Chemistry Regents Page 1 of 7Mr. Markic

Chapter 25 - Nuclear Chemistry

The study of ____________ ____________ and their uses in chemistry

Radiation -• The penetrating rays & particles emitted by a radioactive source

Radioactivity -• Process that occurs when nuclei change ____________Radioisotopes -• Atoms containing ____________ ____________

Recall… What are isotopes?o

How do nuclear reactions differ from chemical reactions?

Chemical Reactions• Atoms become stable by ____________or

____________electrons • Rate ____________changed by catalysts,

temperature, pressure, etc. • Mass & charge ____________

Nuclear Reactions• Nuclei become stable by undergoing changes

that ____________of energy • Rate ____________be changed• Charge conserved, mass ___________________

o A ____________small amount of mass is converted into energy

Nuclear Stability• Most nuclei are stable • The stability of a nucleus depends on its neutron to proton

____________• The stable nuclei are in the ‘________________________’

Radioactive Decay• Occurs in atoms with too _______or too _______neutrons • The nucleus will attempt to become more stable by releasing energy

through ______________• Transmutation -

o when the unstable nucleus of one element is ____________ into a stable nucleus of a different element

o Occurs naturally and artificially

Types of Radioactive Decayo Alpha particles, o Beta particles, o Positron emission o Gamma rays,

Chemistry Regents Page 2 of 7Mr. MarkicAlpha Decay – α• Gives off ____________ particles (helium nuclei) • Occurs when the neutron: proton ratio is low• A radioactive element gives off 2 protons and 2 neutrons

o The charge on the nucleus ________________________• Alpha particles do not travel far & are not very penetrating due to their large mass and charge

o Sheet of paper or surface of skin stops them

24094Pu → 236

92U + 42He (α)

o Atomic # - o # of protons -o Mass # -o # of neutrons -

Beta Decay – β• Gives off ____________ particles (electrons)• Occurs when the neutron: proton is high• A neutron breaks apart into a proton, which stays in the nucleus, and an electron which is released

o 10n → 1

1H + 0-1e (β)

o Charge on the nucleus ________________________• More penetrating than α particles

o Can pass through paper, but are stopped by aluminum foil or thin pieces of wood

22888Ra → 228

89Ac + 0-1e (β)

o Atomic # -o # of protons -o Mass # -o # of neutrons -

Positron Emission• Gives off a ____________ (positive electron)• Occur when the neutron: proton is low• A proton breaks apart into a neutron, and a unit of positive charge

o 11p → 1

0n + 0+1e (positron)

o Charge on the nucleus ________________________

2211Na → 22

10Ne + 0+1e

o Atomic # -o # of protons -o Mass # -o # of neutrons -

Chemistry Regents Page 3 of 7Mr. MarkicGamma Decay – γ• Gives off high energy photons called ________________________• _____________________________________________ • Very dangerous - extremely penetrating

o Pass easily through paper, wood, & the human bodyo Some can be stopped by several meters of concrete or several centimeters of lead

24094Pu → 240

94Puo Atomic # -o # of protons -o Mass # -o # of neutrons -o Nucleus changes only in its energy state

Summary of Radiation

Particle Mass Charge Symbol Penetrating Power

Alpha 4 amu 2+ 42He, α Low

Beta 0 amu 1- 0-1

e, β ModeratePositron 0 amu 1+ 0

+1e Moderate

GammaRay

0 amu None γ High

Nuclear Equations• Similar to chemical equations…• Mass and charge must balance on both sides

• 147N + 4

2He → 178O + 1

1H

• By using the concept of conservation of charge and mass number, you can identify a missing particle in an equation.

Sample Exercises1. What product is formed when radium-226 undergoes alpha decay?

2. What element undergoes alpha decay to form lead-208?

3. What product is formed when Mg-27 decays by beta emission?

Chemistry Regents Page 4 of 7Mr. Markic4. 3

1H _____ + 0-1e

5. 93Li 9

4Be + _____

6. 146C 0

-1e + _____

7. 24195Am 4

2He + _____

8. 167N 16

6O + _____

9. What forms when francium-220 decays?

10. What forms when potassium-37 decays?

11. What forms when potassium-42 decays?

12. 63Li + 1

0n 42He + _____

13. 2713Al + 4

2He _____ + 01e

14. 2714Si 0

-1e + _____

15. 21483Bi 4

2He + _____

16. 6629Cu 66

30Zn + _____

17. 23592U 90

38Sr + _____ + 10n + 40

-1e

Half-Life• Time required for ____________ of the nuclei of a radioactive sample to decay into products

• After each half-life, __________of the existing radioactive atoms have decayed into atoms of a new element

Chemistry Regents Page 5 of 7Mr. MarkicTable N• Each element has a characteristic half-life

• Anywhere from a fraction of a second to billions of years

• ½ life is ________________________

Uses for Half-Life• Dating• Nuclear medicine

Half-Life Calculations• No formula; make a ________________________ chart• Always start with time = ____________• If no mass is given, start with ________________________

Sample Problems1. Most chromium atoms are stable, but Cr-51 is an unstable isotope with a half-life of 28 days.

a. What fraction of a sample of Cr-51 will remain after 168 days?

b. If a sample of Cr-51 has an original mass of 52.0g, what mass will remain after 168 days?

2. How much was present originally in a sample of Cr-51 if 0.75mg remains after 168 days?

3. According to Reference Table N, how much of a 100. microgram (μg) sample of nitrogen-16 will remain after 28.52 seconds of decay?

4. In 5.49 seconds, 1.20g of Ar-35 decay to leave only 0.15g. What is the half-life of Ar-35?

5. Na-24 has a half-life of 15 hours. How much Na-24 will remain in an 18.0g sample after 60 hours?

Chemistry Regents Page 6 of 7Mr. Markic6. How many half-lives are required for a radioisotope to decay to 1/32 of its initial value?

7. What fraction of 226Ra will be left after 4797 years?

Artificial Transmutations• Particle Accelerators -

o Collision of a proton or α particle with a target nucleuso Uses magnetic or electrostatic fields to accelerate particles & overcome the repulsive forces

• Neutron Collisions -o Occurs when a neutron collides with a target nucleuso Used to prepare radioactive nuclei from stable nuclei

▫ 23892U + 1

0n → 23992U

▫ 5927Co + 1

0n → 6027Co

▫ 3216S + 1

0n → 3215P + 1

1H

Nuclear Fission• ____________ of the nucleus into smaller fragments

o Occurs when the nucleus is bombarded with ____________

o Uranium-235 and Plutonium-239 are the only fissionable isotopes

________________________________________________

• Releases huge amounts of ____________• Very exothermic

o 1 kg U-235 = 20,000 tons of dynamiteo Atomic bombs & nuclear reactors

Nuclear Reactors• Uses controlled fission• Energy from fission reaction heats the coolant• Heated coolant is used to produce steam• Steam turns a turbine• Turbine drives a generator to produce electricity

Chemistry Regents Page 7 of 7Mr. Markic

Nuclear Fusion• Light nuclei ____________ to produce a nucleus of greater mass• Solar fusion – H nuclei fuse to make He nuclei

________________________________________________

Fusion as an Energy Source• Produces ____________ energy than fission• Occurs only at high temperatures (over 40,000,000ºC) • More appealing than fission because:

o Availability/low cost of light isotopeso Products are generally not ________________________

Detecting Radiation• Radiation ____________ be seen, heard, felt, or smelled• Ionizing radiation -

o radiation with enough energy to knock electrons off atoms of the bombarded substance to produce ions

o Can be detected by Geiger counters, scintillation counters, and film badges

Using Radiation• Medicine -

o Diagnostic toolso Treatment for cancero Help to determine mechanisms for chemical

reactionso Trace movements of atoms in biological

systems

• Radiation in the body should:o Have a ____________ half-lifeo Be ____________ eliminated from the body

Other Uses of Radiation• Tracers -

o Any radioisotope used to follow the path of a substance

o Ex. Used in agriculture to test the effects of herbicides, pesticides, and fertilizers

• Dating -o Using half-lives, ‘age’ of objects can be

determinedo C-14 → C-12: o U-238 → Pb-206:

• Industrial Applications –o Used to measure the thickness or

strength of a material based on radioactive absorption

• Food Irradiation –o Kills insects, bacteria, & moldo Prevents the ‘sprouting’ of fruits &

vegetables