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The Early Atom
Chapter 22
Journey in the microscopic world
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Division of matter: can it be done indefinitely?
What are the basic constituents of matter?
19th century: atoms are considered as basic entities
many type of atoms: H, He, Li, C, O, Al, Fe,…….
the known atoms combine to create complex molecules
the great variety of substances are a result of different combination of a
few type of atoms
 Ordering the atoms after their weight
(Dmitri Mendeleev: periodic table of the elements)
- periodic properties of atoms
- atoms can be arranged in a table,
where each column has similar
chemical and physical properties.
 Existence of periodicity: a
clue that there might be an
underlying structure to atoms ….
Atomic Spectra
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When electric current passes through a gas, the gas gives off a
characteristic color
- passing the light through a prism to spread out the colors, the light
splits up into a pattern of discrete colored lines
- this lines are always the same for a given gas (and different from the
patterns produced by other gases): called emission spectrum
Spectral lines also appear when white light passes through a cool gas
- dark lines in the continuous rainbow spectrum
- the particular wavelengths are absorbed: absorption spectra
lines in the absorption and emission spectra coincide (more lines in the
emission spectrum)
Origin of the characteristic spectra unexplained !!!
Cathode
Rays:
the
Electron
High voltage applied across two electrodes in a partially evacuated
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glass tube produces an electric current and light in the tube.
If we use good vacuum a yellowish green glow is visible from one end
of the glass tube
indications that rays were coming from the negative electrode
(cathode -rays)
- deflected by electric and magnetic field as negatively charged particles
- fluorescent screen glowed wherever it was struck by the cathode rays
- if no electric or magnetic field is applied, thet travel in straight lines
 J.J. Thomson studies quantitatively the deflection of the cathode rays
- measures the charge-to-mass ratio (independent of the gas and
electrodes) 1800 times smaller than the one for the known ions!!!
- It should be a new particle! Thomson propose that is the constituent
of the atoms. Named: ELECTRON
- Thomson proposes that it has the charge of the Hydrogen ion, and a
mass 1800 smaller than that
Charge of the Electron
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Measured by R. Millikan (1910)
 Millikan produced varying charges on droplets (by cathode
rays) and studies there movement in electric field.
 finds that charges on each droplet were a multiple of an
identical chunk or quanta: 1.6 x 10-19 C, the charge given
by an electron to the droplet
 mass of the electron: 9.11 x 10-31 kg (1800 smaller than
the Hydrogen atom)
First model of the atom:
Thomson’s model
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Thomson proposed that atoms have internal structure
What does an atom look like?
Atoms has electrons inside
Atoms should be electrically neutral --> it should have also
a positively charged part.
Thomson proposed the “plum-pudding” model
- the atom consisting of some
sort of positively charged
material with electrons stuck
in it, like plums in a pudding
- cannot explain the discrete
emission and absorption spectra,
and the repetitive characteristics
in the periodic table
Rutherford’s Model
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An experiment that radically changed our view of the atom
- Bombarding materials with alpha particles (He++) (determining the
shape of an object…)
- The target was a thin gold plate
- expected that the deflection of the alpha particles is small, since
electrons have 7300 times smaller mass, and the positive charge is
uniformly distributed
Surprise! A small number of alpha particles were scattered in backward
direction (like hitting a wall….)
To explain the experimental results Rutherford proposed a new model
(the solar system model of the atoms)
- positive charge located in a very tinny place: the nucleus
- electrons orbit the nucleus like Planets the Sun
The model have the same problems as the one proposed by Thomson!
+ electrons in circular motion should constantly radiate
electromagnetic waves and should quickly loose its energy!
Radiating objects
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Germination of the next model: from the properties of the radiation
emitted by hot objects
all objects glow, emit electromagnetic radiation
at low and normal temperatures this radiation is invisible
all objects at temperatures different from zero emit a continuous
spectra of electromagnetic radiation
Distribution of the intensity of the electromagnetic radiation versus the
wavelength --> spectral distribution curve
Total intensity of the radiation ~ T4 (Stefan’s law)
The position of the maximum in the spectra, changes with temperature:
maxT=const (Wien’s displacement law)
The spectral distribution curve does not depend on the radiating
material (if we use a cavity) only on T --> universal shape
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no theory could describe the whole shape of the spectral distribution
curve--> crisis in classical physics
1900 Max Planck: a solution which describes well the distribution
BUT!!! Planck uses a hypothesis in total contradiction with classical
physics
- Atomic oscillators (whose vibration produce the radiation) can
vibrate only with certain discrete energies!
- Electromagnetic radiation caries this energy also in bundles, or
quanta, this energy quanta is: E=hf (h: the famous Planck
constant,
f: the frequency)
h=6.63 x 10-34 Js
- every atomic oscillator can have an energy which is a multiple of
this quanta (if it oscillates with frequency: f)
Discreteness of energy in disagreement with classical physics
Electromagnetic waves caries energy in discrete units --> back to the
particle model for light
The Photoelectric effect
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Another unexplained phenomenon in 1900
When light is shined on metallic objects, electrons are ejected from
their surface
- the electrons come off with a range of energies
up to a maximum energy, which depends on the
color of the light
- the intensity of the light does not determine the
maximum energy of the out-coming electrons, it
determines only their number
- experimental results in contradiction with the wave-model of light
Conflict solved by A. Einstein, using Planck’s hypothesis
- light carries energy in bundles, called photons
- the energy of a photon for an electromagnetic wave with
frequency f is:
E=h f
- a photon can give either no energy or gives its entire energy to an
atom, which can transfer this energy to an electron
New conflict: light is BOTH particle and wave
Old conflict: still no good model for the light
Bohr’s atomic model
1913 Niels Bohr, proposes a new model, incorporating Planck’s discrete
energies and Einstein’s photon
 Bohr’s model accept Rutherford’s picture + new postulates
 Extra postulates:
1. The angular momentum of the electron is quantized.
h
L n
The possible angular momentum is given as:
2
Consequence: - restriction on the possible radii and speeds
r n r
(r1=5.3 x 10-11 m is the smallest radius, and n an integer
called quantum number)
- the kinetic, potential and total energy of the electrons is also quantized
2. The electron does not radiate when it is on one of the allowed orbits
3. The atom can receive and emit only photons with well defined energies.
The energy of the photon equals the energy differences between the
possible states of the electrons.
- emitting and absorbing photons
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n
2
n
1
Explaining the atomic spectra
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Bohr’s model accounted for the existence of spectral lines
drawing energy-level diagrams
exciting the electron and return to a ground state
explaining the emission and absorption spectra
The Periodic Table
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Bohr’s theory give a fairy picture of the Hydrogen atom
It can not explain completely, however, the atoms with more than one
electron (complications due to the interactions between electrons)
Occupancy of the energy levels by the electrons
- first level: max. 2 electrons
- second level: max. 8 electrons
- third level: max. 8 electrons
- …..(will learn how on….)
atoms in the periodic table arranged after the number of the electrons
chemical properties of atoms are determined by the electrons in their
outermost shells
atoms “like” and tend to fill up the outermost shell completely with
electrons --> this is the driving force for making molecules
noble elements: the outer shell completely filled with electrons
- inert, no chemical activity (group 8
in the periodic table)
atoms from group one (alkaline metals)
are very active (tend to give up the single
electron in the outer shell)
atoms from group 7 (also very active)
(tend to receive one electron to fill up
completely the outermost shell)
X Rays
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1895 Wilhelm Roentgen: new radiation from cathode-ray tubes
this radiation had big penetration depth, very different from cathoderays, it is invisible, observable only from their effect on substances
it can be produced when cathode-rays enter in different substances
with high energy
it cannot be deviated by magnetic or electric fields (no charge)
X rays --> due to their unknown nature in 1895
X rays are electromagnetic waves, with very high frequency
important applications in medicine --> can reveal internal structures
an X ray spectrum typically consist of two parts:
- a continuous spectrum (produced by the
deceleration of the bombarding electrons)
- a set of discrete spikes (X-rays photons
emmited by the bombarded material)
energy of the X ray photons increases as the
atomic number of the bombarded element
is increasing
Summary
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emission and absorption spectrum
cathode rays are electrons; charge of an electron: 1.6 x 10-19 C
mass of the electron: 9.11 x 10-31 kg
cathode rays led to the discovery of X rays (electromagnetic waves)
Thomson’s atomic model: the “plum-pudding” model
Rutherford shows that the positive charge of an atom is concentrated in
a very tinny place: the nucleus (100,000 times smaller than the atom)
Rutherford’s model “solar system” model of the atom
all objects emit electromagnetic radiation; characteristic spectral
distribution curves; Wien and Stefan’s law
Planck’s theory about this heat radiation--> first clues that energies at
the atomic level are quantized
The energy quanta: E= h f (h=6.63 X 10-34 Js, the Planck constant)
Electromagnetic waves (and light also) carries energy in quanta, called
photons. Energy of a photon with frequency f: E= h f
Einstein explains the photoelectric effect
Bohr’s model for the H atom incorporates Planck.s discrete energies
and Einstein’s photons into the Rutherford’s model; it is based on three
extra postulates
The Bohr model was succecsfull in accounting for many atomic
observations, especially the emission and absorption spectra for
hydrogen and ordering of the chemical elements
Home-work assignment
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Part I. 575/1-10; 576/11-15,17-19,21-24;
578/1-12
Part II. 576/26,28; 577/30,32-38,40-42
577/45-51,54-55,57; 578/58/60,14-18,21,23