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Atomic Models
Historical Development
Ancient Greece
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In the fourth century B.C., Greek
philosopher Democritus inferred
that substances were made of
invisible units called atoms.
 “Atom” is derived from the Greek
word meaning “unable to be
divided.”
 Democritus, though, could not
provide evidence that atoms really
existed.
Centuries of Work
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Throughout the centuries that
followed, other theories were
proposed:
 Emphasis was put on making
careful and repeated
measurements.
 More reliable data were collected.
Centuries of Work
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Beginning in the 1800s, newer models
were devised:
 More discoveries about electrons,
protons, and neutrons.
 The model of the atom was
continuously refined and revised.
Dalton Model
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In the early 1800's, John Dalton developed the
first scientific model about atoms:
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All matter composed of tiny particles
Particles can’t be divided into smaller particles.
Atoms of each element are exactly alike.
Atoms of different elements are different masses.
Atoms of different elements join to form different
compounds.
Dalton imagined atoms to be solid spheres.
Modern Atomic Theory
Several changes have been made to Dalton’s theory:
Dalton said:
Atoms of a given element are identical in size,
mass, and other properties; Atoms of other
elements differ in size, mass, and other properties.
Modern Theory states:
Atoms of an element have a characteristic average
mass which is unique to that element.
Modern Atomic Theory
Dalton said:
Atoms cannot be subdivided, created or destroyed
Modern Theory states:
Atoms cannot be subdivided, created or destroyed
in ordinary chemical reactions. However, these
changes can occur in nuclear reactions.
Thomson Model
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At the end of the 1800's J.J.Thomson discovered that
atoms were not just solid spheres:
 He discovered the electron in 1897
 This meant that atoms contained even smaller,
subatomic particles.
In 1903, he developed a model that imagined negative
electrons in a ball of positive material.
The model was called Thomson’s
“Plum Pudding Model.”
Discovery of the Electron
In 1897, Thomson used a cathode ray tube to deduce the
presence of negatively charged particles.
Cathode ray tubes pass electricity through a gas that is
contained at a very low pressure.
Conclusions from the Study of
the Electron
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Cathode rays have identical properties
regardless of the element used to produce
them. All elements must contain identically
charged electrons.
Atoms are neutral, so there must be a
positive particle in the atom to balance the
negative charge of the electrons.
Electrons have so little mass that the atoms
must contain other particles that account for
most of the mass.
Rutherford's Model
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Ernest Rutherford made the discovery of protons in
1908 In 1911 – he did the “Gold Foil Experiment”:
 He tested the theory that protons and electrons
were evenly scattered throughout the atom.
 He aimed a positively charged particle beam at a
sheet of gold foil.
 Most passed through, but some, surprisingly
bounced back.
 The particles struck something larger than a
single proton.
Gold Foil Experiment
Rutherford’s model
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Rutherford’s model further
refined the atom’s structure:
 An atom is mostly empty space.
 The small positive nucleus is in the center
 Most of an atom’s mass is contained in
the small nucleus.
 The nucleus is surrounded by negative
orbiting electrons
James Chadwick
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Recognized that the presence of
protons and electrons explained the
neutral charge of the atom but did not
account for total mass of an atom
James Chadwick
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In 1932, Chadwick discovered another
particle within the nucleus
It’s mass was approximately the same
as a proton
He named it the neutron
Bohr Model
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In 1913, Danish scientist Niels Bohr explained
that Electrons surround the nucleus in distinct
energy levels:
 Energy levels are arranged in concentric
circles like layers of an onion.
 Each electron has a certain amount of
energy, which keeps it moving in its level.
Bohr's model has been called the “planetary
model”. It compares electrons to planets and
the nucleus to the sun
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Bohr Model
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Each energy level is further away
from the nucleus.
The farther a level is from the
nucleus, the higher the energy an
electron needs to stay in that level.
By absorbing or releasing specific
amounts of energy, an electron can
move from one energy level to the
next.
An electron can't exist between
energy levels.
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Modern Electron Cloud Model
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Although many of the basic ideas
of the Bohr atomic model still hold
true, scientists now know that
electrons do not actually orbit the
nucleus as in the planetary model.
 The electron cloud model is
now used to describe atoms.
 Electrons dart about in a
constantly changing path.
 Electron paths form a region
called an electron cloud.
Modern Electron Cloud Model
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Example: The idea can be related to the
blur of a fan as the blades move, they seem
to fill in the spaces between them, just as
fast-moving electrons fill the space around
the nucleus.
Modern Electron Cloud Model
A.K.A. The Quantum Mechanics Model
Electrons act more like an energy wave
than a tiny particle [like getting hit by a
wave, not a rock]
Only the probable location of an electron
can be determined, not the exact
location.
Electron Orbitals
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Like in the Bohr model, electrons are arranged in energy
levels within an electron cloud.
Electrons are subject to the laws of quantum mechanics. This
means they carry only certain quantities of energy:
 These strengths define the energy levels.
 Electrons with the lowest energy are in energy levels
closest to the nucleus.
 Electrons with the highest energy are in energy levels
farthest from the nucleus.
 Each energy level can be further broken down into
sublevels, or orbitals.
 Orbitals help identify electron placement.
Orbitals form a number of simple to bizarre
3-dimensional shapes, depending on their energy.