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Atomic Theory
Chem. 11
Dalton’s Atomic Theory
• 1 All matter is made up
of invisible particles called
atoms
• 2 All atoms of one
element are identical. The
atoms of any one element
are different from those of
all other element.
• 3 Chemical change is the
union or separation of
atoms
• 4 Atoms combine in
small whole number ratios
to form compounds.
Thomson’s Model
• Joseph Thomson
realized that the
accepted model, of his
time, did not take into
account electrons and
protons.
• Thomson created a new
model, known as the
“plum pudding atom.”
Thomson’s Model
• His model contains
negatively charged
electrons, raisons, which
are stuck into a lump of
positively charged
protons, dough.
• The model does not
contain any neutrons as
they were not discovered
during Thomson’s time.
Thomson’s Model
• Fails to explain about
the number of protons,
their arrangement in
the atom, or the ease
with which atoms are
stripped of electrons to
form ions
Rutherford’s Model
• Ernest Rutherford
performed the Gold Foil
experiment, which led
to his discovery of the
nucleus
• Rutherford proposed
the nuclear atom in
which electrons
surround a dense
nucleus.
Rutherford’s Model
• The Gold foil experiment
involved the firing of
radioactive particles
through a thin gold foil
and detecting the
particles using screens
coated with zinc sulfide.
• Rutherford discovered
that though most of the
particles travelled straight
through the foil, some
particles were deflected.
Rutherford’s Model
• He concluded that the atom
is mainly empty space with
all the positive charge and
mass being located in the
center known as the
nucleus.
• An argument with
Rutherford’s model is what
is keeping the negatively
charged electrons from
being attracted to the
positively charged nucleus?
• Later experiments showed
that the nucleus was
composed of protons and
neutrons
Bohr’s Model
• Niels Bohr developed a
new atomic model to
answer the question of
why electrons do not
collapse into the nucleus
in Rutherford’s model.
• He proposed that
electrons are arranged in
concentric circular paths,
know as orbits, around
the nucleus.
Bohr’s Model
• His model is fashioned
after the motions of the
planets around the sun.
• It is often referred to as
the planetary model.
• Bohr proposed that the
electrons in a particular
path have a fixed
energy; the electrons
do not lose energy and
do not fall into the
nucleus
Energy Levels
• The Energy Level of an
electron is the region
around the nucleus where
it is likely to be moving.
• The lowest rung of the
ladder corresponds to the
lowest energy level
• An electron can move
from one energy level to
another, but it cannot
stop between levels
Energy Levels
• An electron must lose
or gain just the right
amount of energy to
move from one energy
level to another.
• The amount of energy
gained or lost by every
electron is not the
same.
• The energy levels in an
atom are not equally
spaced.
Energy Levels
• A Quantum of energy is the amount of energy
required to move an electron from its present
energy level to the next higher level.
• The energies of electrons are said to be
quantized
• In general, the higher an electron is placed on
the energy ladder, the larger the diameter of
the circular path of that electron
Quantum Mechanical Model
• The Quantum Mechanical
Model is the modern
description of the
electrons in atoms
• The model leads to
quantized energy levels
for an electron, but it
does not define the exact
path an electron takes
around the nucleus
Quantum Mechanical Model
• The model is concerned
with the likelihood of
finding an electron in a
certain position.
• It is similar to a cloud;
the most dense portion
is where the probability
of finding an electron is
larger.