radioactivity and half-life · half-life calculation # 3 •cobalt-60 is a radioactive isotope used...

Nuclear Chemistry

Nuclear Chemistry

• The study of changes that occur in the nucleus of an ATOM!!!

• Radioactivity - the process in which an unstable atomic nucleus emits charged particles and/or energy (also called nuclear decay)

• During "nuclear decay", an atom of one element can change into an atom of a different element.

Chemical vs. Nuclear ReactionsChemical Reactions Nuclear Reactions

Occur when bonds are broken

Occur when nuclei emit particles and/or rays


Occur when bonds are broken Occur when nuclei emit particles and/or rays

Atoms remain unchanged, although they may be rearranged

Atoms often converted into atoms of another element





Involve only valence electrons

May involve protons, neutrons, and electrons





Involve only valence electrons May involve protons, neutrons, and electrons

Associated with small energy changes

Associated with large energy changes





Involve only valence electrons May involve protons, neutrons, and electrons

Associated with small energy changes Associated with large energy changes

Reaction rate influenced by temperature, particle size, concentration, etc.

Reaction rate is not influenced by temperature, particle size, concentration, etc.

Radioactivity

• An unstable atomic nucleus emits a form of radiation (alpha, beta, or gamma) to become stable.

• In other words, the nucleus decays into a different atom.

Types of Nuclear Radiation

• Alpha particles

• Beta particles

• Gamma rays

Radioactivity

• Alpha Particle – Helium nucleus

• Beta Particle – electron

• Gamma Ray – high-energy photon

Types of Nuclear Radiation

• Alpha particles - positively charged, made up of 2 protons and 2 neutrons (same as a helium nucleus)

• Alpha decay - one product is ALWAYS a Helium nucleus 4

2 He

• (ex) 22288Ra 4

2 He + 21886 Rn

Mass #Atomic #

Look up element #86

Beta Particles

• Beta particles – electron emitted by an unstable nucleus, negatively charged particle

• Beta decay - one product is 0-1e (because of it’s negative charge, it is assigned an atomic number of -1)

• (ex) 146C 0

-1e + 147N

• (once again, the sums of mass number and atomic number of the products must equal the reactant)

• Once you find the atomic number, you look for the element on your periodic table!!!

Gamma Decay

• Gamma decay- penetrating ray of energy emitted by an unstable nucleus

• They have no mass and no charge• Like X-rays and visible light, gamma rays

travel at the speed of light• During gamma decay, the atomic number and

mass number of the atom remain the same, but the energy of the nucleus decreases

• X-rays are emitted by electrons outside the nucleus, while gamma rays are emitted by the nucleus.

http://en.wikipedia.org/wiki/Nucleus_%28atomic_structure%29

Decay Particles Penetrating Ability• alpha particle

– Least penetrating (can be stopped by a sheet of paper or clothing)

• beta particle– More penetrating that alpha (pass through paper, but stopped by a thin

sheet of metal

• gamma ray– Much more penetrating that alpha and beta (takes several centimeters of

lead or several meters of concrete to stop gamma radiation)

Effects of Nuclear Radiation

• Background radiation – nuclear radiation that occurs naturally in the environment– Examples – radioisotopes in air, water, rocks, plants and

animals

• Nuclear radiation can ionize atoms, so it can damage the cells and tissues of your body– Bonds holding proteins and DNA may be broken

• Examples – Alpha particles: radon gas (can be inhaled and causes lung

cancer)

– Beta particles can damage tissues more than alpha particles

– Gamma rays can expose all organs to ionization damage

Half-Life

• Amount of time it takes for one half of a sample of radioactive atoms to decay

Medical Applications of Half-Life

Nuclide Half-Life Area of Body

I–131 8.1 days Thyroid

Fe–59 45.1 days Red Blood Cells

Sr–87 2.8 hours Bones

Tc–99 6.0 hours Heart

Na–24 14.8 hours Circulatory System

Half-Life Calculation #1

• You have 400 mg of a radioisotope with a half-life of 5 minutes. How much will be left after 30 minutes?

To calculate:• Start with 400 mg.

• The half-life is 5 minutes, so how much will you have after 5 minutes?

– 200 mg

• Now, how much will you have at the 10 minute mark? (remember that after 5 minutes, you will half of what you started with at that particular time.)

– 100 mg

• Keep going until you have reached 30 minutes!

• You can also use fractions…you try it!

Half-Life Calculation #2

• Suppose you have a 100 mg sample of Au-191, which has a half-life of 3.4 hours. How much will remain after 10.2 hours?

Half-Life Calculation # 3• Cobalt-60 is a radioactive

isotope used in cancer treatment. Co-60 has a half-life of 5 years. If a hospital starts with a 1000 mg supply, how many mg will need to be purchased after 10 years to replenish the original supply?

Half-Life Calculation # 4

• A radioisotope has a half-life of 1 hour. If you began with a 100 g sample of the element at noon, how much remains at 3 PM? At 6 PM? At 10 PM?


• How many half-lives have passed if 255 g of Co-60 remain from a sample of 8160 g?


• Suppose you have a sample containing 400 nuclei of a radioisotope. If only 25 nuclei remain after one hour, what is the half-life of the isotope?


• If a radioactive element has diminished by 7/8 of its original amount in 30 seconds, what is its half-life?

Answers to Half-Life Calculations

• Half-Life Calculation #1

– 6.25 mg


– 12.5 mg


– 750 mg



– 12.5 g, 1.5625 g, 0.09765625 g


– 5 half-lives



– 15 minutes


– 10 seconds

Bell ringer 11/18/13

1. What are 2 ways to detect radiation?

2. What is half-life?

3. You have 400 mg of a radioisotope with a half-life of 5 minutes. How much will be left after 30 minutes?

32

Video on Fusion and Fission

• http://www.youtube.com/watch?v=-Qliifidcuw

• Another one:

• http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa/atoms_radiation/nuclearact.shtml

http://www.youtube.com/watch?v=-Qliifidcuw

http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa/atoms_radiation/nuclearact.shtml

Fission and Fusion

• Fission- splitting of atomic nuclei. Tons of energy produced from a small mass

• Fusion- combining atomic nuclei. Requires high temp, stars do this.

Fission

• NOT a natural process

• Occurs as a result of neutron bombardment

• heavy nucleus (mass# > 200) splits into two similar sized pieces

• wide range of nuclei produced

• releases lots of energy b/c smaller nuclei more stable

• 235U and 239Pu most important

Fission

• CONTROLLED:

– Nuclear Power Plant

• UNCONTROLLED:

– In reactor = meltdown!

– Atomic bomb!

Fission and Fusion

Fusion• nuclei fuse together

• for very light elements, stability increases with increasing mass #

• energy released when two light nuclei combine to larger, more stable nucleus

• thermonuclear reaction - occurs only at very high tempslowest temperature required for fusion: 40,000,000 K

• occurs constantly in sun

• Plasma is an example (atoms have been stripped of their electrons)

• FusionHas it occurred on earth?

Hydrogen bomb

Comparing and Contrasting

Section 10.4

a. is the splitting of a large nucleus into two smaller fragments

b. is widely used as an alternative energy source

c. is the fusing of two small nuclei into one larger nucleus

d. is still being researched and developed as an alternativeenergy source

Radiation in our lives

• Background radiation • cosmic rays in atmosphere• radon from granite rock (lung cancer)• Foods• bananas, Brazil nuts• Medical exposure• Dental x-ray• Cancer treatment (Cobalt-60)• Detecting art forgeries• Analyzing gunpowder residue• Agricultural research, diagnose disease• Smoke detectors (Americium-241)

What do we do with Nuclear Waste?

• Bury it underground in a spot that is geologically stable!!

• SC has a nuclear waste site.


1. Suppose you have a 100 mg sample of Au-191, which has a half-life of 3.4 hours. How much will remain after 10.2 hours?

2. Classify the following as acid or base:

a) HCl

b) KOH

3. What are the names of the above substances? 43


1. Explain fission.

2. Explain fusion.

3. Explain 2 ways they are similar.

4. Explain 2 ways they are different.

5. Name a pro and a con about nuclear radiation.

6. What is Mg(OH)2?

7. What is H2SO4?

radioactivity and half-life · half-life calculation # 3 •cobalt-60 is a radioactive isotope used...

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