Unit Two Part B

Nuclear Chemistry

Radioactivity

There are two main types of radioactivity: Natural and Induced

Natural Radioactivity

Occurs in natureUsually large, unstable nucleiOccurs in three ways:

α

Particle (alpha particle)

β

Particle (beta particle)

γ

Ray (gamma ray)

Positron--0

1e+

Alpha Decay

A helium nucleus is released from the nucleus. ( )

The mass decreases by 4The atomic number decreases by 2

(Because the He nucleus has 2p+

and 2no)

Alpha radiation can be stopped by a piece of paper. Cannot penetrate skin. Not dangerous.

He4

2

Alpha Decay Example

Notice that the uranium has changed into a new element, thorium.

HeThU 4

2

234

90

238

92+→

Beta DecayAn electron is released from the nucleus when a neutron becomes a proton.

The mass is unaffected. (the mass of a neutron is roughly equal to the mass of a proton)The atomic number is increased by 1.Harder to stop and more dangerous.

e0

1−

Beta Decay Example

eNC 0

1

14

7

14

6 −+→

Notice that carbon has changed into nitrogen.

Gamma Decay

Pure energy is released from the nucleus.The mass and atomic number are unaffected.Stopped by lead. The most harmful to living tissue.

Gamma Decay Example

γ+→ SrSr 8738

8738

*No new element formed. Gamma radiation (energy) released.

Induced RadioactivityParticles are slammed together to cause transmutation of stable elements. (Nuclear Bombardment)Discovered by Rutherford in 1919.Everyday radiation…from mitchrad

physics.

Radioactive Decay of U-238

HeThU 4

2

234

90

238

92+→

•Uranium-238 becomes Thorium-234

•Transmutation by Alpha Decay

Half-Life

The time it takes for half of the atoms in a given radioactive sample to decay into a more stable isotope. This number is different for each kind of isotope of any kind of element.Can be calculated because atoms decay at a predictable rate.Half lives can range from fractions of a second to millions of years.

TYPES AGAIN!!!

Half-Life

Two formulas will help you solve half life problems:

1. Half-Life = Total time

n

2. Final Mass = Total Mass2 n

(n=#cycles)

Example Problems

The half-life of technetium is 6.00 hours. What mass of Tc-99 remains from a 10.0 gram sample after 24.0 hours.

Since 24.0 hours is 4 half-life cycles, the original 10.0 gram sample is divided four times.

Final Mass = Total Mass2n

10.0 g

= 10.0g

= 0.625 g Tc-99 remain24 16

How about another one???

A 50.0g sample of N-16 decays to 12.5g in 14.4s. What is its half-life?

Half-Life = Total Timen

Half-Life = 14.4s2

Half-Life = 7.2s

50.0g 1 half = 25.0g 2 half = 12.5 g

Sure, one more…

why not?

There are 5.0g of I-131 left after 40.35 days. How many grams were in the original sample if its half-life is 8.07 days?

Final Mass = Total Mass2 n

1st

: How many cycles have occurred?

40.35 / 8.07 = 5 cycles.2nd: Rearrange the formula to solve for the

original total mass. Total Mass = Final Mass x 2n

So, solve it already!!!

3rd: SolveTotal Mass = 5.0g x 25

Total Mass = 5.0g x 32

Total Mass = 160.0g

Shall we check it???(of course)160.0g Total MassAt the end of one half life = 80.0g (8.07days)At the end of two cycles = 40.0g (16.14 days)At the end of three cycles = 20.0g(24.21 days)At the end of four cycles = 10.0g(32.28 days)At the end of five cycles = 5.0g(40.35 days)

Since decay occurs at a predictable rate, we can use the ratio of decayed to undecayed

isotopes to…

Determine the age of Organic Matter with Carbon –

14 (Up to 30,000 yrs)

Determine the age of Rocks (and therefore other earth structures) with Uranium –

238 (Millions of yrs.)

Using Radioisotopes

More uses for radioisotopes…

Tracers

used to detect structure and function of organs (thyroid, gall bladder, GI tract, etc…)Can also be used to track movement of silt in rivers and nutrient uptake in plants.Cancer treatmentFood preservationSensors in Smoke DetectorsStarters in Fluorescent lampsNuclear fuel for power plants

Detection of Radioactivity

Detected with a Geiger Counter. (When ions strike the cylinder of the Geiger counter, it emits an audible click.)

Detection of Radiation

Dosimeter

–

measures the total amount of radiation that a person has received. Works because photographic film is sensitive to radiation. Usually is worn like a badge. The film is later developed and the exposure to radiation can be measured.Unit used to measure radiation exposure in humans is the rem. (Stands for Roentgen Equivalent for Man) Roentgen discovered X-rays.

Biological Effects of Radiation

o

Destruction of tissue especially blood and lymph which cells multiply rapidly.

o

Causes various cancers.o

Direct damage to an organism is

called Somatic Damage.o

Damage that affects reproductive

cells is called Genetic Damage. This leads to birth defects in offspring.

Nuclear Fission

A large nucleus is split into two smaller nuclei of approximately equal mass.The fission of 4.5g of U-235 will satisfy the average person’s energy needs for an entire year. (Equal to about 15 tons of coal.)

Nuclear Fission

The total mass of the products in a fission reaction is slightly less than the mass of the starting materials.Law of Conservation of Matter does not apply to fission reactions!This small amount of “missing”

mass is

converted into a huge amount of energy. (E = mc2) c=300,000,000m/s

Nuclear Fission

A fission reaction can produce a Chain Reaction

because each reaction

releases high speed neutrons, each capable of starting another fission reaction.Chain reactions make the fission process sustainable for use in Nuclear Power Plants.

Nuclear FusionTwo small nuclei join to form a large nucleus.Some mass is converted into energy (even more than fission reactions)Difficult to produce and control. To overcome the repulsion between nuclei, they must be heated to 40 million kelvins. For this reason, they are sometimes called Thermonuclear Reactions.

Nuclear Fusion

Thermonuclear reactions create the energy produced by the sun and other stars.Thermonuclear reactions are the source of the destructive power of a hydrogen bomb.Not (yet?) sustainable for use in nuclear power plants.