Average Atomic Mass

•What is average atomic mass?

•How do you calculate it?

Review Who was the first scientist to say that

matter was made up of tiny particles? How do we write ions and isotopes in a

shorthand notation? Why is it necessary to have both the ion

and isotope represented in a symbol when determining number of subatomic particles?

Cations are _______ ions and anions are ________ ions.

Radioactivity

•How did Becquerel discover radiation?

•What particles did Rutherford discover dealing with radioactivity?

Radioactive elements•Where are protons, neutrons, and electrons each found in an atom?

•If we are changing the composition of an atom’s nucleus, what particles change?

•These particles are also referred to as nucleons.

Nuclear Reactions

•Change the composition of an atoms nucleus.•Strong force: holds the nucleus together.

**A greater number of protons requires a greater number of neutrons to hold them together.

Stability of atoms

Stable atoms (nuclei) have an equal number of protons and neutrons.

Unstable nuclei have more neutrons than protons.

Which of these do you think is more likely to undergo a nuclear reaction?

Nuclear Reactions and Radioactivity

•Unstable nuclei are more likely to undergo nuclear reactions.

•These isotopes are radioactive.

•What elements did we hear about already that are radioactive?

Radioactivity The _______ the element, the

more unstable it is.

All elements have at least 1 radioactive isotope.

All isotopes of elements with atomic numbers greater than 83 are radioactive. Why?

Spontaneous Emission of Radiation

Radioactive decay: unstable nuclei will spontaneously emit radiationThe original nucleus emits radiation and

forms at least one new nucleus

Total mass number and atomic number must be conserved

Types of Radioactive Decay

Nuclear Reactions and Radioactivity

•Alpha Emission: a helium nucleus is emitted.•Restricted to very heavy nuclei

•Beta Emission: an electron is emitted from the nucleus.•A neutron is converted into a proton and electron.

•Gamma Emission: a photon is emitted.•Nucleus is changing from an excited state to ground energy state.

Nuclear Reactions and Radioactivity

http://library.thinkquest.org/3471/radiation_types_body.html?tql-iframe#alpha

Alpha (α) Beta (β) Gamma (γ)

Charge +2 -1 0

Mass (amu) 4.0015 0.0005 0

Symbol(s)

Stopped by Paper Heavy clothing or aluminum foil

Several centimeters of

lead

Nuclear Equations

226 222 488 86 2Ra Rn α

•Total mass number and atomic number must be conserved.

Radium undergoes alpha decay to form radon.

Nuclear Equations

14 06 1C ________ β

238 092 0U ________ γ

185 479 2Au _________ α

1)

2)

3)

Practice1. What element undergoes alpha decay

to form radium-226?

2. What type of decay occurs when plutonium-242 forms uranium-238?

3. What is produced when selenium-81 undergoes beta decay?

Vocabulary

nuclear reaction strong force nucleons radioactive decay alpha particle beta particle gamma ray nuclear equation

Half-life•The time required for the number of atoms to reach half the original value.

•How do you think the number of atoms changes?

•What is the unit for a half-life?

Half-life

Suppose we have 12 grams of an isotope that undergoes radioactive decay. How many grams will be left after:•1 half-life?

•2 half-lives?

•3 half-lives?

Directions•Take a pen and the lab paper to your bench.

1) Count the number of candies in your cup.

2) Dump the candies onto a paper towel.

3) Remove the candies that land marked side down.

4) Record the number of candies left.

5) Repeat procedure until no candy remains

GraphN

umbe

r of

can

dies

Number of tosses

http://lectureonline.cl.msu.edu/~mmp/applist/decay/decay.htm

Warm-up

What subatomic particles are effected when dealing with nuclear reactions?

What are the three types of radioactive decay? List the names and the symbols (in isotopic notation) used to depict them.

What type of radioactive decay does Ac-227 undergo to form Th-227?

Uranium Radioactive Decay Series

Half-life•The time required for the number of atoms to reach half the original value.

•How do you think the number of atoms changes?

•What is the unit for a half-life?

Half-life•Each isotope has its own characteristic half-life.

•These can range from fractions of a second to millions of years.

Half-life

Suppose we have 400 atoms. How many atoms will be left after:•1 half-life?

•2 half-lives?

•3 half-lives?

Half-life graph

Half-lives used for dating materials

•If we know the original amount of a material and can count how much is currently left, we can determine how old something is using its half-life.

•Why is this possible?

•What is the most common form of radioactive dating?

Radiocarbon dating•Used to determine how long something has been dead for.

•All living things contain C-12 and C-14 at a constant, known ratio. When something dies this ratio changes.•Which one of these isotopes is unstable?

•Which one will undergo radioactive decay?

Radiocarbon dating•Carbon-14 has a known half-life of 5730 years.

•For dating: Detect the amount of C-14 in something that has died and compare this to the amount of C-12 present.

•After 4 half-lives there is not enough C-14 to detect, so potassium-40 is used.•Half-life = 1.28 billion years

http://www.youtube.com/watch?v=GfiNewvZA4I&feature=related

Warm-up •What is a half-life?

•What are the units for half-lives?

•What radioactive isotope of carbon is detected and used for radiocarbon dating?

•Why is this isotope radioactive but not the other carbon isotope?

Detecting Radioactivity

•Geiger counter: measures radiation in objects

•Dosimeter: measures radiation in people

•Film badges are also worn by people working with radiation to monitor exposure.•Why does this work for detecting radiation?

Uses for radioactive particles•Smoke detectors: emit alpha particles for smoke detection

•CAT SCAN: uses X-rays to analyze body

Uses for radioactive particles•MRI and NMR: detects body’s absorption of radio waves

•Detection and treatment of various medical conditions.

Nuclear Energy•Nuclear fusion and nuclear fission create large amounts of energy.

•Early on scientists believed this energy could be used as an energy source. The only problem was being able to control the reactions and energy produced.

Nuclear Fusion•Occurs when two small nuclei fuse (join) to form larger more stable nuclei.

•These reactions require very high temperatures.•Occur on the sun and in hydrogen bomb

•Hard to control energy produced from these reactions.

Nuclear Fusion

Nuclear Fission•Occurs when a heavy nucleus spits into two smaller nuclei.

•Easier to control energy produced from these reactions so this is used in nuclear power plants.

•Products have a large neutron-to-proton ratio (radioactive waste).

Nuclear Fission

Nuclear Power Plants

Limerick Nuclear Power Plant

Why are natural disasters such as earthquakes even more dangerous in areas with nuclear power plants?

Japan disaster

explosion

Ghost Town

Important Dates•Homework: page 15 in packet and 3-4 practice and review sheet

•Test next Monday (models of the atom, atoms, ions, isotopes, nuclear reactions, radioactive decay, nuclear fission/fusion)