Nuclear Chemistry

Chapter 22 Notes

The Nucleus• Nucleons – the particles found

in the nucleus of an atom; protons and neutrons.

• In nuclear chemistry, the atoms are called nuclides. Each one is identified by its name and mass number.– Radium-288– Ra288

88

Mass Defect• For a helium atom, we have 2 protons, 2

neutrons and 2 electrons. – 2 protons: 2*1.007276 amu = 2.014552 amu– 2 neutrons: 2*1.008665 amu = 2.017330

amu– 2 electrons: 2*0.0005486 amu = 0.001097

amu

• The sum of these is 4.0032979 amu.

Mass Defect• We’d expect an atom of helium-4 to

have a mass of 4.032979 amu.• It actually has a mass of 4.00260

amu, which is 0.03038 amu less than expected.

• This difference is known as the mass defect.

• Where does this “lost” mass go? Isn’t mass a conserved quantity?

Nuclear Binding Energy

• We can use E=mc2 to calculate the amount of energy “held” by the mass that was lost.– Convert the mass to kilograms first

• m = 5.0446x10-29 kg

– E = (5.0446x10-29 kg)*(3.00x108 m/s)2

= 4.54x10-12 J

• The nucleus of each atom is held together by this nuclear binding energy.

Nucleons and Nuclear Stability

• What makes a nucleus stable?– It doesn’t undergo nuclear reactions.– It has enough neutrons to overcome the

repulsion of the protons.– It lies on the band of stability.

• Band of Stability: the stable nuclei cluster over a range of neutron-proton ratios

Band of Stability

Nuclear Reactions• Nuclear reaction – a

reaction that affects the nucleus of an atom

• Transmutation – a change in the identity of a nucleus as a result of a change in the number of its protons

Radioactive Decay• Radioactive decay –

the spontaneous disintegration of a nucleus into a slightly lighter nucleus, accompanied by emission of particles, electromagnetic radiation or both

Radioactive Decay• Nuclear radiation – particles or

electromagnetic emitted from the nucleus during radioactive decay

• Radioactive nuclide – an unstable nucleus that undergoes radioactive decay

Alpha Particles• Contain two protons and two neutrons• Very low penetrating power – stopped

by a piece of paper• Written as: αor Heor α 4

242

Beta Particles• High speed electrons• Medium penetrating

power – stopped by thin metal foil

• Released when a neutron decays into a proton and an electron

• Written asβor eor β 0

1-01-

Gamma Rays• High speed radiation

without mass or charge

• Strong penetrating power – partially stopped by thick lead and/or thick concrete

• Causes tissue damage

• Represented byγ

Nuclear Reaction Equations

• Shows the changes occurring within the nuclei

• The types of atoms will change, but two things must be constant:– The sum of the mass number– The sum of the atomic number

Nuclear Reaction Equations

HeThU 42

23490

23892

Types of Radioactive Decay

• Alpha Decay• Beta Decay• Positron Emission• Electron Capture

Alpha Decay• Releasing alpha particles – very

heavy nuclei getting rid of mass to increase stability

• Alpha decay of radium-226

226 88 Ra + 4

2 He222 86 Rn

Beta Decay• Elements above the band of stability

are unstable because they have too many neutrons– One neutron is converted into a proton

and an electron– The electron is released from the nucleus

• Beta decay of carbon-14

14 6 C +

0-1 β14

7 N

Positron Emission

15 8 O + 0

+1 β ?157 N? ?

• Elements below the band of stability are unstable because they have too many protons– One proton is converted into a neutron by releasing

a positron– A positron is a beta particle with a positive charge– The positron is released from the nucleus

• Positron emission of oxygen-15

Electron Capture• Another way of stabilizing a nucleus

with too many protons• Capturing an electron from the inner

orbitals– The electron combines with a proton to

make another neutron

• Electron capture of rubidium-81

8137 Rb + 0

-1 e 8136 Kr

Gamma Emission• Gamma rays are released when

nucleons drop to a lower energy level– Similar to the light released when

electrons drop to a lower energy level

• Released whenever another type of radioactive decay occurs

Half-Life• Half-life, t½: the time required for half

the atoms in a radioactive material to decay

• Half-life is an indication of the stability of a nuclide– Short half-life means very unstable

(microseconds)– Long half-life means very stable (billons

of years)

Half-Life Equation

2

1t

t

o 2

1mm

mass

Original mass

Time

Half-life

Example 1• The half-life of polonium-210 is 138.4

days. How many milligrams of polonium-210 remain after 415.2 days if you started with 2.0 mg of the isotope?

Example 1• The half-life of polonium-210 is 138.4

days. How many milligrams of polonium-210 remain after 415.2 days if you started with 2.0 mg of the isotope?

2

1t

t

o 2

1mm

Example 1• The half-life of polonium-210 is 138.4

days. How many milligrams of polonium-210 remain after 415.2 days if you started with 2.0 mg of the isotope?

days4.138

days2.415

2

1mg0.2m

Example 1• The half-life of polonium-210 is 138.4

days. How many milligrams of polonium-210 remain after 415.2 days if you started with 2.0 mg of the isotope?

mg25.0m

Example 2• The half-life of uranium-238 is

4.46x109 years. If 10.0 billion years ago a sample contained 4.00 grams of uranium-238, how much does it have today?

Example 2• The half-life of uranium-238 is 4.46x109

years. If 10.0 billion years ago a sample contained 4.00 grams of uranium-238, how much does it have today?

2

1t

t

o 2

1mm

Example 2• The half-life of uranium-238 is 4.46x109

years. If 10.0 billion years ago a sample contained 4.00 grams of uranium-238, how much does it have today?

years

years

gm9

10

1046.4

1000.1

2

100.4

Example 2• The half-life of uranium-238 is 4.46x109

years. If 10.0 billion years ago a sample contained 4.00 grams of uranium-238, how much does it have today?

g845.0m

Decay Series• Decay series – a series of radioactive

reactions that begins with a radioactive nuclide and ends with a stable one

• Parent nuclide – the heaviest particle in a decay series

• Daughter nuclides – all of the nuclides produced in a decay series

• Page 710 shows the decay of uranium-238 into lead-206

Artificial Transmutations

• Artificial transmutations – causing a nucleus to undergo a radioactive reaction– Hit the nucleus with high speed particles– The particles go into the nucleus,

causing it to become unstable– The unstable nucleus undergoes

radioactive decay to return to stability

Fermilab (Illinois)4 miles in diameter!

*photo from Google Map

Transuranium Elements

• Elements with atomic numbers higher than 92 (past uranium)– Very unstable – they decay within

microseconds– Only found in laboratories now– Were probably found in the very

beginning of the universe, but they all decayed a long, long time ago

Radiation Exposure• The energy in nuclear radiation can

be transferred to atoms in your cells, causing damage to your body.

• The amount of radiation is measured in units called rems

• Being exposed to a higher amount of radiation = more damage, therefore you’d be more likely to get radiation poisoning or cancer

Radiation Detection• Film badges – use exposure to

approximate radiation exposure• Geiger-Mϋller counters – count the

ions made by radiation to measure its strength

• Scintillation counters – measure the light made by radiation to measure its strength

Applications of Nuclear Radiation

• Radioactive dating – use the concentration of radioactive nuclides to approximate the age of an object.

• Radioactive tracers – use radioactive atoms in medical procedures to follow their pathway through the body

• In agriculture, radioactive atoms can be used to test the effectiveness of fertilizers.

Nuclear Waste• Nuclear waste – radioactive products

of nuclear reactions• It must be contained to protect

humans from the radiation– Storage: store on-site if the half-life is

short (ponds or dry casks)– Disposal: put the waste somewhere

isolated, with no intentions of ever touching it again (Yucca Mountain)

Nuclear Fission• Splitting of nucleus

into fragments• Involves heavy

atoms breaking down into lighter atoms

• Fat man and little boy were fission bombs

Nuclear fission of uranium-235

10 n 235

92 U

10 n

+

9236 Kr 141

56 Ba+ +

Notice change from heavy atom to lighter atoms

3

Fission Chain Reaction• Multiple fission reactions; initiated by

new neutrons released• LOTS OF ENERGY PRODUCED• Example: atomic bomb, nuclear

power plant

Nuclear Power Plant• Generates steam by

nuclear fission; steams drives turbines to produce electricity– Pros: less fuel, fewer

pollutants– Cons: radioactive waste,

more catastrophic plant accidents

Nuclear Fusion• Combining two or more small nuclei

into one stable nucleus

• Example: the Sun (or stars)

hydrogen

Helium +

ENERGYFUSION

Hydrogen Bomb