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Fall 2015
Chapters 2 & 3
Atoms,
Atomic Structure and
Electronic structure
Atoms
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Fall 2015
Atoms: The Greek Idea
~384 B.C.E., Aristotle:
All matter is
composed of four
elements and all
matter is continuous,
not atomistic.
Aristotle declared matter to be
infinitely divisible.
Atoms: The Greek Idea
~ 450 B.C.E., Leucippus and
Democritus
Atomos: The point at which matter can no
longer be subdivided.
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Atomic Theory of Matter
The theory that atoms
are the fundamental
building blocks of
matter reemerged in
the early 19th century,
championed by John
Dalton.
1803
Dalton’s Postulates
1. Each element is composed of extremely small
particles called atoms.
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Dalton’s Postulates
2. Atoms of an element cannot be created, destroyed, broken into
smaller parts or transformed into atoms of another element
•
The discovery of nuclear processes showed that it was possible to
transform atoms from one element into atoms of another. But we
don't consider processes that affect the nucleus to be chemical
processes. The postulate is still useful. A slightly more restrictive
wording is "Atoms cannot be created, destroyed, or transformed into
other atoms in a chemical change".
Dalton’s Postulates
2. Atoms of an element cannot be created, destroyed, broken into
smaller parts or transformed into atoms of another element
• The discovery of nuclear processes showed that it was possible to
transform atoms from one element into atoms of another. But we
don't consider processes that affect the nucleus to be chemical
processes. The postulate is still useful. A slightly more restrictive
wording is "Atoms cannot be created, destroyed, or transformed into
other atoms in a chemical change".
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Dalton’s Postulates
3. All atoms of a given element are identical to one another in
mass and other properties, but the atoms of one element are
different from the atoms of all other elements.
Actually most elements occur in nature as mixtures of two or more
kind of atoms called isotopes that have slightly different masses
but same nuclear charges.
In modern atomic theory, the postulate has been amended to
read: "Elements are characterized by the nuclear charge of their
atoms".
Dalton’s Postulates
3. All atoms of a given element are identical to one another in
mass and other properties, but the atoms of one element are
different from the atoms of all other elements.
Actually most elements occur in nature as mixtures of two or more
kinds of atoms called isotopes that have slightly different masses
but same nuclear charges.
In modern atomic theory, the postulate has been amended to
read: "Elements are characterized by the nuclear charge of their
atoms".
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Dalton’s Postulates
4. When elements react, their atoms combine in simple, wholenumber ratios.
Dalton’s Postulates
5.When elements react, their atoms sometimes combine in
more than one simple, whole-number ratio.
Nitrogen and oxygen combine to produce more than one product
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Law of Constant Composition
Joseph Proust (1754–1826)
• Also known as the law of definite
proportions.
• The elemental composition of a pure
substance never varies.
1799
Regardless of the source, copper carbonate has
the same composition.
Malachite (mineral)
Cu(OH)2CO3
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The Berzelius experiment illustrates the Law of
Definite Proportions.
Law of Conservation of Mass
The total mass of substances present at
the end of a chemical process is the same
as the mass of substances present before
the process took place.
1785
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Law of Conservation of Mass
Law of Multiple Proportions
“If two elements form more than one compound between
them, then the ratios of the masses of the second
element which combine with a fixed mass of the first
element will be ratios of small whole numbers.”
Or…
”If two elements form 2 different compounds, the mass
ratio of the elements in one compound is related to the
mass ratio in the other by a small whole number”.
1804
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Atomic structure
Electricity and the Atom
Electrolyte: A compound that conducts
electricity when molten or dissolved in water.
Electrodes: Carbon rods or metallic strips that
carry electrical current.
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E
Electrolysis
Anode: A positive
electrode.
Cathode: A negative
electrode.
1800:
1807:
A. Volta invents the electrochemical cell
H. Davy carries out the electrolysis of potassium hydroxyde
Ions
Ion: An atom or group of atoms with a
charge.
Anion: A negative ion.
Cation: A positive ion.
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Discovery of the Electron
• Streams of negatively charged particles were
found to exit from cathode tubes.
• J. J. Thomson is credited with their discovery
(1897).
1897
Discovery of the Electron
1897, Joseph John Thomson:
Determined the charge:mass ratio of cathode
rays (discovered electrons).
Charge/mass = 1.76 × 108 C/g
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The Electron
Thomson used a magnetic field to bend the
cathode rays.
The amount the cathode ray bent from the
straight line using the magnetic field allowed
Thomson to calculate
the e/m ratio= 1.76  108 coulombs/g.
CRT TV
(Cathode Ray Tube)
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CRT TV
(Cathode Ray Tube)
Refresh rate: 60-100 Hz
Sir William Thomson
(1824-1907)
J. J. Thomson
(1856-1940)
British mathematical physicist
British physicist
1892:
Knighted by Queen Victoria.
Titled « Baron Kelvin of Largs » (Lord Kelvin)
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Goldstein’s Experiment:
Positive Particles
1886, Goldstein:
Observed positive
rays using a
perforated cathode.
1886
Millikan's discovery
1913
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Millikan's discovery
Robert Millikan determined the charge on the electron to be
-1.60  10- 19 coulombs.
For convenience, electronic charge is expressed as a multiple of that
charge rather than coulombs. Thus the charge of the electronic charge is 1.
Millikan's discovery
1) The drop is allowed to fall and its terminal velocity v1 in the
absence of an electric field is calculated.
The drag force (ie friction force/air resistance) acting on the drop, Fd,
is
terminal
velocity of the
falling drop
radius of the
drop
viscosity of the air
(eta)
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Millikan's discovery
W  m g
 d V  g
2) For a perfectly spherical droplet the apparent weight, W,
can be written as
density of air
radius of the
drop
gravity
acceleration
constant
density of the
oil
Millikan's discovery
3) At terminal velocity the oil drop is not accelerating.
This implies
W F
d
4 3
r g    air   6rv1
3
 r
9v1
2 g    air 
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Millikan's discovery
4) Now the electrical field is turned on, and the electric force on the
drop, FE, is
FE  qE
electrical field
charge on the
oil drop
E
For parallel plates,
So:
FE  q 
V
d
V
d
potential difference
distance between
the plates
Millikan's discovery
5) By adjusting V until the oil drop remains steady, we get
FE  W
q
V 4 3
 r g    air 
d 3
4
d
q  r 3 g    air 
3
V
charge on the
oil drop
q was found to be a multiple of -1.60  10-
19
coulombs.
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Electron mass
The electron mass was determined indirectly
to be m= 9.10938 x 10-28g.
X-Rays
1895, Wilhem
Roentgen:
Using a cathode ray
tube, Roentgen
discovered X-rays.
shotgun pellets visible without surgery
3/38
1895
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Radioactivity
• The spontaneous emission of radiation by a
radioactive atom.
• First observed by Henri Becquerel (1896)
• Also studied by Marie and Pierre Curie.
Radioactivity
• Three types of radiation were discovered by
Ernest Rutherford:
 particles
  particles
  rays

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Radioactivity
Model of the atom
J. J. Thomson who discovered the electron,
proposed the “plum pudding model” of the atom
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Rutherford Gold Foil Experiment
Using an apparatus similar to that shown below,
Ernest Rutherford discovered the atomic
nucleus.
Rutherford Gold Foil Experiment
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The nuclear atom
• Rutherford postulated a very small, dense
nucleus with the electrons around the outside
of the atom.
• Most of the volume of the atom is empty
space.
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Other subatomic particles
• Protons were discovered by Rutherford
(1919).
• Neutrons were discovered by James
Chadwick in (1932).
Subatomic particles
• Protons and electrons are the only particles that have a
charge.
• Protons and neutrons have essentially the same mass.
• The mass of an electron is so small we ignore it.
• 1 amu = 1.66054 x 10–24 g.
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Symbols of elements
Elements are symbolized by one or two letters.
Atomic Number
All atoms of the same element have the same
number of protons:
The atomic number (Z)
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Atomic Mass
The mass of an atom in atomic mass units (amu)
is the total number of protons and neutrons in
the atom.
Isotopes
• Atoms of the same element with different
masses.
• Isotopes have different numbers of neutrons.
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6C
12
6C
13
6C
14
6C
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Isotopes of Hydrogen
Electronic
structure
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Quantum theory
Branch of physics which deals with physical phenomena at
microscopic scales.
Quantum theory is the theoretical basis of modern physics
that explains the nature and behavior of matter and energy
on the atomic and subatomic level.
It provides a mathematical description of much of the
behavior and interactions of energy and matter.
The wave nature of light
• The electronic structure of an atom refers to
the arrangement of electrons.
• Interaction of light (electromagnetic radiation)
with matter has provided us with a lot of
information about the electronic structure of
atoms.
• Visible light is a form of electromagnetic
radiation
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• All waves have a characteristic wavelength,
l(lambda), amplitude, A and frequency n.
• The speed of a wave is given by its
frequency (hertz) multiplied by its
wavelength.
• For light, speed is c = n l
Electromagnetic radiation moves through a vacuum
with a speed of 3.00 x 108 m/s.
Modern atomic theory arose out of studies of
the interaction of radiation with matter.
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• The electromagnetic spectrum is a display
of the various types of electromagnetic
radiation arranged in order of increasing
wavelength.
• Example: visible radiation has wavelengths
between 400 nm (violet) and 750 nm (red).
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Line Spectra and Bohr Model
• Radiation composed of only one
wavelength is called monochromatic.
• Radiation that spans a whole array of
different wavelengths is called continuous.
• When radiation from a light source, such
as a light bulb, is separated into its
different wavelength components, a
spectrum is produced.
Continuous Spectrum
White light can be separated into a continuous spectrum of colors.
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A rainbow is a continuous spectrum of light produced by the
dispersal of sunlight by raindrops
Line Spectrum
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Line Spectra Of Na and H
Electron Arrangement:
The Bohr Model
Flame tests: Different elements give different colors to
a flame.
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Bohr’s Model
• Bohr noted the line spectra of certain
elements and assumed the electrons
were confined to specific energy states.
These were called orbits.
Bohr’s Model
• Bohr’s model is based on three postulates:
1. Only orbits of specific radii, corresponding
to certain definite energies, are permitted for
electrons in an atom.
2. An electron in a permitted orbit has a
specific energy and is in an "allowed" energy
state.
3. Energy is only emitted or absorbed by an
electron as it moves from one allowed
energy state to another.
(The energy is gained or lost as a photon).
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Energy Level in Hydrogen Atom
Limitations of the Bohr Model
• The Bohr Model has several limitations:
• It cannot explain the spectra of atoms other than
hydrogen.
• Electrons do not move about the nucleus in
circular orbits.
However, the model introduces two important
ideas:
• The energy of an electron is quantized:
electrons exist only in certain energy levels
described by quantum numbers.
• Energy gain or loss is involved in moving an
electron from one energy level to another.
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Quantum Mechanics and Atomic Orbitals
• Schrödinger proposed an equation containing both wave
and particle terms.
Solving the equation leads to wave functions
• The wave function describes the electron’s matter wave.
• The square of the wave function,  2, gives the
probability of finding the electron.
That is,  2 gives the electron density for the atom.
•  2 is called the probability density.
Electron density
A region of high electron density is one
where there is a high probability of finding
an electron.
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• A collection of orbitals with the same value
of n is called an electron shell.
• A shell is made of one or more subshells.
• Each subshell is designated by a number
and a letter.
Representations of Orbitals
• All s orbitals are spherical.
• As n increases, the s orbitals get larger.
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s Orbitals
p Orbitals
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d Orbitals
f Orbitals
• When n is equal to 4 or larger, there are seven f
orbitals for which l=3.
• The shape of f orbitals are very complicated than
those of d orbitals
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Many-Electron Atoms
Orbitals and Their Energies
• In a many-electron atom, for a given value
of n, the energy of an orbital increases
with increasing value of l (2s and 2p).
• Orbitals of the same energy are said to be
degenerate.
Ordering of Orbital energy
levels
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Electron Configurations
• Electron configurations tell us how the
electrons are distributed among the
various orbitals of an atom.
• The most stable configuration, or ground
state, is that in which the electrons are in
the lowest possible energy state.
Assigning Electronic Configuration of
a given atom
• The following sequence is used:
• 1s,2s,2p,3s,3p,4s,3d,4p,5s,4d,5p,6s,4f, 5d,
6p,7s,5f,6d.....
• You begin with the first orbital, 1s, and
add electrons until the maximum
number for that orbital is reached,
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Assigning Electronic Configuration of
a given atom
• 1s
2s 2p
3s 3p
4s 3d 4p
5s 4d 5p
6s 4f 5d 6p
7s 5f 6d 7p
Energy ordering rule
Klechkowski’s rule
Madelung’s rule
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Electron Configuration of Lighter
Elements
Transition Metals
• After Ar the d orbitals begin to fill.
• After the 3d orbitals are full the 4p orbitals
begin to fill.
• The ten elements between Sc and Zn are
called the transition metals, or transition
elements.
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Transition Metals
Lanthanide and Actinide Elements
• The 15 elements corresponding to the
filling of 4f orbitals are called lanthanide
elements (or rare earth elements).
• The 15 elements corresponding to the
filling of 5f orbitals are called actinide
elements.
• Most actinides are not found in nature
(they are synthesized).
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Electron Configurations and the Periodic
Table
• The periodic table can be used as a guide
for electron configurations.
• The period number is the value of n.
• Groups 1A and 2A have their s orbitals
being filled.
• Groups 3A – 8A have their p orbitals being
filled.
• The s-block and p-block of the periodic table
contain the representative, or main-group,
elements.
• Transition metals have their d orbitals being
filled.
• The lanthanides and actinides have their f
orbitals being filled.
• The actinides and lanthanide elements are
collectively referred to as the f-block metals.
• Note that the 3d orbitals fill after the 4s orbital.
Similarly, the 4f orbitals fill after the 5d orbitals.
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Electron Configuration of
lanthanide
• La [Xe] 6s2 5d1 4f 0
Lu [Xe] 6s2 5d1 4f 14
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The Periodic Table
Electron Configurations and the
Periodic Table
The periodic table is considered by many to be
the most predictive tool in all of chemistry.
It is composed of vertical columns called groups
or families and horizontal rows called periods.
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Electron Configurations and the
Periodic Table
Groups (families): Vertical columns in the
periodic table. Groups contain elements with
similar chemical properties.
Periods: Horizontal rows in the periodic table.
Elements in a period demonstrate a range of
properties from metallic (on the left) to
nonmetallic (on the right).
Electron Configurations and the
Periodic Table
Valence electrons:
Valence electrons are the electrons in the
outermost principle energy level of an atom.
These are the electrons that are gained, lost,
or shared in a chemical reaction.
Elements in a group or family have the same
number of valence electrons.
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Electron Configurations and the
Periodic Table
Some groups in the periodic table have special
names:
• Alkali Metals: Group 1A
– Valence electron configuration: ns1
• Alkaline Earth Metals: Group 2A
– Valence electron configuration: ns2
• Halogens: Group 7A
– Valence electron configuration: ns2np5
• Noble Gases: Group 8A
– Valence electron configuration: ns2np6
Electron Configurations and the
Periodic Table
• Metals, Nonmetals, and Metalloids:
– Metals
• Metallic luster, conduct heat and electricity,
malleable, and ductile. Examples are sodium and
copper.
– Nonmetals
• Dull luster, nonconductors, and brittle.
Examples are sulfur and bromine.
– Metalloids
• Demonstrate properties of both metals and
nonmetals. Examples are silicon and arsenic.
3/98
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Electron Configurations and the
Periodic Table
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