Radiation and Radioactivity Nuclear Change Radiation and Radioactivity

Change We have learned about We are now going to learn about Physical change – when atoms retain their identities and chemical compositions Chemical change – when atoms are rearranged forming new substances We are now going to learn about Nuclear change – when atoms form new atoms with different identities and properties. A change in the atom’s nucleus occurs!

Comparison of Chemical & Nuclear Chemical Reaction Nuclear Reaction Occur when bonds are broken and formed Involve only valence electrons Small energy changes Atoms keep the same identity Occur when nuclei combine, split, and emit radiation Can involve protons, neutrons, and electrons Large energy changes Atoms are converted into different elements

Check for understanding What type of change are the following? H2O (l)  H2O (s) 2H2 (g) + O2  2H2O (l) 21 H (g) + 21H (g)  42He (g) He (g)  He (l)

Discussion What do you think of when we talk about radioactivity? Can you give some examples of materials that are radioactive? What are some uses for radioactivity?

Common radioactive elements Uranium Plutonium Radon Radium Bismuth Carbon

Uses of Radioactivity Medicine Weaponry Sun Industry Power X-rays, PET scans, MRI Bombs Sterilizing bandages Sun Cancer treatment Industry Power Mass spectroscopy Nuclear power plants generate 1/5 of our electricity Carbon dating

Essential Questions Who discovered radiation or radioactivity? Why do we have radiation? What are the different types of radiation?

Who discovered of radioactivity? In 1895, Wilhelm Roentgen discovered the existence of X-rays, though the mechanism behind their production was not yet understood. He named the radioactive rays after the traditional unknown X.

Radioactivity In 1896, Henri Becquerel discovered that uranium salts emitted rays that resembled X-rays in their penetrating power.

Radioactivity Marie Curie was a French-Polish physicist and chemist, famous for her pioneering research on radioactivity. She coined the word, radioactivity. Marie continued work on uranium salts and in 1898, determined that the radiation was dependent on the quantity of the substance not an interaction with other molecules.

Radioactivity In 1899, Ernest Rutherford, a British scientist, began to classify radiation: alpha (a), beta (b), and gamma (g).

Why do we have radiation? For most stable atoms, the number of protons roughly equals the number of neutrons. But, if an isotope has too many more neutrons than protons, the nucleus becomes unstable. When this happens, the nucleus needs to shed some neutrons in the form of decay.

Remember - The Atom The atom consists of two parts: 1. The nucleus which contains: protons neutrons 2. Orbiting electrons.

Because of isotopes, there are many types of uranium: 235 92 U 238 92 Mass Number Number of protons Number of neutrons 235 Mass Number Number of protons Number of neutrons 238 92 92 143 146

Band of Stability Graph of the number of protons against the number of neutrons. Stable atoms have the ideal ratio of protons to neutrons Unstable will release radiation until they are stable

Isotopes Unstable isotopes can become stable by releasing radiation in the form of energy and different types of particles.

Types of Radioactive Decay Radioactive decay results in the emission of either: Alpha decay – When the nucleus spins off 2 protons and 2 neutrons Beta decay – When a neutron in the nucleus converts to a proton and electron and then keeps the additional proton while shooting off the electron. Gamma decay – accompanies the other two types of decay but does not change the mass or charge.

Types of Radioactive Decay Radiation Type Symbol(s) Charge

Radiation

Types of Radioactive decay Rutherford’s experiment http://www.youtube.com/watch?v=vuGvQjCOdr0 (1:44)

Alpha Decay When an alpha particle, containing two protons and two neutrons, is ejected from the nucleus. An alpha particle is identical to the nucleus of a helium atom The mass number decreases by 4 and the atomic number decreases by 2.

Alpha Decay Rn 222 86 He 4 2 Ra 226 88

Beta Decay Beta particle is a fast moving electron which is emitted from the nucleus of an atom undergoing radioactive decay. Beta decay occurs when a neutron changes into a proton and an electron.

Beta Decay As a result of beta decay, the nucleus has one less neutron, but one extra proton. The mass number stays the same and the atomic number increases by 1 and

Beta Decay b -1 At 218 85 Po 218 84

Gamma Decay Gamma rays are not charged particles like a and b particles. Gamma rays are electromagnetic radiation with high frequency. When atoms decay by emitting a or b particles to form a new atom, the nuclei of the new atom formed may still have too much energy to be completely stable. Gamma decay does not result in the creation of a new element!

Other Types of Nuclear Reactions positron – proton - neutron -

Nuclear Equations Because we know that mass is conserved in every chemical reaction, we know that the conservation of matter is maintained even in radioactive decay! So, can we use this information to write balanced nuclear equations?

X Y + He Ra Rn + He Alpha Decay A Z A - 4 Z - 2 4 2 226 88 222 86 4 2 unstable atom alpha particle New more stable element Ra 226 88 Rn 222 86 + He 4 2

Alpha Decay Rn 222 86 He 4 2 + Po 218 84 He 4 2 U 234 92 + Th 230 90

Alpha Decay He 4 2 + Pb 214 82 Po 218 84 He 4 2 + Ra 226 88 Th 230 90

Beta Decay X A Z Y Z + 1 + e -1 Po 218 84 Rn 85 + e -1

Beta Decay Bi 210 83 Po 84 + e -1 Th 234 90 Pa 91 + e -1

Beta Decay Pb 214 82 Bi 83 + e -1 Tl 210 81 Pb 82 + e -1

Check for understanding Work on problems on worksheet: Radioactive Decay Worksheet Then with a partner, use the cards to solve the decay series of uranium. Be sure to indicate the type of decay in each step.

Radioactive Decay Rates Calculating half lives

Radioactive decay Radioactive decay rates are measured in half-lives Half life – the time required for one half of the nuclei to decay into its products The decay continues until there is a negligible amount of the radioactive isotope remaining.

Calculating Half–lives Example An Isotope of cesium-137 has a half-life of 30 years. If 1.0 g of cesium-137 disintegrates over a period of 90 years, how many grams of cesium-137 would remain?

The solution Time (yrs) Mass (g) 1.0 30 0.5 60 0.25 90 0.125

Essential Question So you’ve talked about carbon-14 dating, how does it actually work?

Carbon Dating In the 1950s W.F. Libby and others (University of Chicago) devised a method of estimating the age of organic material based on the decay rate of carbon-14. Carbon-14 dating can be used on objects ranging from a few hundred years old to 50,000 years old.

How does Carbon-14 dating work? http://videos.howstuffworks.com/discovery/29401-assignment-discovery-carbon-dating-artifacts-video.htm C 14 6 N 7 + b -1

Decay Rate Formula N = N0(1/2)t/T Where: N = the remaining amount N0 = the initial amount t = elapsed time T = duration of the half life

Practice 1. The half life of cobalt-57 is 270 days. How much of a 5.000mg sample will remain after 810 days? N = ?? N0 = 5.000 mg t = 810 days T =270 days

Practice 2. Krypton-85 is used in indicator lights of appliances. The half-life of kypton-85 is 11 years. How much of a 2.000 mg sample remains after 33 years? N = ?? N0 = 2.000 mg t = 33 years T =11 years