Nuclear Chemistry•Nuclear chemistry is the study of the changes of the nucleus of atoms.

•Nuclear Reactions involve changes within the nucleus where as chemical reactions involve the loss, gain or sharing of electrons.

The Nucleus

• Remember that the nucleus is made up of protons and neutrons. The are collectively called nucleons.

Radioactivity

• A stable nucleus holds together well. An unstable nucleus will decay or break down, releasing particles and/or energy in order to become stable.

• An atom with an unstable nuclei is considered “radioactive”.

Nuclear TransformationsNuclear transformations can be induced by accelerating a particle and colliding it with the nuclide.

These particle accelerators are enormous, having circular tracks with radii that are miles long.

There are several ways radioactive atoms can decay into different atoms!

Transmutation:

• Type of nuclear reaction that will change the number of protons and thus will create a different element.

• Atoms with an atomic number larger than 92 are created through this process

Alpha DecayLoss of an -particle (a helium nucleus)

• Atomic number decreases by 2 and mass number decreases by 4• Penetrating Power: LOW: Can be blocked by clothing or thin paper• Example

He42

U23892

Th23490 He4

2+

α42

OR

Alpha Decay

http://education.jlab.org/glossary/alphadecay.gif

Alpha Decay

Uranium Thorium

Beta DecayLoss of a -particle (a high energy electron)

• Atomic number increases by 1 and mass number stays the same. A neutron becomes a proton and a high speed electron that is discharged from the nucleus.• Penetrating Power: Medium: Can be blocked by thin metal or wood• Example

I13153

Xe13154 e0

−1 +

0−1 e0

−1or

Beta Decay

Beta Decay

Thorium Protactinium

Gamma EmissionLoss of a -ray (high-energy radiation that almost always accompanies the loss of a nuclear particle)

• Atomic number and mass number stays the same• Penetrating Power: High: Can only be blocked by thick metal or thick concrete• Example

I13153

I13153 e +

00

Radioactivity

• Radioactive isotopes decay at a characteristic rate measured in half life.

• A half life is the time required for half of the amount of radioactive atoms to decay. The time ranges from seconds to millions of years

 

Examples• Beta decay of zircomium-97

 

• Alpha decay of americium-241

• Alpha decay of uranium-238

• Complete this:

Np23593

Pu23994 ____ +

Common Radioactive Isotopes

Isotope Half-Life Radiation Emitted

Carbon-14 5,730 years

Radon-222 3.8 days

Uranium-235 7.0 x 108 years

Uranium-238 4.46 x 109 years

Radioactive Half-Life

• After one half life there is 1/2 of original sample left.

• After two half-lives, there will be

1/2 of the 1/2 = 1/4 the original sample.

Graph of Amount of Remaining Nuclei vs Time

A=Aoe-t

A

Half Life CalculationsHOW TO’s

1. To calculate the number of half lives, divide the half life (T1/2) into the total time (T). 

T/T1/2 = # of half lives 

2. Use the equation to calculate remaining amount left over after a certain number of half lives have passed.

• Amt remaining = (initial amt) (.5)n (# of half lives)

Example

You have 100 g of radioactive C-14. The half-life of C-14 is 5730 years.

• How many grams are left after one half-life?

• How many grams are left after two half-lives?

Examples

• Suppose you have 20 grams of sodium-24. Its half-life is 15 hours. How much is left over after 60 hours.

Examples

• Uranium-238 has a half life of 4.46 x 109 years. How long will it take for 7/8th of the sample to decay?

Examples

• The half life of radium-222 is 38 s. How many grams of a 12.0 g sample are left after 114 s?

Examples

A sample of 3x107 Radon atoms are trapped

in a basement that is sealed. The half-life of

Radon is 3.83 days. How many radon atoms

are left after 31 days?

answer:1.2x105 atoms

Nuclear Fission: How does one tap all that energy?

• Large atoms split into smaller atoms that generate huge amounts of energy.

• Carried out in nuclear reactors.

• Could result in a chain reaction of fission like the atomic bomb

Nuclear Fission

• Bombardment of the radioactive nuclide with a neutron starts the process.

• Neutrons released in the transmutation strike other nuclei, causing their decay and the production of more neutrons.

• This process continues in what we call a nuclear chain reaction.

Nuclear Fission

• If there are not enough radioactive nuclides in the path of the ejected neutrons, the chain reaction will die out.

• Therefore, there must be a certain minimum amount of fissionable material present for the chain reaction to be sustained: Critical Mass.

Nuclear ReactorsIn nuclear reactors the heat generated by the reaction is used to produce steam that turns a turbine connected to a generator.

Nuclear Reactors

• The reaction is kept in check by the use of control rods.

• These block the paths of some neutrons, keeping the system from reaching a dangerous supercritical mass.

Nuclear Fusion

• Fusion would be a superior

method of generating power.The good news is that the

products of the reaction are

not radioactive.The bad news is that in order to achieve fusion, the

material must be in the plasma state at several million kelvins.

Tokamak apparati like the one shown at the right show promise for carrying out these reactions.

They use magnetic fields to heat the material.

Nuclear Fusion

• Smaller atoms are combine to form a large atom.

• Occurs in the sun and stars

• Generates huge amounts of energy