Nuclear TransformationNuclear Transformation

Prentice-Hall Chapter 25.2Prentice-Hall Chapter 25.2

Dr. YagerDr. Yager

ObjectivesObjectives

DescribeDescribe the type of decay a radioisotope the type of decay a radioisotope undergoes.undergoes.

Solve problemsSolve problems that involve half-life. that involve half-life.

IdentifyIdentify the two ways transmutation can the two ways transmutation can occur.occur.

Radon in the EnvironmentRadon in the Environment

Radon-222 is a Radon-222 is a naturally occurring naturally occurring radioisotope found in radioisotope found in the soil in some areas the soil in some areas which can collect in a which can collect in a closed house and closed house and pose a health risk.pose a health risk.

The protons and neutrons of a nucleus are called nucleons.

A nuclide is a general term applied to a specific nucleus with a given number of protons and neutrons.

Nuclides can be represented in two ways. One way shows an element’s symbol with its atomic number and mass number.

A

ZX

atomic number

mass number

Nuclear Strong ForceNuclear Strong Force In 1935, the Japanese physicist Hideki Yukawa

proposed that a force between protons that is stronger than the electrostatic repulsion can exist between protons.

Later research showed a similar attraction between two neutrons and between a proton and a neutron.

This force is called the strong force and is exerted by nucleons only when they are very close to each other.

All the protons and neutrons of a stable nucleus are held together by this strong force.

Although the strong force is much stronger than electrostatic repulsion, the strong force acts only over very short distances.

Although forces due to charges are weaker, they can act over greater distances.

In smaller nuclei, the nucleons are close enough for each nucleon to attract all the others by the strong force.

In larger nuclei, some nucleons are too far apart to attract each other by the strong force.

If the repulsion due to charges is not balanced by the strong force in a nucleus, the nucleus will break apart.

The Strong ForceThe Strong Force

In the nucleus, the nuclear force acts only over a distance of a few nucleon diameters.

Arrows describe magnitudes of the strong force acting on the protons.

The The neutron-to-proton rationeutron-to-proton ratio determines determines the type of decay that occurs.the type of decay that occurs.

If there are too many neutrons relative to If there are too many neutrons relative to protons, a neutron will turn into a proton protons, a neutron will turn into a proton ((-decay)-decay)

If there are too many protons, a proton If there are too many protons, a proton will turn into a neutron (will turn into a neutron (positron emissionpositron emission))

A A positronpositron is a particle with the mass of is a particle with the mass of an electron but a positive charge.an electron but a positive charge.

What determines the type of decay a radioisotope undergoes?

Approximately 350 isotopes of 90 elements Approximately 350 isotopes of 90 elements are found in our solar system. About 70 of are found in our solar system. About 70 of these isotopes are radioactive.these isotopes are radioactive.

An additional 1,500+ isotopes have been An additional 1,500+ isotopes have been made in the laboratory, most of which are made in the laboratory, most of which are unstable (radioactive).unstable (radioactive).

For elements with atomic number < 83, For elements with atomic number < 83, most isotopes are stable. These nuclei are in most isotopes are stable. These nuclei are in a region called the a region called the band of stabilityband of stability..

Predicting Nuclear StabilityPredicting Nuclear Stability Except for Except for 11H and H and 33He, all stable nuclei have He, all stable nuclei have

# neutrons # neutrons >> # protons # protons

A nucleus with N/Z (neutrons/protons) that is too large A nucleus with N/Z (neutrons/protons) that is too large or too small is unstable.or too small is unstable.

small nuclei: N/Z ~ 1small nuclei: N/Z ~ 1

large nuclei: N/Z ~ 1.5large nuclei: N/Z ~ 1.5

Nuclei with even numbers of protons & neutrons are Nuclei with even numbers of protons & neutrons are more stablemore stable

No atoms with atomic number > 83 and mass number > No atoms with atomic number > 83 and mass number > 209 are stable209 are stable

Isotope TriviaIsotope Trivia

Calcium has six stable isotopes (Calcium has six stable isotopes (4040Ca – Ca – 4848Ca)Ca)

Tin has 10 stable isotopes (most of any element)Tin has 10 stable isotopes (most of any element)

Heaviest stable isotope: Heaviest stable isotope: 209209Bi bismuth’s only stable isotope w/126 neutronsBi bismuth’s only stable isotope w/126 neutrons

Half LifeHalf Life

A A half-lifehalf-life ( (tt1/21/2)) is the time required for one- is the time required for one-half of the nuclei of a radioisotope sample to half of the nuclei of a radioisotope sample to decay to products.decay to products.

After each half-life, half of the existing After each half-life, half of the existing radioactive atoms have decayed into atoms of radioactive atoms have decayed into atoms of a new element.a new element.

Half-lives can range from fractions of a second to billions of years.

Stable Isotope

Decay of Uranium-238

Carbon-14 DatingCarbon-14 Dating

The ratio of Carbon-14 to The ratio of Carbon-14 to stable carbon in the stable carbon in the remains of an organism remains of an organism changes in a predictable changes in a predictable way that enables the way that enables the archaeologist to obtain archaeologist to obtain an estimate of its age.an estimate of its age.

The half-life (The half-life (tt½½)) of of C-14 is 5,730 years. C-14 is 5,730 years.

How long is three half-lives?How long is three half-lives?

3 x (5,730 years) = 17,190 years3 x (5,730 years) = 17,190 years

How much of 8 grams are left after three How much of 8 grams are left after three half-lives?half-lives?

8 g x ½ x ½ x ½ = 1 g8 g x ½ x ½ x ½ = 1 g

Limitations of C-14 DatingLimitations of C-14 Dating

Two factors limit dating with carbon-14.

C-14 cannot be used to date objects that are completely composed of materials that were never alive, such as rocks or clay.

After four half-lives, the amount of radioactive C-14 remaining in an object is often too small to give reliable data.

C-14 is not useful for dating specimens that are more than about 50,000 years old.

Anything older than 50,000 years must be dated on the basis of a radioactive isotope that has a half-life longer than that of carbon-14.

Potassium-40, which has a half-life of 1.28 billion years, represents only about 0.012% of the potassium present in Earth today.

Potassium-40 is useful for dating ancient rocks and minerals.

Potassium-40 Useful for DatingPotassium-40 Useful for Dating

Transmutation ReactionsTransmutation Reactions

The conversion of an atom of one element to an The conversion of an atom of one element to an atom of another element is called atom of another element is called transmutationtransmutation..

Transmutation can occur by radioactive decay.Transmutation can occur by radioactive decay.

Transmutation can also occur when particles Transmutation can also occur when particles bombard the nucleus of an atom.bombard the nucleus of an atom.

The first artificial transmutation reaction involved bombarding nitrogen gas with alpha particles.

The elements in the periodic table with The elements in the periodic table with atomic numbers above 92, the atomic atomic numbers above 92, the atomic number of uranium, are called the number of uranium, are called the transuranium elementstransuranium elements.. All transuranium elements undergo All transuranium elements undergo

transmutation.transmutation.

None of the transuranium elements occur None of the transuranium elements occur in nature, and all of them are radioactive.in nature, and all of them are radioactive.

Transuranium Elements

Transuranium Transuranium elements are elements are synthesized in synthesized in nuclear reactors nuclear reactors and nuclear and nuclear accelerators.accelerators.

During nuclear decay, if the atomic During nuclear decay, if the atomic number decreases by one but the mass number decreases by one but the mass number is unchanged, the radiation number is unchanged, the radiation emitted is emitted is

a.a. a positron.a positron.

b.b. an alpha particle.an alpha particle.

c.c. a neutron.a neutron.

d.d. a proton.a proton.

During nuclear decay, if the atomic During nuclear decay, if the atomic number decreases by one but the mass number decreases by one but the mass number is unchanged, the radiation number is unchanged, the radiation emitted is emitted is

a.a. a positron.a positron.

b.b. an alpha particle.an alpha particle.

c.c. a neutron.a neutron.

d.d. a proton.a proton.

When potassium-40 (atomic number 19) When potassium-40 (atomic number 19) decays into calcium-40 (atomic number 20), decays into calcium-40 (atomic number 20), the process can be described as the process can be described as

a.a. positron emission.positron emission.

b.b. alpha emission.alpha emission.

c.c. beta emission.beta emission.

d.d. electron capture.electron capture.

When potassium-40 (atomic number 19) When potassium-40 (atomic number 19) decays into calcium-40 (atomic number 20), decays into calcium-40 (atomic number 20), the process can be described as the process can be described as

a.a. positron emission.positron emission.

b.b. alpha emission.alpha emission.

c.c. beta emission.beta emission.

d.d. electron capture.electron capture.

If there were 128 grams of radioactive If there were 128 grams of radioactive material initially, what mass remains material initially, what mass remains after four half-lives? after four half-lives?

a.a. 4 grams4 grams

b.b. 32 grams32 grams

c.c. 16 grams16 grams

d.d. 8 grams8 grams

If there were 128 grams of radioactive If there were 128 grams of radioactive material initially, what mass remains material initially, what mass remains after four half-lives? after four half-lives?

a.a. 4 grams4 grams

b.b. 32 grams32 grams

c.c. 16 grams16 grams

d.d. 8 grams8 grams

When transmutation occurs, the _____ When transmutation occurs, the _____ always changes. always changes.

a.a. number of electronsnumber of electrons

b.b. mass numbermass number

c.c. atomic numberatomic number

d.d. number of neutronsnumber of neutrons

When transmutation occurs, the _____ When transmutation occurs, the _____ always changes. always changes.

a.a. number of electronsnumber of electrons

b.b. mass numbermass number

c.c. atomic numberatomic number

d.d. number of neutronsnumber of neutrons

Transmutation occurs by radioactive decay Transmutation occurs by radioactive decay and also byand also by

a.a. extreme heating.extreme heating.

b.b. chemical reaction.chemical reaction.

c.c. high intensity electrical discharge.high intensity electrical discharge.

d.d. particle bombardment of the nucleus.particle bombardment of the nucleus.

Transmutation occurs by radioactive decay Transmutation occurs by radioactive decay and also byand also by

a.a. extreme heating.extreme heating.

b.b. chemical reaction.chemical reaction.

c.c. high intensity electrical discharge.high intensity electrical discharge.

d.d. particle bombardment of the nucleus.particle bombardment of the nucleus.