section 4.4 unstable nuclei and radioactive decay

Section 4.4Section 4.4

Unstable Nuclei and

Radioactive Decay

ObjectivesObjectives

• Explain the relationship between unstable nuclei and radioactive decay.

• Characterize alpha, beta, and gamma radiation in terms of mass and charge.

RadioactivityRadioactivity

• Chemical Reactions– Atoms of one element do not change into atoms

of another element during a chemical reaction.– Why?


• Chemical reactions involve the rearrangement of atoms – not their “destruction.”

• Chemical reactions involve only an atom’s electrons – the nucleus remains unchanged.


• 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.


• 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.


• 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 RadiationTypes of Radiation

• Research, begun in the late 1880’s, used electrically charged plates. Scientists were able to identify 3 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 4

2α or 42He

(the helium nucleus)

– Example of alpha decay

Alpha RadiationAlpha 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 RadiationAlpha Radiation

22688Ra --> 222

86Rn + 42He

• 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 ProblemsPractice Problems

Complete the following nuclear equations foralpha decay:

1 24195Am ---->

2 _____ ----> 42He + 235

92U

3 23592U ---->

4 _____ ----> 42He + 206

84Po


• 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 0-1ß

or 0-1e.

– Example of beta decay

Beta RadiationBeta 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:

146C ---> 0

-1e + 147N.

• Note that, in the equation, atomic numbers and mass numbers are conserved.

Beta RadiationBeta 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 ProblemsPractice Problems

Complete the following nuclear equations for beta decay.

1. 6027Co ---->

2. _____ ---> 0-1e + 231

91Pa

3. 9740Zr --->

4. _____ ---> 0-1e + 66

30Zn


• 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 0

0 ϒ or just ϒ – Gamma rays account for most of the energy lost during the radioactive decay process.

Unstable* NucleiUnstable* 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.

section 4.4 unstable nuclei and radioactive decay

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