Nuclear Transformation Prentice-Hall Chapter 25.2 Dr. Yager

Objectives  Describe the type of decay a radioisotope undergoes.  Solve problems that involve half-life.  Identify the two ways transmutation can occur.

Radon in the Environment Radon-222 is a naturally occurring radioisotope found in the soil in some areas which can collect in a closed house and 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. atomic number mass number

Nuclear 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 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 neutron-to-proton ratio determines the type of decay that occurs.  If there are too many neutrons relative to protons, a neutron will turn into a proton (  -decay)  If there are too many protons, a proton will turn into a neutron (positron emission) A positron is a particle with the mass of an electron but a positive charge. A positron is a particle with the mass of an electron but a positive charge. What determines the type of decay a radioisotope undergoes?

 Approximately 350 isotopes of 90 elements are found in our solar system. About 70 of these isotopes are radioactive.  An additional 1,500+ isotopes have been made in the laboratory, most of which are unstable (radioactive).  For elements with atomic number < 83, most isotopes are stable. These nuclei are in a region called the band of stability.

Predicting Nuclear Stability  Except for 1 H and 3 He, all stable nuclei have # neutrons > # protons # neutrons > # protons  A nucleus with N/Z (neutrons/protons) that is too large or too small is unstable. small nuclei: N/Z ~ 1 small nuclei: N/Z ~ 1 large nuclei: N/Z ~ 1.5 large nuclei: N/Z ~ 1.5  Nuclei with even numbers of protons & neutrons are more stable  No atoms with atomic number > 83 and mass number > 209 are stable

Isotope Trivia  Calcium has six stable isotopes ( 40 Ca – 48 Ca)  Tin has 10 stable isotopes (most of any element)  Heaviest stable isotope: 209 Bi bismuth’s only stable isotope w/126 neutrons 209 Bi bismuth’s only stable isotope w/126 neutrons

Half Life  A half-life ( t 1/2 ) is the time required for one- half of the nuclei of a radioisotope sample to decay to products.  After each half-life, half of the existing radioactive atoms have decayed into atoms of a new element.

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

Stable Isotope Decay of Uranium-238

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

The half-life ( t ½ ) of C-14 is 5,730 years. How long is three half-lives? 3 x (5,730 years) = 17,190 years 3 x (5,730 years) = 17,190 years How much of 8 grams are left after three half-lives? 8 g x ½ x ½ x ½ = 1 g 8 g x ½ x ½ x ½ = 1 g

Limitations 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 Dating

Transmutation Reactions  The conversion of an atom of one element to an atom of another element is called transmutation.  Transmutation can occur by radioactive decay.  Transmutation can also occur when particles 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 atomic numbers above 92, the atomic number of uranium, are called the transuranium elements. The elements in the periodic table with atomic numbers above 92, the atomic number of uranium, are called the transuranium elements. All transuranium elements undergo transmutation. All transuranium elements undergo transmutation. None of the transuranium elements occur in nature, and all of them are radioactive. None of the transuranium elements occur in nature, and all of them are radioactive. Transuranium Elements

Transuranium elements are synthesized in nuclear reactors and nuclear accelerators.

During nuclear decay, if the atomic number decreases by one but the mass number is unchanged, the radiation emitted is a. a positron. b. an alpha particle. c. a neutron. d. a proton.

During nuclear decay, if the atomic number decreases by one but the mass number is unchanged, the radiation emitted is a. a positron. b. an alpha particle. c. a neutron. d. a proton.

When potassium-40 (atomic number 19) decays into calcium-40 (atomic number 20), the process can be described as a. positron emission. b. alpha emission. c. beta emission. d. electron capture.

When potassium-40 (atomic number 19) decays into calcium-40 (atomic number 20), the process can be described as a. positron emission. b. alpha emission. c. beta emission. d. electron capture.

If there were 128 grams of radioactive material initially, what mass remains after four half-lives? a. 4 grams b. 32 grams c. 16 grams d. 8 grams

If there were 128 grams of radioactive material initially, what mass remains after four half-lives? a. 4 grams b. 32 grams c. 16 grams d. 8 grams

When transmutation occurs, the _____ always changes. a. number of electrons b. mass number c. atomic number d. number of neutrons

When transmutation occurs, the _____ always changes. a. number of electrons b. mass number c. atomic number d. number of neutrons

Transmutation occurs by radioactive decay and also by a. extreme heating. b. chemical reaction. c. high intensity electrical discharge. d. particle bombardment of the nucleus.

Transmutation occurs by radioactive decay and also by a. extreme heating. b. chemical reaction. c. high intensity electrical discharge. d. particle bombardment of the nucleus.