Transcript Radioactivity

Nuclear Chemistry
Targets:
1.I CAN Utilize appropriate scientific vocabulary
to explain scientific concepts.
2.I CAN Distinguish between fission and fusion
and where each process may be used.
3.I CAN Construct balanced nuclear equations
(alpha, beta, gamma, neutron, etc.).
4.I CAN Complete a decay series for a given
element.
5.I CAN Perform calculations to determine how
much of a radioactive substance will remain
based on the half-life and time elapsed.
6.I CAN Perform calculations to determine how
much time has elapsed based on starting and
ending amounts in a half-life problem.
• Changes in matter originating in the
nucleus of an atom
• All atoms have• p+ and n0 (nucleons)
• p+ number is the atomic number = Z
• p+ + n0 = mass number = A
The Nucleus
• The nucleus is comprised of the two nucleons
(a nuclear particle) protons and neutrons.
• The number of protons is the atomic number.
• The number of protons and neutrons together
is the mass of the atom.
• Nuclide, is the nucleus of an atom having a
specific atomic number and atomic mass #
Isotopes
• Elements with the same atomic #, but
different mass # due to gaining or losing
of neutrons
• Ex: There are three naturally occurring
isotopes of uranium:
– Uranium-234
– Uranium-235
– Uranium-238
Radioactivity
• It is not uncommon for some nuclides of an
element to be unstable, or radioactive.
• We refer to these as radionuclides
• Radionuclides are unstable and spontaneously
emit particles and electromagnetic radiation
(EM)
• Radioisotopes- atoms containing these
radionuclides
• Transmutation- process by which one element
is converted to another element by the
spontaneous emission of radiation
Large Nuclei
• Nucleus is held together by strong force
• Large nuclei tend to be unstable because
the force is not strong enough to hold it
together
• These nuclei break apart and decay
• All nuclei with 83 or more protons are
radioactive
• Almost all elements with more than 92
protons don’t exist naturally
Neutron-Proton Ratios
• Any element with more than
one proton (i.e., anything but
hydrogen) will have
repulsions between the
protons in the nucleus.
• A strong nuclear force helps
keep the nucleus together
• Neutrons play a key role
stabilizing the nucleus.
• Therefore, the ratio of
neutrons to protons is an
important factor.
Neutron-Proton Ratios
For smaller nuclei
(Z  20) stable
nuclei have a
neutron-to-proton
ratio close to 1:1.
Neutron-Proton Ratios
As nuclei get
larger, it takes a
greater number of
neutrons to
stabilize the
nucleus.
Stable Nuclei
• Nuclei above this belt
have too many
neutrons.
• They tend to decay
by emitting beta
particles.
Stable Nuclei
• Nuclei below the belt
have too many protons.
• They tend to become
more stable by positron
emission or electron
capture.
What is Radioactivity?
• When an unstable nucleus emits one or
more particles or energy
• When these particles are emitted, the
element changes to another isotope or to
a different element
• Nuclear radiation refers to radiation
resulting from nuclear changes
Types of Nuclear
Radiation
• Alpha particles
• Beta particles
• Gamma rays
Alpha Particles
• Symbol is
• Actually particles made of 2 p+ and 2 n0,
same as a He nucleus
• +2 charge, most massive of all nuclear
radiation, mass = 4 amu
• Do not travel far, can be stopped by a
sheet of paper
• Can be very dangerous inside human
body – illness and disease
Alpha Particle Emission
Beta Particles
• Symbol is Greek letter, beta (β)
• High speed negatively charged particles
that come from the nucleus – hmmm…
• Neutron (neutral) actually decays to form
a p+ and an e-, e- is ejected from nucleus
• Travel farther than α, but can be
stopped by 3 mm of Al, or 10 mm
of wood
• Can cause damage inside cells
Beta Decay
mk
M
Gamma rays
• Gamma Radiation, γ
• Not made of matter, no charge
• Waves of electromagnetic energy (photons)
• Usually emitted from nucleus when alpha or beta
decay occurs
• A nucleus de-excites by emitting a high energy
gamma ray photon
• High energy, can be stopped by 60 cm of Al or 7
cm of Pb
• Cause less damage than alpha because it has no
charge
Gamma Radiation
Damage Caused
Penetrating Power
Nuclear Decay
• When unstable nuclei emit alpha or beta
particles, what changes?
– Number of protons or neutrons
– New element or new atomic mass
Change During Alpha Decay
• Nucleus gives up 2 p+ and 2 n0
• All mass numbers add up, all atomic
numbers add up
Change During Beta Decay
• Neutron is changed to a proton and an
electron and emitted
• Increases atomic number due to
conversion of a neutron to a proton and
an electron
• So neutron is lost and proton is gained
Gamma Emission
Loss of a -ray (high-energy radiation that
almost always accompanies the loss of a
nuclear particle)
0
0
238 U
92
→
238
Th
90

+ 4He
2
+ 0 (gamma ray)
Classwork/Homework
• WS with example problems
Decay Rates
• Half-life = the time in which half a
radioactive substance decays
• After 1 half-life, half of the substance is
unchanged
• After 2 half-lives, ¼ of the substance is
unchanged
• Half-life table on p. 226
Half-Life
• Measure how quickly a substance decays
• Can be anywhere between nanoseconds
to billions of years, depending on nuclear
stability
• C-14 is used to find the age of relatively
recent materials
• C-14 is taken in in tiny fractions while
alive in some molecules of CO2
• C-14 goes through beta decay, so ratio is
compared in living and nonliving things
Half-Life Example
• The half-life of iodine-131 is 8 days.
• If you start with 36 grams of I-131, how
much will be left after 24 days?
• 36 g  1 half-life  8 days  18 g
• 18 g  2 half-lives  16 days  9 g
• 9 g  3 half-lives  24 days  4.5 g
Classwork/Homework
• WS – Nuclear Decay
• Half-Life problems on the back