NUCLEAR CHEMISTRY

Unit 15

Overview Nuclear Chemistry

IsotopesNuclear force

Radioactive decayAlpha, beta, gamma

decayPositron emissionElectron capture

Nuclear Stability

Radiometric DatingHalf-life

Nuclear fusion Nuclear fission Nuclear energy

Mass DefectNuclear binding

energy

Nuclear Chemistry Involves the change in the nucleus of an atom Nuclear reactions are everywhere

Produce sunlightCreate elements (synthetic and natural in stars)Radiation therapy (cancer treatment)Generate electricityNuclear weapons

World Energy Use

The Nucleus Remember – the nucleus is comprised of

the two nucleons (protons and neutrons) Atomic Number – number of protons Mass Number – number of protons and

neutrons together It is effectively the mass of the atom

Nuclear Symbols

C126

Mass number (p+ + no)

Atomic number(number of p+)

Element symbol

Isotopes

Not all atoms of the same element have the same mass due to different numbers of neutrons in those atoms

Example: There are three naturally occurring isotopes of uranium:○ Uranium-234○ Uranium-235○ Uranium-238

Nuclear Force

Strong nuclear forceHolds protons and

neutrons in nucleus very close together

Strongest force known

Nuclear Force Nucleus is not stable when atoms

experience certain ratios of protons to neutrons

Unstable atoms decay and emit radiationRadioactive decay

Elements with more than 83 protons (bismuth) are naturally radioactive

Radioactive Decay

Radionuclides: Radioactive elements During radioactive decay

The makeup of the nucleus changesThe number of protons may change

○ Means that the element has changed

Natural Radioactive Isotopes Radon-222

Comes from decomposition of Uranium rocks2nd leading cause of lung cancerComes up through cracks in basements

Radium-226Some radium salts glow in the darkEarly 1900s used to be used as paint for watches and clocks

(workers licked paint brushes and got cancer – “radium girls”)

Uranium-238Rocks create radon gasUsed in radioactive dating

Potassium-40One of few light radioactive elementsProduces argon that is found in atmosphere

Other Common RadioisotopesIsotope Use

14C Archaeological dating24Na Circulatory system testing for obstruction32P Cancer detection

51Cr Determination of blood volume

59Fe Measurements of red blood cell formation and lifetimes

60Co Cancer treatment131I Measurement of thyroid activity

153Gd Measurement of bone density226Ra Cancer treatment

3H Archaeological dating235U Nuclear reactors and weapons238U Archaeological dating

241Am Smoke detectors

Measuring Radioactivity One can use a device like this Geiger counter to

measure the amount of activity present in a radioactive sample.

The ionizing radiation creates ions, which conduct a current that is detected by the instrument.

Radioactive Decay(3 Most Common Types)

Alpha ( , a He)2 protons, 2 neutrons

Beta ( , b e)High energy electron

Gamma (g)Electromagnetic radiationHigh energy photons

Alpha, Beta, Gamma Radiation

Alpha Decay:

Loss of an -particle (a helium nucleus)

He or 42

U23892 Th

23490 He

42+

42

Beta Emission:

Loss of a -particle (a high energy electron)

0−1 e0

−1or

I13153 Xe

13154 + e

0−1

Gamma Emission: Loss of a -ray High-energy radiation that almost always

accompanies the loss of a nuclear particleNot usually written in nuclear equation

00

0023490

42

23892 ThHeU

Positron Emission:

Loss of a positron (a particle that has the same mass as but opposite charge of an electron)

C116 B

115 + e

01

01 e0

1or

Has a very short life because it is destroyed when it collides with an electron, producing gamma rays:

e + e 01

0-1

00

Positron Emission

A positron can convert a proton to a neutron

p11 n

10 + e

01

Electron Capture

Capture by the nucleus of an electron from the electron cloud surrounding the nucleusAddition of an electron to a proton in the nucleusAs a result, a proton is transformed into a neutron

p11 + e

0−1 n

10

Nuclear Stability

Several factors predict whether a particular nucleus is radioactiveNeutron-to-proton ratioRadioactive seriesMagic NumbersEvens and Odds

Neutron-Proton Ratios The strong nuclear force helps keep the

nucleus from flying apartProtons repel each otherNeutrons help the strength of the nuclear force

As protons increase, neutrons have to counter-act increasing proton-proton repulsionsIn low atomic number elements (1-20) protons and

neutrons are approximately equalIn high atomic number elements number of neutrons

much larger than protons Neutron-proton ratio helps stabilize nucleus

Neutron-Proton Ratios

For smaller nuclei (Atomic Number 20) stable nuclei have a neutron-to-proton ratio close to 1:1.

As nuclei get larger, it takes a greater number of neutrons to stabilize the nucleus.

Neutron-Proton Ratios

Stable Nuclei

The shaded region in the figure shows what nuclides would be stable, the so-called belt of stability.

Stable Nuclei Nuclei above this belt

have too many neutrons.

They tend to decay by emitting beta particles.

(If an isotopes mass number is greater than its atomic weight, the same trend will happen example C)16

6

Stable Nuclei Nuclei below the belt

have too many protons. They tend to become

more stable by positron emission or electron capture.

(If an isotopes mass number is less than its atomic weight, the same trend will happen example C)11

6

Stable Nuclei

There are no stable nuclei with an atomic number greater than 83.

These nuclei tend to decay by alpha emission.Decreases both protons and neutrons

Radioactive Series

Large radioactive nuclei cannot stabilize by undergoing only one nuclear transformation.

They undergo a series of decays until they form a stable nuclide (often a nuclide of lead).

Often occur in nature

Magic Numbers Nuclei with 2, 8, 20, 28, 50, or 82 protons

or 2, 8, 20, 28, 50, 82, or 126 neutrons tend to be more stable than nuclides with a different number of nucleons.These are called the “Magic Numbers”

Evens and Odds Nuclei with an even number of protons and

neutrons tend to be more stable than nuclides that have odd numbers of these nucleons.

Kinetics of Radioactive Decay

Radioactive decay is a 1st order process Remember this equation:

= t1/2 0.693

k

Radiometric Dating Half life can help determine the age of different

objects

Carbon-14Half life of 5,715 yearsCan determine age of organic materials up to about

50,000 years old

Radiometric Dating Uranium-238

Half life of 4.5×109 yearsUsed to determine age of Earth (measured rocks)

○ Oldest rock found is almost 4.5 billion years old

Nuclear Fusion

Elements can be man-made by bombarding nuclei with particlesAlpha particles accelerated and collided with

nucleusNeutrons bombard nucleus

Bombard nuclei to create transuranium elementsHeavy elements beyond uranium on

periodic table

Particle Accelerators Nuclear transformations can be induced by accelerating a

particle and colliding it with the nuclide These particle accelerators are enormous, having circular

tracks with radii that are miles long

Nuclear Fission The splitting of heavy nuclei

(Fusion is the combination of light nuclei) Process begins by bombarding heavy nucleus

with a neutron 2 main commercial uses

Nuclear WeaponryNuclear Energy

Nuclear Fission About 2 neutrons are produced for each fission

These 2 neutrons cause 2 additional fissions○ Which cause 2 more fissions each

Which cause 2 more fissions each…

This is called a chain reaction

Nuclear Fission

Chain reactions can escalate quicklyIf the reaction is not controlled, it results in a

violent explosion because of the release of too much energy too quickly

Nuclear Energy We can control fission reactions and use it to

create energy

Nuclear Energy Fission reactions are carried out

in nuclear reactorsThe reaction is kept in check by the use of

control rodsThese block the paths of some neutrons,

keeping the system from escalating out of control

The heat generated by the reaction is used to produce steam that turns a turbine connected to a generator

Video: http://www.youtube.com/watch?v=VJfIbBDR3e8

Debates on Nuclear Energy Pros…

Cleaner energy than coal and fossil fuel plants

Doesn’t add to global warming

High amount of electricity can be generated in one plant

Cheaper to run a nuclear facility than a fossil fuel plant

Cons…Nonrenewable source of

energyProduces nuclear waste

that must be stored for thousands of years

Accidents (Chernobyl, Three Mile Island, Fukushima)○ http://www.youtube.com/watch?v=

eGI7VymjSho

Very expensive to build a nuclear facility (about $10 billion per reactor)

Nuclear Energy

We can measure the energy

associated with nuclear reactions

E = mc2

E = energy (J)

m = change in mass (kg) during reaction (mass of products-mass of reactants)

c = speed of light (3.0×108 m/s)

When a system loses mass, it is exothermic (-E)

When a system gains mass, it is endothermic (+E)

Nuclear Energy The mass change in chemical reactions is so small

that we treat them as though mass is conservedEx: Mass change for exothermic process of combustion

of 1 mol of CH4 is -9.9×10-9 grams

Mass change in nuclear reactions is measureableEx: Mass change accompanying decay of 1 mol of

uranium-238 is 50,000 times greater than combustion of CH4

Nuclear Energy (example)

For example, the mass change for the decay of 1 mol of uranium-238 is −0.0046 g.

The change in energy, E, is then

E = (m) c2

E = (−4.6 10−6 kg)(3.00 108 m/s)2

E = −4.1 1011 J

Mass Defect When protons and neutrons form a nucleus, the mass

of the nucleus is less than the sum of the masses of its constituent protons and neutrons

Example: Helium (He) – 2 protons, 2 neutrons

Protons and Neutrons Mass of Nucleus

Mass of 2 protons (2×1.0073 = 2.0146) 4.0015 amu

Mass of 2 neutrons (2×1.0087 = 2.0174)

Total mass = 4.0320 amu

Difference = 4.0320 – 4.0015 = 0.0305 amu (mass defect)

Mass Defect

To measure the energy associated with the mass defect use

E = mc2

Example: Helium (He) – 2 protons, 2 neutrons

E = (5.1×10-29 kg)(3.0×108 m/s)2

E = 4.6×10-12 J

NOTE: 1 gram = 6.022×1023 amu

Nuclear Binding Energy

Energy required to separate a nucleus into its individual nucleons (protons and neutrons)

Also use E = mc2

The larger the binding energy, the more stable the nucleus toward decomposition