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Chapter 3
Notes
(rev. 10/14/09)
Democritus
Democritus was a Greek philosopher who “…was one of two founders of
ancient atomist theory”.
“The atomists held that there are smallest indivisible bodies from which
everything else is composed, and that these move about in an infinite void
space”.
“The atomists held that there are two fundamentally different kinds of
realities composing the natural world, atoms and void. Atoms, from the
Greek adjective atomos or atomon, ‘indivisible,’ are infinite in number and
various in size and shape, and perfectly solid, with no internal gaps”
.
Text http://plato.stanford.edu/entries/democritus/
image http://reich-chemistry.wikispaces.com/file/view/demo._atom_model.gif/97847685/demo._atom_model.gif
• Atoms:
Smallest particle of an element that retains
the chemical identity of
that element.
• Principles of Chemical Behavior:
* Lavoisier: Law of Conservation of Matter
* Proust: Law of Constant Composition
• a compound always contains the same elements in
the same proportions by mass.
John Dalton’s:
Atomic Theory of Matter
•
•
•
•
4 Postulates:
elements are composed of atoms
all atoms of an element are identical, but
different from atoms in other elements
atoms are neither created nor destroyed
a given compound always has the same
relative number and kind of atoms.
Michael Faraday
• atoms contain particles that have an
electrical charge
Ben Franklin
• studied electricity
• he determined the following:
– there are 2 kinds of charge
positive and negative
– 2 like charges repel each
other
– opposites charges attract
each other
– excess negative charge can
be discharged as static
electricity
Ben Franklin
• Do you remember
Ben Franklin’s
famous kite
experiment?
J.J. Thomson
• called the negative particles electrons
• determined the charge to mass ratio of an electron
• The Plum Pudding Model is Thomson’s name for his
model of the atom
JJ Thomson’s Plum Pudding Model
of the Atom
• www.tamucc.edu
Cathode Ray Tube (CRT)
• negative end is the cathode
• positive end is the anode
A cathode ray is radiation streaming from a
cathode to an anode in a CRT
• it is a stream of particles
• a magnet can deflect the ray
• cathode ray particles have a
negative charge
•
http://www.chem.uiuc.edu/clcwebsite/cathode.html
Robert Millikan
• measured the charge of an electron using
the Oil Drop Experiment.
• x-rays gave the oil a negative electron
• 1.60x10 -19 coulomb is the charge of an
electron
• using Thomson’s charge to mass ratio, he
determined the mass of the electron
is 9.11x10 -28 g
Robert Millikan’s
Oil Drop Experiment
• Robert Millikan
received the Nobel
Prize for his work
•
www.68pair.com
Henri Becquerel
• discovered that uranium
exhibits radioactivity
• the chemical properties
of an element change as
it gives off radiation
Ernest Rutherford
• alpha particles have a +2 charge
• beta particles are high
speed electrons
• gamma rays are not
composed of particles
Rutherford
• Gold Foil Experiment (alpha scattering)
• he determined that an atom’s positive charge,
and most of its mass, was concentrated in the
core
(most of the atom is empty space)
• In 1920, Ernest Rutherford postulated that there
were neutral, massive particles in the nucleus of
atoms.
• he named the core of the atom
“the nucleus”
Rutherford’s Gold Foil Experiment
• Over 98% of the particles went straight
through
• Approx. 2% of the particles were deflected
• Approx. 0.01% of
the particles bounced
off the gold foil
www.sci.tamucc.edu
Rutherford’s Experiment
www.sci.tamucc.edu
James Chadwick
• Chadwick proved the existence of
neutrons.
http://dev.physicslab.org/img/c2df54c3-7f43-4f21-8c54-36f23218c5f5.gif
Rutherford’s Nuclear Model
This is an image of a
lithium atom.
• The atom contains a tiny dense center called
the nucleus
• The nucleus is essentially the entire mass of
the atom
•
http://upload.wikimedia.org/wikipedia/commons/thumb/e/e1/Stylised_Lithium_Atom.svg/180px-Stylised_Lithium_Atom.svg.png
The Nucleus
• The nucleus is positively charged
• The amount of positive charge in the
nucleus balances the negative charge of
the electrons
• The electrons move around in the
empty space of the atom surrounding
the nucleus
Neils Bohr
• Planetary Model of the Atom
•http://jila-amo.colorado.edu/research/images/bohr.gif
Quantum Mechanical Model
This image is a representation of Schrodinger’s Quantum
Mechanical Model of the atom.
This model shows the nucleus in the center surrounded by
electrons in different energy levels, but there is no distinct
energy level shown as in the Bohr Model.
http://www.physicalworld.org/restless_universe/figs/fig_1_30.gif
Modern Atomic Theory
* atoms are composed of protons, neutrons and electrons
* Recently, scientists discovered these particles contain
particles (i.e. quarks, gluons)
* A nucleus is a positively charged central core containing
protons and neutrons
* An individual proton has a charge that is equal to the
charge of an electron, but the charge is positive
* In a neutral atom: the number of protons equals the
number of electrons
* a proton is much more massive than an electron
Charge and Mass
• a proton’s charge is +1
• an electron’s charge is -1
• mass is expressed in atomic mass
units (amu)
• the mass of a proton is approx. 1 amu
• the mass of a neutron is approx. 1 amu
• the mass of an electron is
approx. 0 amu
Henry Mosely
• an atom’s identity comes from the number of protons in
its nucleus
• the number of protons is called the atomic number
• every element has a unique atomic number
• the atomic number is written above the chemical
symbol on the periodic table
• since atoms are electrically neutral, the number of
positive charges equals the number of negative charges
• the number of neutrons equals the mass number minus
the atomic number
• # of no = mass # - atomic #
IONS
• when an atom loses or gains one or more
electrons it acquires a net charge and is
called an ion
• if there are more electrons than protons,
the ion has a negative charge
• charge of ion = (# of p+) - (# of e-)
• charge can be written as +2 or 2+
• place the charge to the upper right
corner of the chemical symbol
ISOTOPES
• atoms that have the same number of
protons, but different numbers of neutrons
• the chemical properties of an element
depend primarily on its electrons and
protons
• isotopes of the same element, with
different numbers of neutrons have
essentially the same chemical properties
ISOTOPES
the major difference between 2 isotopes is
their mass
• isotopes with more neutrons have higher mass
than those with fewer neutrons
• mass # = (isotope’s # of p+) + (isotope’s # of n0)
• to identify an isotope you add the mass number
after the element’s name (i.e. chlorine -37)
• number of neutrons = mass # - atomic #
Carbon-12
• The entire periodic table is based on the
carbon-12 isotope.
Example
mass number
Carbon-12
p+
6
e6
nº
6
Carbon-14
6
6
8
2 extra neutrons
Iodine-125
• What is the difference between this
isotope of iodine and the iodine shown on
the periodic table?
• How many neutrons does iodine-125
have?
Atomic Mass in Brackets
• Atomic mass (the weighted average of all
naturally occurring isotopes) is not given
for those atoms which have no stable
isotopes.
• In these cases the mass number of the
most stable isotope is reported, often in
brackets, for example Technetium (98).
•
http://www.digipac.ca/chemical/molemass/isotopes.htm
Practice
• Try 3-3 Rev & Rein WS
• # 12,13,14
MASS OF AN ATOM
* amu can be defined as the mass of a proton
or neutron
* Generally:
mass of an atom =
number of protons + number of neutrons
* an amu is defined in terms of an arbitrary
standard:
a carbon-12 atom
* scientists set the mass of a carbon-12 atom to
be exactly equal to 12 amu
* therefore, one amu is 1/12 of the mass of a
carbon-12 atom
* 1 amu = 1/12 (mass of 126C atom) = 1.66 x 10-24
g
MASS OF AN ATOM
• when determining the mass of a large number of
atoms:
• remember that elements have different isotopes
• determine the mass of an “average” element’s
atom
• the isotopes of some elements are found in
constant ratios
• these ratios are called fractional abundances
• atomic mass is the average mass of an
element’s atoms
• the atomic mass for each element is written on
the periodic table below the element symbol
RADIOACTIVITY
• nuclear reactions change the composition of
an atom’s nucleus.
• alpha, beta and gamma radiation are produced
by nuclear reactions
• alpha and beta radiation consist of particles that
are emitted from the nucleus
• Radioactivity is the spontaneous emission of
radiation from an atom
• atoms with stable nuclei are not radioactive
PURPOSE OF NEUTRONS
• not all combinations of protons and neutrons
create a stable nucleus
• protons in the nucleus should repel each other,
but there is a strong nuclear force (that is an
attractive force) that holds the nucleus together
• the nuclear force is only strong between
subatomic particles that are extremely close
• the presence of neutrons in the nucleus adds a
net attractive force to the inside of the nucleus
• think of neutrons as the glue that holds the
nucleus together
STABLE OR NOT?
• elements with:
• atomic number 1-20: stable nuclei have almost
equal numbers of protons and neutrons
• atomic number 21-83: nuclei need more
neutrons than protons to be stable
• atomic number >83: no number of neutrons is
sufficient to hold the nucleus together
indefinitely.
• All nuclei with atomic numbers greater than 83
are radioactive.
UNSTABLE NUCLEI
• nuclei are unstable if they contain too few
or too many neutrons
• nuclei that have excess neutrons are likely
to emit beta radiation
• Generally, isotopes that are much
heavier (contain more neutrons) or
much lighter (contain fewer neutrons)
than the most common isotope are
likely to be radioactive.
TYPES OF RADIOACTIVE DECAY
* radioactive elements emit different kinds of
radiation (three types are alpha, beta and
gamma)
* these types of radiation can be
distinguished by their:
charge
mass
penetrating power
ALPHA PARTICLES
* consist of a stream of high-energy alpha
particles
* alpha particle consists of 2 protons and 2
neutrons
(it is identical to the helium-4 nucleus)
* these particles do not have much penetrating
power
* these particles travel only a few centimeters in
air and can be stopped by paper or clothing
* not normally a health hazard
BETA RADIATION
* consists of a stream of high-speed electrons
* the electrons come from changes in the nucleus
* in the process that produces beta radiation, a neutron
changes into a proton and an electron
* the proton remains in the nucleus and the electron (now
the beta particle) is propelled out of the nucleus at high
speed
* the mass number for a beta particle is zero because an
electron has a very small mass compared with a proton
or neutron
* beta radiation is approx. 100 times more penetrating
than alpha
* it can penetrate clothing and harm skin
GAMMA RAYS
• gamma rays are not particles
• they are a very energetic form of light that
we cannot see
• it can penetrate deeply into solid material
(it can only be stopped by concrete or
lead)
RADIOACTIVE DECAY
• Radioactive Decay occurs when an
atom emits alpha or beta particles or
gamma rays.
• The term decay is used because the
original nucleus decomposes (decays)
to form a new nucleus, releasing
radiation in the process.
• Use a nuclear equation to show the
radioactive decay.