Transcript Early Models of the Atom

The Evolution of Early Atomic
Models
Early Models of Atomic Structure
The work of Thomson,
Rutherford and Bohr…
Thomson’s Model
Discovered the
electron while
studying cathode
ray tubes in 1897.
He received the
Nobel Prize in 1906
Thomson’s cathode ray tube
(a vacuum tube with 2 electrodes)
His discovery of the electron…
Thomson found that the cathode ray
was a beam of negative particles
(electrons) and so atoms were not
indivisible.
Thomson’s “Plum Pudding” Model
of the Atom
He described his
atomic model as
negative charged
electrons scattered
in a lump of
positively charged
material, like
raisins scattered in
plum pudding ( a
popular dessert at
the time).
Rutherford’s Model of the Atom
Rutherford’s Gold
Foil experiment led
to the development
of his atomic
model in 1911
He received the
Nobel prize in
1908.
The Gold Foil Experiment
Rutherford fired a
beam of positively
charged particles
(called alpha
particles) at a
sheet of gold foil a
few particles thick.
The Gold Foil Experiment
Rutherford was
expecting results in
line with
Thomson’s model,
with the stream of
positive particles
passing through
the foil.
The Gold Foil Experiment
Instead, he
observed that
some of the alpha
particles were
repelled, while
most went through
the foil unchanged.
There was only one explanation…
A dense, very
positive charge
was condensed
into one place,
called the nucleus
The rest of the
atom had to be
mostly empty
space
Rutherford’s Model of the Atom
Understanding the basics of
Modern Atomic Theory
We need to understand wave
properties, and light energy
Wave Properties
Wavelength – λ, is the distance
between two like points on a wave
Wave Properties
Frequency, v, describes the number
of wave cycles per second.
The unit of frequency is
cycles/second (s-1), or the Hertz (Hz)
Wave Properties
Amplitude is the maximum height of
a wave, measured from the origin of
the wave
A wave has zero amplitude at certain
intervals along the wave,called nodes
The Electromagnetic Spectrum
Short wavelength………long wavelength
High frequency……………low frequency
High energy……………………low energy
How are wavelength and frequency
related?
How are frequency and energy
related?
What type of electromagnetic radiation
has the lowest frequency?
The Electromagnetic Spectrum
All electromagnetic radiation
(including visible light) travels at the
same speed.
The speed of light(c) = 3.0 x 108m/s
3.0 x 1010cm/s
3.0 x 1017nm/s
What travels faster, x- rays or visible
light?
Questions:
Which has the highest frequency, red
light or green light?
Which has the longest wavelength,
x-rays or microwaves?
Which has the highest energy, yellow
light or infrared?
Questions:
What mathematical relationship can
you draw about wavelength and
frequency?
The wavelength and frequency of
light are inversely related.
C = λv
Speed of light = wavelength x
frequency
( Since c = 3.0 x 108 m/s, you will be
asked to solve for wavelength or
frequency.)
λ = c/v
v= c/λ
The Bohr Model
In 1912, Niels Bohr
adapted
Rutherford’s model
to Planck’s
quantum theory
and so developed
his theory of
atomic structure
Atoms can give off light
Bohr’s model explained the atomic
emission spectrum of hydrogen. For this
he received the Nobel Prize in 1922. His
atomic model is based on these ideas.
The atomic emission spectrum of an
element is emission of particular
frequencies (colors) of light by energized
atoms of that element
Each atom’s emission spectrum is unique
The emission spectrum of
hydrogen
The most prominent spectral lines are
violet, blue, blue-green, and red.
Which of the lines has the lowest
frequency?
Which of the lines has the shortest
wavelength?
Atomic Emmision Spectra
Atoms can give off light
Firework colorants
Red: strontium,
lithium
orange: calcium
Gold: iron
Green: barium
Blue: copper
Purple: strontium
+ copper
Silver: magnesium
Bohr’sModel
In 1913, Bohr
proposed his model
of the atom. He
determined that
electrons can be
located in certain
discrete energy
states, called
energy levels
Bohr related his model to a ladder
As a person can stand on one rung of a
ladder or the next, yet is is impossible for
a person to stand between the rungs… an
electron can be found in one energy level
or the next, but not between levels.
The only way for the electron to jump to
the next level is for it to have a quantum
leap, which is the leap from one energy
level to another.
The energy of the electron has a definite
value in a stationary orbit. The electron
can jump from one stationary orbit to
another.
It it jumps from an orbit of lower energy
E1 to an orbit of higher energy E2 , it
aborbs a photon.
If it jumps from an orbit of higher energy
E2 to an orbit of lower energy E1 , it emits
a photon.
Which jump would produce the
highest energy light?
The Energy Levels of Hydrogen
Quantum- the amount of energy
required to move an electron from
one level to another
The Bohr Model of the Atom
Quantized energy
levels
Electron moves in
a circular orbit
Electron jumps
between levels by
absorbing or
emitting photon of
a particular
wavelength
Quantized Energy Levels
– Since only certain energy changes occur
for each type of element an atom must
contain discrete energy levels.
Bohr’s atomic model was ultimately
not successful
Bohr’s model considered the electron as a
particle, and classical physics shows that a
charged particle accelerating around a
circular path would lose energy, and so
the electrons would fall into the nucleus.
The modern model of the atom considers
the electron, not as a particle, but as a
matter-wave.