Unit 9

Nuclear Reactions

Nuclear Reactions vs Typical Reactions

• Typical reactions involve changes in the outer electronic structures of atoms or molecules.

• Nuclear reactions result from changes taking place within atomic nuclei

Nuclear Equations

• The subscript represents the number of protons

• The superscript represents the mass number (number of protons + neutrons)

• A nuclear equation must be balanced with respect to nuclear charge (atomic number) and nuclear mass (mass number)

Radioactive

• A radioactive nucleus spontaneously decomposes with the evolution of energy.

• Few occur in nature• Many can be “induced” in the laboratory by

bombarding stable nuclei with high-energy particles

Modes of Decay

• Alpha particle emission (α)– An ordinary helium nucleus is given off

Modes of Decay

• Beta Particle Emission (β)– Produces an electron– Converts a neutron to a proton– Occurs with nuclei that contain “too many

neutrons” for stability

Modes of Decay

• Gamma Radiation Emission (γ)– High energy photons– Changes neither the mass number nor the

atomic number, it is ordinarily omitted in writing the nuclear equations

– Co-60 is used for cancer therapy

Artificially Produced Nuclei

• Artificially produced nuclei can show alpha, beta, and gamma emission, but can also decompose by…

• Positron emission– Identical to an electron except that it has a +1

charge– Nuclei that have too many protons for stability

Artificially Produced Nuclei

• K-electron capture– An electron in the innermost energy level

(n=1) “falls” into the nucleus– More common with heavy nuclei, because the

n=1 level is closer to the nucleus

Bombardment Reactions

• More than 1500 radioactive isotopes have been prepared in the lab by bombardment reactions.– Stable nucleus converted to one that is

radioactive, which in turn decays to stable products.

– The first isotopes made were in 1934 by Iren Curie and her husband, Frederic Joliot

Bombardment Reactions

• Transuranium elements, those with higher atomic numbers that uranium have been synthesized in the last 50 years.

• The heavy elements (107-112) were prepared in the late twentieth century. Most half-lives are a few microseconds

• January 1999- element 114 was formed with a half-life of 30 seconds. Considered to be a long-lived atom

How do we use radioactive matter in our lives?

• Medicine– Look on page 510 Table 19.2. – Small list of isotopes that test for very specific

things

How do we use radioactive matter in our lives

• Chemistry– Archaeology (dating remains, etc…)– C-14– Since the nuclear testing began, the carbon

dating technique has become less effective due to radioactive fallout.

• Commercial Applications– Smoke Alarms– Food preservation

Rate of Radioactive Decay

• All Radioactive Decay is a first-order process

Rate=k[x]

Half-life= 0.693/k

ln[A]f-ln[A]o=-kt

Rate of Radioactive Decay

Because of the way in which rate of decay is measured, it is often described by the activity (A) of the sample

A=kN

A=activity

K= rate constant

N=number of radioactive nuclei present

Rate of Radioactive Decay

• Activity can be expressed…– Becquerel (Bq).

• 1 Bq= a atom/s

– *****Curies (Ci)• 3.70 x 1010 atoms/s

Rate of Radioactive Decay

• The half life of radium-226 is 5.05 x 10 10 s– Calculate k– What is the activity in curies of a 1.00 g

samples of Ra-226?– What is the mass in grams of a sample of Ra-

226 that has an activity of 1.00 x 109 atoms/min?

Carbon Dating

• A tiny piece of paper taken from the Dead Sea Scrolls, believed to date back to the first century A.D., was found to have an activity per gram of carbon of 10.8 atoms/min. Taking Ao to be 13.6 atoms/min, estimate the age of the scrolls. The half life of carbon is 5730 years.– Calculate k– Calculate t

Geiger Counter

• http://www.youtube.com/watch?v=7jeKFOpzcAU

• 1 count = 1 atom

Mass-Energy Relations

• ∆E= c2∆m– ∆E= energy products – energy reactants– C= speed of light– ∆m= mass products- mass reactants

Due to units we will use… we want E in KJ

∆E= 9.00 X 1010 kJ/g x ∆m

Mass-Energy Relations

• For the radioactive decay of radium-226 to give an alpha particle + another nucleus, calculate the ∆E in KJ when 10.2 g of radium decays.

Nuclear Binding

• It is always true that a nucleus weighs less than the individual protons and neutrons of which it is composed.

• This number can be negative, because some of our mass numbers are rounded

***mass number= protons + neutrons

Nuclear Binding

• Calculate the binding energy C-14, in KJ/mole.

Nuclear Fission

• This process was discovered in 1938. One year before WW II broke out.

• Atomic Bomb a product of nuclear fission– First tested in New Mexico July 16, 1945– Hiroshima- August 6, 1945– Nagasaki- August 9, 1945

• August 14, 1945, the war ended

• Fission-evolution of energy

Nuclear Fission

• With fission, scientist mainly use U-235 and Pu-239. (can be done with the heavier elements also)

• They bombard these elements with neutrons. It causes several chain reactions every time a U-235 nucleus splits. With the energy formed, it causes an explosion.

• Nuclear Power Plants use nuclear fission to produce electricity

Nuclear Fusion

• Energy is evolved when very light nuclei combine with one another to form a heavier nucleus.

• Produces more energy than fission– The sun is constantly having fusion of

hydrogen.

If fusion produces more energy, why don’t we use it?

• Extremely high activate energies (remember the lower the more collision= more collisions= reaction takes place)

• Scientist are working on using magnetic fields to produce enough velocity to overcome the AE.

• They have done this on a small level but the it only lasted one nanosecond (10-9s)