Transcript Early Atomic Models

Early Atomic Models
• Democritus
– World is made of two
things: empty space
and indivisible
particles, called atoms
– There are different
atoms for every
material in the world
• Aristotle
– Proposed that matter
was continuous and not
made of smaller
particles of matter
– He called this
continuous flow of
matter hyle
– This was accepted until
17th century
John Dalton (1800s)
• Elements consisted of very tiny particles,
called atoms, which cannot be broken into
smaller particles by chemical processes
• All atoms of an element were identical and
they had the same mass
• Compounds consisted of atoms of different
elements combined together
John Dalton’s Model
• Atoms were tiny, indivisible, indestructible
particles
• Each one had a certain mass, size and
chemical behavior
Dalton used these to prove:
• Law of Definite Proportions- specific
substances always contain elements in the
same ratio by mass
– Example: NO, NO2, N2O
• Law of Conservation of Matter- matter
cannot be created or destroyed. A chemical
reaction simply changes the way atoms are
grouped together.
J.J. Thompson (1890s)
• showed that atoms of any element have
negative particles with a mass.
• Theorized that positive and negative
charges were distributed through the atom,
called the plum pudding model.
Cathode Ray Tube Parts
Tube- sealed glass tube from which most of the air is
removed. It contains an a metal screen with material that
emits light when struck by electrons
Cathode- negative electrode where electrons originate
Anode- positive electrode which electrons travel towards
For a cathode ray tube to work you must have power source
• Cathode Ray Tube
Experiment
• Scientists found that the beam would bend
when it was in the presence of a magnet.
Therefore, he concluded that all atoms have
particles with a mass.
• The beam would bend toward a positively
charged plate. Therefore he concluded that
atoms have particles with a charge.
Experiment continued..
• Finally, Thomson measured the charge to mass ratio and
found that it was very large. This meant the particles were
a thousand times smaller than an atom.
• He found that charge to mass ratio was the same for every
element he tried and concluded that all elements have this
negatively charged particle that has mass.
• WE CALL THE PARTICLE THOMSON
DISCOVERED THE ELECTRON.
THIS WORK WON JJ THOMSON THE PHYSICS NOBEL
PRIZE IN 1906.
The Plum Pudding Model
• JJ Thomson- Protons were found later in a modified
cathode ray tube
• From this Thomson came up with the plum pudding
model. This showed that atoms are composed of negative
charges spread in a positve “pudding” .
Other Experiments
• Millikan- Obtained first accurate measurement of
an electrons charge using the oil drop experiment
• Millikan Experiment
• Roentgen discovered X-rays
• Bequerel discovered radioactivity
• Chadwick- discovered neutrons (1932)
Lord Rutherford and the Gold
Foil Experiment (1909)
• Used gold foil experiment to find positively
charged “core” of the atom which he called
the nucleus
• Found that the rest of the atom is composed
of empty space
Gold Foil Experiment
Rutherford-Bohr Atom“Planetary Model”
• Suggested electrons orbit nucleus the way
that the planets orbit the sun.
• The more energy an electron has, the farther
it will exist from the nucleus.
We use a lot of the same principles in our
modern day atomic theory, but not that
electrons orbit the nucleus!