Download Chem Ch. 4.4

Section 4.4
Unstable Nuclei and
Radioactive Decay
Objectives
• Explain the relationship between unstable
nuclei and radioactive decay.
• Characterize alpha, beta, and gamma
radiation in terms of mass and charge.
Radioactivity
• Chemical Reactions
– Atoms of one element do not change into atoms
of another element during a chemical reaction.
– Why?
Radioactivity
• Chemical reactions involve the
rearrangement of atoms – not their
“destruction.”
• Chemical reactions involve only an atom’s
electrons – the nucleus remains unchanged.
Radioactivity
• Nuclear Reactions
– These are reactions that involve an atom
of one element changing into an atom of
another element. These reactions involve
the change of an atom’s nucleus.
Radioactivity
• In the late 1890’s, radioactivity was
discovered.
– Radioactivity is a process in which some
substances spontaneously emit radiation.
– Radiation is the name for the energy (“rays”)
and particles emitted by radioactive material.
– Scientists discovered that by emitting radiation,
atoms of one element could change into atoms
of another element.
Radioactivity
• Why does radioactivity occur?
– Radioactive atoms emit radiation because their
nuclei are unstable*.
– Unstable systems gain stability by losing
energy.
– Unstable nuclei lose energy by emitting
radiation in a spontaneous process called
radioactive decay.
– Radioactive decay continues until stable,
non-radioactive atoms form. They are often of
a different element.
Types of Radiation
• Research, begun in the late 1880’s, used
electrically charged plates. Scientists were
able to identify 3 types of radiation.
Types of Radiation
• Alpha Radiation
– Deflected to negatively
charged plates.
– Made up of alpha
particles: 2 protons and 2
neutrons with a +2 charge.
– Symbol for an alpha
particle is 42α or 42He (the
helium nucleus)
– Example of alpha decay
Alpha Radiation
• Note what happens to the atomic number of
an element upon emission of an alpha
particle.
• The atom giving up the alpha particle has its
atomic number reduced by two. This
results in the atom becoming a different
element. In the example, Radium-226
undergoes alpha decay to form Radon-222.
Alpha Radiation
226
222 Rn +
Ra
-->
88
86
4
2He
• This type of equation is known as a nuclear
equation.
• The atomic numbers and mass numbers of
all particles involved are shown.
• Note that both atomic number and mass
number are conserved in a nuclear equation.
Practice Problems
Complete the following nuclear equations for
alpha decay:
1 24195Am ---->
2 _____ ----> 42He + 23592U
3 23592U ---->
4 _____ ----> 42He + 20684Po
Types of Radiation
• Beta radiation
– Deflected towards the positively
charged plate
– Consists of fast-moving
electrons called beta particles.
They have a 1- charge.
– Symbol for a beta particle is 00
1ß or -1e.
– Example of beta decay
Beta Radiation
• The atom giving up the beta particle has its
atomic number increased by one.
• This results in the atom becoming a different
element. In the example, carbon-14 undergoes
beta decay to form nitrogen-14, according to
the equation:
14 C ---> 0 e + 14 N.
6
-1
7
• Note that, in the equation, atomic numbers and
mass numbers are conserved.
Beta Radiation
• Since beta radiation is a type of nuclear radiation, the
beta particle is emitted by the nucleus of an atom.
• How, you may ask, is it possible for an electron to come
from a nucleus??
• Scientists believe that neutrons are actually composed
of 2 particles - an electron and a proton.
• In beta decay, the electron is emitted and the proton is
left, increasing the atomic number.
Practice Problems
Complete the following nuclear equations for
beta decay.
1. 6027Co ---->
2. _____ ---> 0-1e + 23191Pa
3. 9740Zr --->
4. _____ ---> 0-1e + 6630Zn
Types of Radiation
• Gamma Radiation
– Also called gamma rays
– It is high-energy electromagnetic radiation. It has no
charge. It also has no mass so its emission does not
change the element.
– However, gamma radiation often accompanies alpha
and beta emission, which do change the element's
identity.
– Symbol for a gamma ray is 00 ϒ or just ϒ
– Gamma rays account for most of the energy lost during
the radioactive decay process.
Types of Radiation
Unstable* Nuclei
• Why are some atoms unstable?
– The primary factor in determining stability is the
ratio of neutrons to protons.
– Atoms containing either too many or too few
neutrons are unstable.
– Unstable nuclei lose enough energy and emit enough
particles during radioactive decay to eventually form
a nucleus with a stable neutron to proton ratio.