Nuclear Chemistry

What is radioactivity?Spontaneous emission of radiation when

the nucleus of an atom breaks down to form a different element.

Background RadiationThe constant level of radioactivity always

present around usCaused by:

Cosmic rays Naturally occurring uranium Radon in the air Radioactive isotopes in food and environment

What is your radiation exposure?National Average: 500 mremBackground Average: 360 mrem

Types of Decay

How do nuclear and chemical reactions differ?

Nuclear reactions involve the nucleus (protons and neutrons)

BUTchemical reactions involve the

transfer and sharing of electrons

Band of Stability

After element 83 (Bismuth)the elements are naturally unstable

and may emit decay particles

The bigger the atom gets and the further from a 1:1 ratio of

protons and neutrons, the less stable the

atom is

What holds the atom together?Electromagnetic Force

Force of repulsion between positive charges of the protons in the nucleus

Strong Force Force present that holds quarks together and

therefore keeps the protons and neutrons tightly packed in the nucleus

When the electromagnetic force wins out, the nuclei break apart!

Review: Types of Decay

Decay Type Symbol Charge Reaction

Particle StrengthCan be

stopped by…

AlphaBeta

Gamma

Nuclear Symbols Review

• The superscript indicates the mass number and the subscript indicates the atomic number.

XAZ

Mass Number Atomic Number

Element Symbol

Steps to writing decay reactions:

1. Write the nuclear symbol for the element that is given.

2. Draw an arrow.3. Identify the type of particle that has decayed

and write it after the arrow.4. Balance the mass number (top) and the

atomic number (bottom). 5. Identify the new element.

Alpha Decay Loss of an alpha particle 4

2He42α

239Pu 94

235U 92

Beta Decay Loss of a beta particle (electron)

0-1 e0

-1 β

42K 19

Beta Positive Decay Loss of a beta positive particle (positron)

42K 19

0+1 e0

+1 β

Gamma Decay Loss of a gamma particle

Does it effect the outcome of the reaction?

Alpha and gamma decay of:

00γ

239Pu 94

Half Life

Half-LifeTime required for half a sample to decay

The stability of the isotope is what determines the rate of decay.

Less Stable = Faster Decay

Half-Life After each half-life, half of

the sample decays. Start = 100%

40 blue particles are present 1 half-life = 50%

20 blue remain 2 half-lives = 25%

10 blue remain 3 half-lives = 12.5%

5 blue remain 4 half-lives = 6.25%

2.5 blue remain

Amount never becomes zero!!

Half-Life After 10 half-lives sample considered

nonradioactive because it approaches the level of background radiation.

Because the amount never reaches zero, radioactive waste disposal and storage causes problems. Would you want radioactive waste stored in your community?

How can we get rid of nuclear radioactive waste?

Half Life PracticeExample 1:

The half-life of mercury-195 is 31 hours. If you start with a sample of 5.00 g, how much of it will still be left after 93 hours?

Half Life Practice ContExample 2:

How many half lives have passed if there is only 1.875 g left of a 30 g sample?

If the half life for this sample is 1 hour, how many total hours have gone by?

Applications

Where do we use nuclear chemistry?

Sun is powered by nuclear reactionsElectricity from nuclear power plantsMedical:

x-rays, cancer treatment, radioactive tracers Industrial:

Sterilize equipment, sterilize (irradiate) food, x-rays of metal equipment (scan for cracks in airplanes)

Military

Radiation ExposureExposure to radiation can cause cell mutations

and illness.Four factors that affect exposure:

Dose (strength of radiation) Exposure time Area exposed Tissue exposed

HOW DO WE BENEFIT FROM NUCLEAR

REACTIONS?

FusionTwo light atoms combine to form a heavier

atom of higher energy

This reaction powers the sun!

FissionOne heavy atom breaks down into two or more

smaller atoms and produces energyThis becomes a chain reaction

(as one atom splits and hits more, and those split and hit more)

Supercritical:creates a great release of energy - atomic bomb

Ping Pong Video

Will a chain reaction always occur?Particles reacting must have critical mass:

The minimum amount of mass needed for the neutron to hit and react with

No critical mass = no reaction

Fission reactions can also be controlled to contain the energy.

Controlled FissionControlled Fission: used for nuclear power

Controlled Diagram:Control Rods to

limit nuclear fission!

Fission heats the water that turns to steam and moves the

turbines