Transcript ch10_sec1_rc

Section 1: What is Radioactivity?
Preview
• Key Ideas
• Bellringer
• Nuclear Radiation
• Nuclear Decay
• Math Skills
• Radioactive Decay Rates
Key Ideas
〉What happens when an element undergoes
radioactive decay?
〉How does radiation affect the nucleus of an
unstable isotope?
〉How do scientists predict when an atom will
undergo radioactive decay?
Bellringer, continued
2. Complete the table below to indicate how many protons and
neutrons are in the nuclei of each atom.
Nuclear Radiation
〉What happens when an element undergoes
radioactive decay?
〉After radioactive decay, the element changes
into a different isotope of the same element or
into an entirely different element.
• radioactive decay: the disintegration of an unstable
atomic nucleus into one or more different nuclides
Nuclear Radiation, continued
• nuclear radiation: the particles that are released from
the nucleus during radioactive decay
• There are different types of nuclear radiation.
– nuclear radiation can contain
• alpha particles
• beta particles
• gamma rays
• neutrons
Types of Nuclear Radiation
Nuclear Radiation, continued
• Alpha particles consist of protons and neutrons.
• alpha particle: a positively charged particle that
consists of two protons and two neutrons and that is
emitted from the nucleus during radioactive decay
• Beta particles are electrons produced from neutron
decay.
• beta particle: an electron or proton that is emitted
from a nucleus during radioactive decay
Nuclear Radiation, continued
• Gamma rays are high-energy electromagnetic radiation.
• gamma ray: a high-energy photon emitted by a
nucleus during fission and radioactive decay
• When atoms decay by emitting a or b particles to form a new atom,
the nuclei of the new atom formed may still have too much energy
to be completely stable. This excess energy is emitted as gamma
rays
• Neutron radioactivity occurs in an unstable nucleus.
• Neutron emission consists of matter that is emitted
from an unstable nucleus.
• Neutrons have no charge.
• Neutrons are able to travel farther through matter
than either alpha or beta particles.
Nuclear Decay
〉How does radiation affect the nucleus of an
unstable isotope?
〉Anytime that an unstable nucleus emits
alpha or beta particles, the number of
protons or neutrons.
• Nuclear-decay equations are similar to those used for
chemical reactions.
Nuclear Decay, continued
• Gamma decay changes the energy of the nucleus.
– There is no change in the atomic number or the
atomic mass of the element.
• The atomic number changes during beta decay.
– The mass number before and after the decay does
not change.
– The atomic number of the product nucleus
increases by 1.
– The atom changes to a different element.
Nuclear Decay, continued
• A beta decay process occurs when carbon-14 decays
to nitrogen-14 by emitting a beta particle.
Nuclear Decay, continued
• Both atomic mass and number change in alpha decay.
– The atomic mass decrease by 4.
– The atomic number decreases by 2.
• The process of the alpha decay of radium-226 is
written as follows.
Math Skills
Nuclear Decay
Actinium-217 decays by releasing an alpha particle. Write
the equation for this decay process, and determine which
element is formed.
1. Write down the equation with the original element on
the left side and the products on the right side.
217
89
Ac  X  He
A
Z
4
2
X = unknown product; A = unknown mass;
Z = unknown atomic number
Math Skills, continued
2. Write math equations for the atomic and mass numbers.
217 = A + 4
89 = Z + 2
3. Rearrange the equations.
A = 217 – 4
Z = 89 – 2
A = 213
Z = 87
4. Rewrite the equation with all nuclei represented.
The unknown decay product has an atomic number of
87, which is francium.
217
89
Ac 
213
87
Fr  42 He
Radioactive Decay Rates
〉How do scientists predict when an atom will
undergo radioactive decay?
〉It is impossible to predict the moment when any
particular nucleus will decay, but it is possible to
predict the time required for half of the nuclei in
a given radioactive sample to decay.
• half-life: the time required for half of a sample of a
radioactive isotope to break down by radioactive decay
to form a daughter isotope
Radioactive Decay Rates, continued
• Half-life is a measure of how quickly a substance
decays.
– Doctors use isotopes with short half-lives to help
diagnose medical problems.
– Scientists can also use half-life to predict how old an
object is.
• Geologists calculate the age of rocks by using the
half-lives of long-lasting isotopes, such as
potassium-40.
Half-Life
• If 100g of carbon-14 decays until only 25g of carbon is left
after 11,460 y, what is the half-life of carbon-14?
Initial mass of sample: 100g
Final mass of sample: 25g
Total time of decay: 11,460 years
Unknown: number of half-lives
Total time of decay = number of half lives X number of years
half-life
Number of years = total time of decay
half-life
number of half-lives
1. Calculate how many half-lives have passed during decay
Final mass of sample = 25.0g = 1/4
Initial mass of sample
100g
After one half life = ½
After two half lives = ½ X ½ = ¼
2. Total time of decay = number of half lives X number of years
half-life
Number of years = total time of decay
half-life
number of half-lives
Number of years = 11,460 years = 5730 years
half-life
2 half-lives
Half Life
• The half life of a radioactive material is the amount of
time required for ½ of the sample to decay into another
element or isotope.
• Half lives are calculated according to the equation:
a = a0(½)x
Half Life
– a = amount of material left at any time
– a0 = amount of material that you begin with
– x = the number of half lives that have passed
since you have begun counting
• This type of decay is said to be exponential since it can
be described graphically as a hyperbola
Alta Physics
Sample Problem
Carbon-14, a radioactive isotope of carbon, has a half life
of 5730 years. If a 20 gram sample of carbon-14 is
allowed to decay for 10,000 years, how much remains at
the end of this period?
Alta Physics
Solution
a = a0(½)x
a0 = 20 grams
x = 10,000 yrs/5730 yrs/half life
= 1.75
So a = 20 grams(½)1.75
= 5.95 grams