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Chapter 3
Notes
 Atoms-
Smallest particle of an element
that retains the chemical identity of
that element.
 Principles
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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
 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:
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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
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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)
 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 Nuclear Model
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The atom contains a tiny dense center called the
nucleus
The nucleus is essentially the entire mass of the
atom
The nucleus is positively charged
The amount of positive charge of the nucleus balances
the negative charge of the electrons
The electrons move around in the empty space of
the atom surrounding the nucleus
www.sci.tamucc.edu
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
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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
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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
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 #
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 rations 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
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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
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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?
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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.