Transcript Types of Radiation - Kasson

Chapter
4
The Philosophers
• Democritus – first to propose the idea that
matter was not infinitely divisible
• Atomos
• Aristotle – rejected this theory
Atomic Structure: Basic Concepts
Topic
2
Dalton’s Atomic Theory
• John Dalton (17661844), an English
schoolteacher and
chemist, studied the
results of experiments
by Lavoisier, Proust,
and many other
scientists.
Atomic Structure: Basic Concepts
Topic
2
Dalton’s Atomic Theory
• The following statements are the main points
of Dalton’s atomic theory.
1. All matter is made up of indestructible atoms.
2. All atoms of one element are exactly
alike, but are different from atoms of other
elements.
3. Atoms unite with other atoms in simple
ratios to form compounds.
Atomic Structure: Basic Concepts
Topic
2
The Electron
• Because of Dalton’s atomic theory, most
scientists in the 1800s believed that the atom
was like a tiny solid ball that could not be
broken up into parts.
• In 1897, a British physicist, J.J. Thomson,
discovered that this solid-ball model was not
accurate.
• Thomson’s experiments used a vacuum tube.
Atomic Structure: Basic Concepts
Topic
2
The Electron
• A vacuum tube has
had all gases pumped
out of it.
• At each end of the tube is a metal piece called
an electrode, which is connected through the
glass to a metal terminal outside the tube.
• These electrodes become electrically charged
when they are connected to a high-voltage
electrical source.
Atomic Structure: Basic Concepts
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2
Cathode-Ray Tube
• When the
electrodes are
charged, rays
travel in the tube
from the negative
electrode, which
is the cathode, to
the positive
electrode, the
anode.
• Because these
rays originate at
the cathode, they
are called cathode
rays.
Atomic Structure: Basic Concepts
Topic
2
Cathode-Ray Tube
• Thomson found that the rays bent toward a
positively charged plate and away from a
negatively charged plate.
• He knew that
objects with like
charges repel each
other, and objects
with unlike charges
attract each other.
Click box to view movie clip.
Atomic Structure: Basic Concepts
Topic
2
Cathode-Ray Tube
• Thomson concluded that cathode rays are
made up of invisible, negatively charged
particles referred to as electrons.
• These electrons had to come from the matter
(atoms) of the negative electrode.
Atomic Structure: Basic Concepts
Topic
2
Cathode-Ray Tube
• From Thomson’s experiments, scientists had
to conclude that atoms were not just neutral
spheres, but somehow were composed of
electrically charged particles.
• Reason should tell you that there must be a
lot more to the atom than electrons.
• Matter is not negatively charged, so atoms
can’t be negatively charged either.
Atomic Structure: Basic Concepts
Topic
2
Cathode-Ray Tube
• If atoms contained extremely light,
negatively charged particles, then they must
also contain positively charged particles—
probably with a much greater mass than
electrons.
Atomic Structure: Basic Concepts
Topic
2
Protons
• In 1886, scientists discovered that a cathoderay tube emitted rays not only from the
cathode but also from the positively charged
anode.
• These rays travel in a direction opposite to
that of cathode rays.
Atomic Structure: Basic Concepts
Topic
2
Protons
• Like cathode rays, they are deflected by
electrical and magnetic fields, but in
directions opposite to the way cathode rays
are deflected.
• Thomson was able to show that these rays
had a positive electrical charge.
• Years later, scientists determined that the rays
were composed of positively charged
subatomic particles called protons.
Atomic Structure: Basic Concepts
Topic
2
Protons
• At this point, it seemed that atoms were made
up of equal numbers of electrons and protons.
• However, in 1910, Thomson discovered that
neon consisted
of atoms of
two different
masses.
Atomic Structure: Basic Concepts
Topic
2
Protons
• Atoms of an element that are chemically
alike but differ in mass are called isotopes
of the element.
• Today, chemists know that neon consists of
three naturally occurring isotopes.
• The third was too scarce for Thomson to
detect.
Atomic Structure: Basic Concepts
Topic
2
Neutrons
• Because of the discovery of isotopes,
scientists hypothesized that atoms contained
still a third type of particle that explained
these differences in mass.
• Calculations showed that such a particle
should have a mass equal to that of a proton
but no electrical charge.
• The existence of this neutral particle, called a
neutron, was confirmed in the early 1930s.
Atomic Structure: Basic Concepts
Topic
2
Rutherford’s Gold Foil Experiment
• In 1909, a team of
scientists led by Ernest
Rutherford in England
carried out the first of
several important
experiments that
revealed an arrangement
far different from the
cookie-dough model of
the atom.
Atomic Structure: Basic Concepts
Topic
2
Rutherford’s Gold Foil Experiment
• The experimenters set up a lead-shielded box
containing radioactive polonium, which
emitted a beam of positively charged
subatomic particles through a small hole.
Click box to view
movie clip.
Atomic Structure: Basic Concepts
Topic
2
Rutherford’s Gold Foil Experiment
• Today, we know that the particles of the
beam consisted of clusters containing two
protons and two neutrons and are called alpha
particles.
• The sheet of gold foil was surrounded by a
screen coated with zinc sulfide, which glows
when struck by the positively charged
particles of the beam.
Atomic Structure: Basic Concepts
Topic
2
The Gold Foil Experiment
Atomic Structure: Basic Concepts
Topic
2
The Nuclear Model of the Atom
• To explain the results of the experiment,
Rutherford’s team proposed a new model
of the atom.
• Because most of
the particles passed
through the foil,
they concluded that
the atom is nearly
all empty space.
Click box to view movie clip.
Atomic Structure: Basic Concepts
Topic
2
The Nuclear Model of the Atom
• Because so few particles were deflected, they
proposed that the atom has a small, dense,
positively charged central core, called a
nucleus.
Atomic Structure: Basic Concepts
Topic
2
The Nuclear Model of the Atom
• The new model of the atom as pictured by
Rutherford’s group in 1911 is shown below.
Atomic Structure: Basic Concepts
Topic
2
Atomic Numbers
• The atomic number of an element is the
number of protons in the nucleus of an atom
of that element.
• It is the number of
protons that
determines the
identity of an element,
as well as many of its
chemical and physical
properties.
Atomic Structure: Basic Concepts
Topic
2
Atomic Numbers
• Because atoms have no overall electrical
charge, an atom must have as many electrons
as there are protons in its nucleus.
• Therefore, the atomic
number of an element
also tells the number
of electrons in a
neutral atom of that
element.
Atomic Structure: Basic Concepts
Topic
2
Masses
• The mass of a neutron is almost the same as
the mass of a proton.
• The sum of the protons and neutrons in the
nucleus is the mass number of that particular
atom.
Atomic Structure: Basic Concepts
Topic
2
Masses
• Isotopes of an element have different mass
numbers because they have different numbers
of neutrons, but they all have the same
atomic number.
Atomic Structure: Basic Concepts
Topic
2
Atomic Mass
• In order to have a simpler way of comparing
the masses of individual atoms, chemists
have devised a different unit of mass called
an atomic mass unit, which is given the
symbol u.
• An atom of the carbon-12 isotope contains
six protons and six neutrons and has a mass
number of 12.
Atomic Structure: Basic Concepts
Topic
2
Atomic Mass
• Chemists have defined the carbon-12 atom as
having a mass of 12 atomic mass units.
• Therefore, 1 u = 1/12 the mass of a carbon-12
atom.
• 1 u is approximately the mass of a single
proton or neutron.
Atomic Structure: Basic Concepts
Topic
2
Information in the Periodic Table
• The number
at the bottom
of each box is
the average
atomic mass
of that
element.
• This number is the weighted average mass
of all the naturally occurring isotopes of
that element.
Basic Concept Questions
Topic
2
Question 1
How does the atomic number of an element
differ from the element’s mass number?
Answer
The atomic number of an element is the number
of protons in the nucleus. The mass number is
the sum of the number of protons and neutrons.
Atomic Structure: Additional Concepts
Topic
2
Calculating Atomic Mass
Atomic Structure: Additional Concepts
Topic
2
Calculating Atomic Mass
• Copper exists as a mixture of two isotopes.
• The lighter isotope (Cu-63), with 29 protons
and 34 neutrons, makes up 69.17% of copper
atoms.
• The heavier isotope (Cu-65), with 29 protons
and 36 neutrons, constitutes the remaining
30.83% of copper atoms.
Atomic Structure: Additional Concepts
Topic
2
Calculating Atomic Mass
• The atomic mass of Cu-63 is 62.930 amu,
and the atomic mass of Cu-65 is 64.928 amu.
• Use the data above to compute the atomic
mass of copper.
Atomic Structure: Additional Concepts
Topic
2
Calculating Atomic Mass
• First, calculate the contribution of each
isotope to the average atomic mass, being
sure to convert each percent to a fractional
abundance.
Atomic Structure: Additional Concepts
Topic
2
Calculating Atomic Mass
• The average atomic mass of the element is
the sum of the mass contributions of each
isotope.
Additional Assessment Questions
Topic
2
Question 2
The table on the next slide shows the five
isotopes of germanium found in nature, the
abundance of each isotope, and the atomic
mass of each isotope.
Additional Assessment Questions
Topic
2
Calculate the atomic mass of germanium.
Additional Assessment Questions
Topic
2
Answer
72.591 amu
Nuclear Chemistry: Basic Concepts
Topic
26
Nuclear Radiation
• Nuclear chemistry is the study of the
structure of atomic nuclei and the changes
they undergo.
Nuclear Chemistry: Basic Concepts
Topic
26
The Discovery of Radioactivity
• In 1895, Wilhelm Roentgen (1845–1923)
found that invisible rays were emitted when
electrons bombarded the surface of certain
materials.
• The emitted rays were discovered because
they caused photographic plates to darken.
Roentgen named these invisible high-energy
emissions X rays.
Nuclear Chemistry: Basic Concepts
Topic
26
The Discovery of Radioactivity
• As is true in many fields, Roentgen’s
discovery of X rays created excitement
within the scientific community and
stimulated further research.
Nuclear Chemistry: Basic Concepts
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26
The Discovery of Radioactivity
• At that time, French physicist Henri
Becquerel (1852–1908) was studying
minerals that emit light after being
exposed to sunlight, a phenomenon
called phosphorescence.
• Building on Roentgen’s work,
Becquerel wanted to determine whether
phosphorescent minerals also emitted
X rays.
Nuclear Chemistry: Basic Concepts
Topic
26
The Discovery of Radioactivity
• Becquerel accidentally discovered that
phosphorescent uranium salts—even when
not exposed to
light—produced
spontaneous
emissions that
darkened
photographic
plates.
Nuclear Chemistry: Basic Concepts
Topic
26
The Discovery of Radioactivity
• Marie Curie (1867–1934)
and her husband Pierre
(1859–1906) took
Becquerel’s mineral sample
(called pitchblende) and
isolated the components
emitting the rays.
Nuclear Chemistry: Basic Concepts
Topic
26
The Discovery of Radioactivity
• They concluded that the darkening of the
photographic plates was due to rays emitted
specifically from the uranium atoms present
in the mineral sample.
• Marie Curie named the process by which
materials give off such rays radioactivity;
the rays and particles emitted by a
radioactive source are called radiation.
Nuclear Chemistry: Basic Concepts
Topic
26
Types of Radiation
• As you may recall, isotopes are atoms of
the same element that have different
numbers of neutrons.
• Isotopes of atoms with unstable nuclei are
called radioisotopes.
Nuclear Chemistry: Basic Concepts
Topic
26
Types of Radiation
• These unstable nuclei emit radiation to
attain more stable atomic configurations
in a process called radioactive decay.
• During radioactive decay, unstable atoms
lose energy by emitting one of several
types of radiation.
Nuclear Chemistry: Basic Concepts
Topic
26
Types of Radiation
• The three most
common types
of radiation are
alpha (α), beta
(β), and
gamma (γ).
Nuclear Chemistry: Basic Concepts
Topic
26
Types of Radiation
• Ernest Rutherford (1871–1937), whom you
know of because of his famous gold foil
experiment that helped
define modern atomic
structure, identified
alpha, beta, and gamma
radiation when studying
the effects of an electric
field on the emissions
from a radioactive source.
Nuclear Chemistry: Basic Concepts
Topic
26
Types of Radiation
• The effect of an electric field on three types
of radiation is shown.
• Positively charged alpha particles are
deflected toward the negatively charged plate.
Nuclear Chemistry: Basic Concepts
Topic
26
Types of Radiation
• Negatively charged beta particles are
deflected toward the positively charged plate.
Nuclear Chemistry: Basic Concepts
Topic
26
Types of Radiation
• Beta particles undergo greater deflection
because they have considerably less mass
than alpha particles.
Nuclear Chemistry: Basic Concepts
Topic
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Types of Radiation
• Gamma rays, which have no electrical charge,
are not deflected.
Nuclear Chemistry: Basic Concepts
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26
Types of Radiation
• An alpha particle (α) has the same
composition as a helium nucleus—two
protons and two neutrons—and is therefore
given the symbol
.
• The charge of an alpha particle is 2+ due to
the presence of the two protons.
Nuclear Chemistry: Basic Concepts
Topic
26
Types of Radiation
• Alpha radiation consists of a stream of
alpha particles.
• Radium-226, an atom whose nucleus
contains 88 protons and 138 neutrons,
undergoes alpha decay by emitting an
alpha particle.
Nuclear Chemistry: Basic Concepts
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26
Types of Radiation
• Notice that after the decay, the resulting atom
has an atomic number of 86, a mass number
of 222, and is no longer radium.
• The newly formed radioisotope is radon-222.
Nuclear Chemistry: Basic Concepts
Topic
26
Types of Radiation
• The particles involved are balanced.
That is, the sum of the mass numbers
(superscripts) and the sum of the atomic
numbers (subscripts) on each side of the
arrow are equal.
Nuclear Chemistry: Basic Concepts
Topic
26
Types of Radiation
• Because of their mass and charge, alpha
particles are relatively slow-moving
compared with other types of radiation.
• Thus, alpha particles are
not very penetrating—a
single sheet of paper stops
alpha particles.
Nuclear Chemistry: Basic Concepts
Topic
26
Types of Radiation
• A beta particle is a very-fast moving electron
that has been emitted from a neutron of an
unstable nucleus.
• Beta particles are represented by the symbol
. The zero superscript indicates the
insignificant mass of an electron in
comparison with the mass of a nucleus.
Nuclear Chemistry: Basic Concepts
Topic
26
Types of Radiation
• The –1 subscript denotes the negative charge
of the particle.
• Beta radiation consists of a stream of fastmoving electrons.
Nuclear Chemistry: Basic Concepts
Topic
26
Types of Radiation
• An example of the beta decay process is the
decay of iodine-131 into xenon-131 by betaparticle emission.
Nuclear Chemistry: Basic Concepts
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26
Types of Radiation
• Note that the mass number of the product
nucleus is the same as that of the original
nucleus (they are both 131), but its atomic
number has increased by 1 (54 instead of 53).
Nuclear Chemistry: Basic Concepts
Topic
26
Types of Radiation
• This change in atomic number, and thus,
change in identity, occurs because the
electron emitted during the beta decay has
been removed from a neutron, leaving
behind a proton.
Nuclear Chemistry: Basic Concepts
Topic
26
Types of Radiation
• Because beta particles are both lightweight
and fast moving, they have greater
penetrating power than alpha particles.
• A thin metal foil is required
to stop beta particles.
Nuclear Chemistry: Basic Concepts
Topic
26
Types of Radiation
• Gamma rays are high-energy (short
wavelength) electromagnetic radiation.
They are denoted by the symbol .
• As you can see from the symbol, both the
subscript and superscript are zero.
Nuclear Chemistry: Basic Concepts
Topic
26
Types of Radiation
• Thus, the emission of gamma rays does not
change the atomic number or mass number
of a nucleus.
• Gamma rays almost always accompany
alpha and beta radiation, as they account for
most of the energy loss that occurs as a
nucleus decays.
Nuclear Chemistry: Basic Concepts
Topic
26
Types of Radiation
• For example, gamma rays accompany the
alpha-decay reaction of uranium-238.
• The 2 in front of the γ symbol indicates that
two gamma rays of different frequencies
are emitted.
• Because gamma rays have no effect on mass
number or atomic number, it is customary to
omit them from nuclear equations.
Nuclear Chemistry: Basic Concepts
Topic
26
Radioactive Decay
• It may surprise you to learn that of all the
known isotopes, only about 17% are stable
and don’t decay spontaneously.