Nuclear Chemistry

Structure and Stability of Nuclei, Fission, Fusion,

and Radiation

Standards11. Nuclear processes are those in which an atomic nucleus changes, including radioactive decay of

naturally occurring and human-made isotopes, nuclear fission, and nuclear fusion. As a basis for understanding this concept:

a. Students know protons and neutrons in the nucleus are held together by nuclear forces that overcome the electromagnetic repulsion between the protons.

11. b. Students know the energy release per gram of material is much larger in nuclear fusion or fission reactions than in chemical reactions. The change in mass (calculated by E = mc2 ) is small but significant in nuclear reactions.

11. c. Students know some naturally occurring isotopes of elements are radioactive, as are isotopes formed in nuclear reactions.

11. d. Students know the three most common forms of radioactive decay (alpha, beta, and gamma) and know how the nucleus changes in each type of decay.

11. e. Students know alpha, beta, and gamma radiation produce different amounts and kinds of damage in matter and have different penetrations.

11. f.* Students know how to calculate the amount of a radioactive substance remaining after an integral number of half-lives have passed.

11. g.* Students know protons and neutrons have substructures and consist of particles called quarks.

Nuclear Chemistry

has to do with an atom’s nucleus

plural of nucleus = nuclei

What’s a nucleus? Let’s reviewAn atom’s nucleus contains almost all of an atom’s mass, but takes up very little of it’s volume.

Subatomic Particlesthe particles that make up an atom

• Protons – high mass, positive charge. Found in nucleus.

• Neutrons – high mass, no charge. Found in nucleus.

• Electrons – low mass, negative charge. Found orbiting around nucleus. (abbreviated e– )

Comparison of Masses

Proton Neutron

Electron

An AtomSize of atom

Size of nucleus

2 protons = He = helium

Basic Electrical Charge Laws

+ and – : Attract(pull

together)

– and – : Repel(push away)

+ and + : Repel(push

away) Like charges repel and Opposites attract

So why don’t the protons in the nucleus fly apart?

nuclear force

electrostatic force

is stronger over short distances

is stronger over long distances

Key11NaSodium22.99

Atomic Number•Number of Protons•Number of Electrons (when atom is neutrally charged)

•Property unique to each element

Key

Average atomic mass*•Weighted Average number of Protons and Neutrons (approximately)

NaSodium22.99

11

IsotopesWhen atoms have the same number of protons, but different numbers of neutrons, they are called isotopes.

Examples: Carbon-14 has 6 protons and 8 neutrons.Carbon-12 has 6 protons and 6 neutrons.

Carbon-14 is found more often in living organisms than in non-living matter. It also undergoes radioactive decay – which is why it is used for fossil dating.

Isotopes

More Examples: Uranium-238 has 92 protons and 146 neutrons.Uranium-235 has 92 protons and 143 neutrons.

Uranium-235 is more rare in nature than Uranium-238, but it also undergoes nuclear chain reactions more easily, which is why small amounts of Uranium-235 are used in nuclear weapons and nuclear power plants.

Some isotopes are more stable than others.

Which isotope is more abundant?Neon-20Neon-22

How many neutrons in each? Neon-20Neon-22 22 – 10 = 12 neutrons

20 – 10 = 10 neutrons

Which isotope is more abundant?Bromine-79Bromine-80

How many neutrons in each? Bromine-79Bromine-80

Lead has two isotopes with the following relative amounts: 80% Lead-207 20% Lead-208What would the average atomic mass calculate to be?

0.8(207) + 0.2(208) = 207.2

Chlorine has two stable isotopes:Chlorine-35Chlorine-37

What are the relative abundances (in %) of each isotope? x(35) + (1-x)(37) = 35.4535x – 37x + 37 = 35.45-2x + 37 = 35.45

-2x = -1.55

-2x = -1.55

x = 0.775

??%100 –??%

–37 –37

-2 -2

x1 – x

Chlorine has two stable isotopes:Chlorine-35Chlorine-37

What are the relative abundances (in %) of each isotope? x(35) + (1-x)(37) = 35.4535x – 37x + 37 = 35.45-2x + 37 = 35.45

-2x = -1.55

-2x = -1.55

x = 0.775

77.5%22.5%

–37 –37

-2 -2

0.7750.225

So why don’t the protons in the nucleus fly apart?

nuclear force

electrostatic force

is stronger over short distances

is stronger over long distances

Sometimes these forces are overcome. Fission – a nucleus breaks apart.(ex. atomic bomb, nuclear power plants, radioactive decay)

Fusion – a nucleus merges with another.(ex. the sun, hydrogen bomb, experimental fusion reactors)

Both fission and fusion release radiation.

It is called radiation because it radiates out.

Common Types of Radiation

α (alpha): helium nucleus at high speed. β (beta): electron at high speed.

γ (gamma): high energy photon.

He42

e0-1

γ00ha

rder

to b

lock

Symbol

Other Types of Radiation

neutron at high speed.

Proton (or hydrogen nucleus) at high speed.

β+ positron at high speed (the anti-matter version of an electron).

n10

p11

β+01

Symbol

Na2311 Na22

11

atomic mass

number of protons

We can use conservation of mass and charge to figure out nuclear reactions just like chemical reactions.

e0-1Th234

90 +

He42U238

92 + ???

???

We can use conservation of mass and charge to figure out nuclear reactions just like chemical reactions.

e0-1Th234

90 +

He42U238

92 +

???

Th23490

We can use conservation of mass and charge to figure out nuclear reactions just like chemical reactions.

e0-1Th234

90 +

He42U238

92 + Th23490

Pa23491

Half-Life• Half-life – how much time has passed

when half of the original amount remains.

• How much of the original remains after two half-lives?

• How much after three half-lives?

• How much after four half-lives?

or 50 %12

or 25 %14

or 12.5 %18

or 6.25 % 116

Table of Half-LivesIsotope Name Symbol Half-LifeUranium-238 238

92U 4.51 × 109 yearsUranium-235 235

92U 7.1 × 108 yearsCarbon-14 14

6C 5,730 yearsStrontium-90 90

38Sr 28 yearsCobalt-60 60

27Co 5.27 years

Table of Half-LivesIsotope Name Symbol Half-LifeThorium-234 234

90Th 24.1 daysProtactinium-234 234

91Pa 6.75 hoursPolonium-218 218

84Po 3.08 minutesRadon-219 219

86Rn 4.00 secondsPolonium-214 214

84Po 1.6 × 10-4 seconds

As a archaeologist you find a dead rat that contains 0.0009 grams of Carbon-14. A rat that died a year ago has 0.01 grams of Carbon-14. How long ago did the rat die? t½ = 5,730 years0.0009 g0.01 g

= 111.1

12

116

132

164

14

18

1 H.L. 2 H.L. 3 H.L. 4 H.L. 5 H.L. 6 H.L.

As a archaeologist you find a dead rat that contains 0.0009 grams of Carbon-14. A rat that died a year ago has 0.01 grams of Carbon-14. How long ago did the rat die? t½ = 5,730 years

111.1

116

18

3 H.L. 4 H.L.

3(5,730) = 4(5,730) =

17,190 years 22,920 years

17,190 – 22,920 years ago

Strontium-90 is one of the “fallout” products from atmospheric testing of nuclear weapons. If there were 50,000 grams of Strontium-90 in the U.S. southwest region when the 1963 testing ban began, how much Strontium-90 currently remains? t½ = 28 years2013 – 1963 =

12

116

132

164

14

18

1 H.L. 2 H.L. 3 H.L. 4 H.L. 5 H.L. 6 H.L.

50 years 50 yr28 yr = 1.8 half-

lives

Radioactive Decay Formulas

A = A0 e-( )tln (2)

t½1

2 t½ = A0

A

ln (2)

ln ( ) t

3 t =A0

Aln ( ) ln (2)

t½

A0 = original amountA = current amountt = current timet½ = half-life

Energy-Mass Equivalence

Sometimes in nuclear change a small amount of the mass disappears. It has been converted to a large amount of energy according to the formula: E = mc2

speed of light: c = 3 × 108 m/s

energy = (mass) × (speed of light)2

J = kg × (m/s)2

Fission Nuclear Reaction

4 e– in valence shell

Which isotope is more abundant?Lead-207Lead-208

How many neutrons in each? Lead-207Lead-208