Transcript Radioactivity - Madison Public Schools

Section 19.1
Radioactivity
Objectives
1. Students will be able to describe the factors that lead to
nuclear stability.
2. Students will be able to describe types of radioactive
decay.
3. Students will be able to write equations to show
radioactive decay.
Section 19.1
Radioactivity
A Review of Atomic Terms
• nucleons – particles found in the
nucleus of an atom
– neutrons
– protons
• atomic number (Z) – number of
protons in the nucleus
• mass number (A) – sum of the
number of protons and neutrons
• isotopes – atoms with identical
atomic numbers but different mass
numbers
• nuclide – each unique atom
Section 19.1
Radioactivity
Nuclear Stability
• Small atoms tend to have
neutron-proton ratios ~
1:1
• The ratio approaches
1.5:1 as the atomic
number increases
• Explained by the
relationship between
strong nuclear forces and
electrostatic repulsion
Section 19.1
Radioactivity
Nuclear Stability
• Strong nuclear forces act
over very short distances
• As the # of protons
increases, the
electrostatic force
between protons
increases
• More neutrons are
needed to stabilize the
nucleus
Section 19.1
Radioactivity
A. Radioactive Decay
• radioactive – nucleus which spontaneously decomposes
forming a different nucleus and producing one or more
particles
• nuclear equation – shows the radioactive decomposition of
an element
Section 19.1
Radioactivity
A. Radioactive Decay
Types of Radioactive Decay
• Alpha-particle decay
• Alpha particle – helium nucleus
– Examples
• Common for heavy isotopes
Section 19.1
Radioactivity
A. Radioactive Decay
Types of Radioactive Decay
• Beta-particle decay
• Beta particle – electron
– Examples
• Net effect is to change a neutron to a proton.
Section 19.1
Radioactivity
A. Radioactive Decay
Types of Radioactive Decay
• Gamma ray release
• Gamma ray – high energy photon
– Examples
• Net effect is no change in mass number or atomic
number.
Section 19.1
Radioactivity
A. Radioactive Decay
Types of Radioactive Decay
• Positron production
• Positron – particle with same mass as an electron but with
a positive charge
– Examples
• Net effect is to change a proton to a neutron.
Section 19.1
Radioactivity
A. Radioactive Decay
Types of Radioactive Decay
• Electron capture
– Example
Section 19.1
Radioactivity
A. Radioactive Decay
Section 19.1
Radioactivity
A. Radioactive Decay
Decay series
Section 19.1
Radioactivity
Sample Problems
1. Silver-116 undergoes beta particle decay.
2. Bismuth-211 undergoes alpha particle decay
3. An isotope undergoes alpha particle decay and
produces the isotope Polonium-218
4. Silver 106 undergoes electron capture.
5. Potassium-38 undergoes positron emission.
Section 19.1
Radioactivity
Detection of Radioactivity and the Concept of Halflife
Objectives
• Students will be able to define the term half-life and
explain how it relates to the stability of the nucleus.
• Students will determine the half-life of an isotope or
the fraction of the isotope that remains given
appropriate data.
Section 19.1
Radioactivity
C. Detection of Radioactivity and the Concept of Halflife
• Half-life – time required for half of the original sample of
radioactive nuclides to decay
Section 19.1
Radioactivity
Half Life Problems:
1. Phosphorus-32 has a half-life of 14.3 days. How
many days will it take for a radioactive sample to
decay to one-eighth its original size.
2. Iodine-131 has a half life of 8.0 days. How many
grams of an original 160 mg sample will remain in
40 days?
3. Carbon-14 has a half life of 5715 years. It is used to
determine the age of ancient objects. If a sample
today contains 0.060 mg of carbon-14, how much
carbon-14 must have been present in the sample
11,430 years ago>
Section 19.1
Radioactivity
B. Nuclear Transformations
• Nuclear transformation – change of one element to
another
Section 19.1
Radioactivity
B. Nuclear Transformations
Discovery of Protons:
• Rutherford performed the first artificial transmutation, by
bombarding nitrogen gas with alpha particles (1919)
14
7
N  He  O  H
4
2
17
8
1
1
Section 19.1
Radioactivity
B. Nuclear Transformations
Discovery of Neutrons
• Chadwick discovered the neutron by bombarding Be
with alpha particles. (1932)
9
4
Be  He  C  n
4
2
12
6
1
0
Section 19.1
Radioactivity
B. Nuclear Transformations
Section 19.1
Radioactivity
B. Nuclear Transformations
Production of Neptunium and Plutonium
U  n U
238
92
1
0
239
92
U  Np e
239
92
239
93
239
93
0
1
Np Pu  e
239
94
0
1
Section 19.1
Radioactivity
B. Nuclear Transformations
• Transuranium elements – elements with atomic numbers
greater than 92 which have been synthesized
Section 19.1
Radioactivity
C. Detection of Radioactivity and the Concept of Halflife
• Geiger-Muller counter – instrument which measures
radioactive decay by registering the ions and electrons
produced as a radioactive particle passes through a gasfilled chamber
Section 19.1
Radioactivity
C. Detection of Radioactivity and the Concept of Halflife
• Scintillation counter – instrument which measures the
rate of radioactive decay by sensing flashes of light that
the radiation produces in the detector