucleus
into 2 or more smaller atoms
Fusion is joining 2 smaller nuclei
into one new larger atom
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