Intro to Nuclear Chemistry

DECEMBER 12

http://www.chem.orst.edu/graduate/pics/Reactor.jpg

How does a nuclear reactor work?

http://www.lanl.gov/science/1663/images/reactor.jpg

How does a small mass contained in this bomb cause……

• Nuclear Bomb of 1945 known as “fat man”

http://www.travisairmuseum.org/assets/images/fatman.jpg

…this huge nuclear explosion?

http://library.thinkquest.org/06aug/01200/Graphics/705px-Nuclear_fireball.jpg

Is there radon in your basement?

http://a.abcnews.com/images/Blotter/abc_1radon_ad_070625_ssh.jpg

Notation

Nucleons

• Protons and Neutrons

• The nucleons are bound together by a strong force called binding force.

Isotopes

• Atoms of a given element with:

same #protons

but

different # neutrons

H H H

http://education.jlab.org/glossary/isotope.html

Isotopes of Carbon

• Isotopes of certain unstable elements that spontaneously emit particles and energy from the nucleus.

• Henri Beckerel 1896 accidentally observed radioactivity of uranium salts that were fogging photographic film.

• His associates were Marie and Pierre Curie.

Radioactive Isotopes

Marie Curie: born 1867, in Poland as Maria Sklodowska

• Lived in France

• 1898 discovered the elements polonium and radium.

http://www.radiochemistry.org/nuclearmedicine/pioneers/images/mariecurie.jpg

Marie Curie a Pioneer of Radioactivity

• Winner of 1903 Nobel Prize for Physics with Henri Becquerel and her husband, Pierre Curie.

• Winner of the sole 1911 Nobel Prize for Chemistry.

RADIOACTIVITY

• Emission of rays and particles from unstable nuclei.

• When a nucleus is emitting rays or particles it is said that is DECAYING or is disintegrating.

Stability of nuclei:

• Depend on the ratio between the neutrons and protons. Too many or too few neutrons lead to an unstable nucleus. All elements with more than 83 protons are unstable.

Transmutation

• When the nucleus of one element is changed into the nucleus of another element. IT CAN ONLY HAPPEN IN A NUCLEAR REACTION!!!

Nuclear Reactions• The chemical properties of the nucleus are

independent of the state of chemical combination of the atom.

• In writing nuclear equations we are not concerned with the chemical form of the atom in which the nucleus resides.

• It makes no difference if the atom is as an element or a compound.

• Mass and charges MUST BE BALANCED!!!

Types ofRadioactive Decay

SeparationAlphaBetaGamma.MOV Separation of Radiation

Emission of alpha particles :

• helium nuclei • two protons and two neutrons • charge +2e  • can travel a few inches through air• can be stopped by a sheet of

paper, clothing.

Alpha Decay

Alpha Decay

Uranium Thorium

Alpha Decay

http://education.jlab.org/glossary/alphadecay.gif

Alpha Decay:

Loss of an -particle (a helium nucleus)

He42

U23892 Th

23490 He

42+

Alpha Decay

• Mass changes by 4

• The remaining fragment has 2 less protons

• Alpha radiation is the less penetrating of all the nuclear radiation (it is the most massive one!)

Alpha decay:

• When a nucleus emits alpha particles.

• * Atomic number decreases by 2.

• * Mass number decreases by 4.

• * Neutrons decrease by 2.

Beta Decay:

Loss of a -particle (a high energy electron)

0

−1 e0

−1or

I13153 Xe

13154 + e

0−1

Beta Decay

• Beta particles : electrons ejected from the nucleus when neutrons decay

( n -> p+ +- )

• Beta particles have the same charge and mass as "normal" electrons.

Beta Decay

• Beta particles : electrons ejected from the nucleus when neutrons decay

n -> p+ +-

• Beta particles have the same charge and

mass as "normal" electrons.

• Can be stopped by aluminum foil or a block of wood.

Beta Decay

• When a neutron becomes a proton and emits an electron.

• * Atomic Number or number of protons increases by 1

• * Number of neutrons decreases by one.

• * Mass number remains the same.

 

Beta Decay

Beta Decay

Thorium Protactinium

Beta Decay

• Involves the conversion of a neutron in the nucleus into a proton and an electron.

• Beta radiation has high energies, can travel up to 300 cm in air.

• Can penetrate the skin

Beta decay

• Write the reaction of decay for C-14

Positron Emission

• When a proton changes to a neutron emits a positron.

• *Atomic number (number of protons)decreases by 1

• *Number of neutrons increase by 1.

• *Mass number remains same

Gamma Emission:

Loss of a -ray (high-energy radiation that almost always accompanies the loss of a nuclear particle)

00

• Gamma radiation electromagnetic energy that is released. 

• Gamma rays are electromagnetic waves.

• They have no mass.• Gamma radiation has no charge.

– Most Penetrating, can be stopped by 1m thick concrete or a several cm thick sheet of lead.

Gamma Decay

3 Main Types of Radioactive Decay

• Alpha

• Beta

• Gamma

Examples of Radioactive DecayAlpha Decay

Po Pb + He

Beta Decay p n + e

n p + e

C N + e

Gamma Decay

Ni Ni + (excited nucleus)

Which is more penetrating? Why?

December 14

• Nuclear stability –

• Half life

• HW review book

• Question 34 to 47

Nuclear Stability

• Depends on the neutron to proton ratio.

Band of Stability

Number of Neutrons, (N)

Number of Protons (Z)

What happens to an unstable nucleus?

• They will undergo decay

• The type of decay depends on the reason for the instability

What type of decay will happen if the nucleus contains too many

neutrons?

• Beta Decay

Example:

C N + e

In N-14 the ratio of neutrons to protons is 1:1

14

7 -1

014

6

• Nuclei with atomic number > 83 are radioactive

Radioactive Half-Life (t1/2 ):

The time required for one half of the nuclei in a given sample to decay.

• After each half life the mass of sample remaining is half.

• Different Isotopes have different half lives. Use table N

Common Radioactive Isotopes

Isotope Half-Life Radiation Emitted

Carbon-14 5,730 years

Radon-222 3.8 days

Uranium-235 7.0 x 108 years

Uranium-238 4.46 x 109 years

Radioactive Half-Life

• After one half life there is 1/2 of original sample left.

• After two half-lives, there will be

1/2 of the 1/2 = 1/4 the original sample.

Graph of Amount of Remaining Nuclei vs Time

A=Aoe-t

A

Example

You have 100 g of radioactive C-14. The half-life of C-14 is 5730 years.

• How many grams are left after one half-life? Answer:50 g

• How many grams are left after two half-lives?

Problem

If 80 g of a radioactive sample decays to 10 g in 30 min what is the element’s half life?

• How many days will take a sample of I-131 to undergo three half life periods?

• What is the total mass of Rn-222 remaining in an original mass 160 mg sample of Rn-222 after 19.1 days?

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.

Transmutations

• To change one element into another.

• Only possible in nuclear reactions never in a chemical reaction.

• In order to modify the nucleus huge amount of energy are involved.

• These reactions are carried in particle accelerators or in nuclear reactors

Nuclear transmutations

• Alpha particles have to move very fast to overcame electrostatic repulsions between them and the nucleus.

• Particle accelerators or smashers are used. They use magnetic fields to accelerate the particles.

Particle Accelerators(only for charged particles!)

These particle accelerators are enormous, having circular tracks with radii that are miles long.

Cyclotron

Nuclear transformations can be induced by accelerating a particle and colliding it with the nuclide.

Neutrons

• Can not be accelerated. They do not need it either (no charge!).

• Neutrons are products of natural decay, natural radioactive materials or are expelled of an artificial transmutation.

• Some neutron capture reactions are carried out in nuclear reactors where nuclei can be bombarded with neutrons.

Mass defect• The mass of the nucleus is always

smaller than the masses of the individual particles added up.

• The difference is the mass defect.

• That small amount translate to huge amounts of energy E = (m) c2

• That energy is the Binding energy of the nucleus, and is the energy needed to separate the nucleus.

Energy in Nuclear Reactions

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 This amount is 50,000 times

greater than the combustion of 1 mol of CH4

Types of nuclear reactionsfission and fusion

• The larger the binding energies, the more stable the nucleus is toward decomposition.

• Heavy nuclei gain stability (and give off energy) if they are fragmented into smaller nuclei. (FISSION)

• Even greater amounts of energy are released if very light nuclei are combined or fused together. (FUSION)

Nuclear Fission

• Nuclear fission is the type of reaction carried out in nuclear reactors.

Nuclear fission:

A large nucleus splits into several small nuclei when impacted by a neutron, and energy is released in this process

Nuclear Fission

• Bombardment of the radioactive nuclide with a neutron starts the process.

• Neutrons released in the transmutation strike other nuclei, causing their decay and the production of more neutrons.

Nuclear Fission

This process continues in what we call a nuclear chain reaction.

Controlled vs Uncontrolled nuclear reaction

• Controlled reactions: inside a nuclear power plant

• Uncontrolled reaction: nuclear bomb

Nuclear Reactors

In nuclear reactors the heat generated by the reaction is used to produce steam that turns a turbine connected to a generator.

Nuclear Reactors• The reaction is kept in

check by the use of control rods.

• These block the paths of some neutrons, keeping the system from reaching a dangerous supercritical mass.

FUSION

• Combining small nucleii to form a larger one.

• Require millions of K of temperature

Fusion

• 1H + 1H 2H + 1e + energy

• 1H + 2H 3He + energy

• 3He + 3He 4He + 21H + energy

• Reaction that occurs in the sun

• Temperature 107 K

• Heavier elements are synthesized in hotter stars 108 K using Carbon as fuel

Nuclear Fusion

• Fusion would be a superior method of generating power.– The good news is that the

products of the reaction are not radioactive.

– The bad news is that in order to achieve fusion, the material must be in the plasma state at several million kelvins.

Nuclear Fusion(thermonuclear reactions)

• Tokamak apparati like the one shown at the right show promise for carrying out these reactions.

• They use magnetic fields to heat the material.

• 3 million K degrees were reached inside but is not enough to begin fusion which requires 40 million K

Fission is the release of energy by splitting heavy nuclei such as Uranium-235 and Plutonium-239

Fusion is the release of energy by combining two light nuclei such as deuterium and tritium

How does a nuclear plant work?• Each fission releases 2 or 3

neutrons• These neutrons are slowed down

with a moderator to initiate more fission events

• Control rods absorb neutrons to keep the chain reaction in check

• The goal of fusion research is to confine fusion ions at high enough temperatures and pressures, and for a long enough time to fuse

• This graph shows the exponential rate of progress over the decadesControlled Fission Chain Reaction

Confinement Progress

• Magnetic Confinement uses strong magnetic fields to confine the plasma

• This is a cross-section of the proposed International Thermo-nuclear Experimental Reactor (ITER)

• Inertial Confinement uses powerful lasers or ion beams to compress a pellet of fusion fuel to the right temperatures and pressures

• This is a schematic of the National Ignition Facility (NIF) being built at Lawrence Livermore National Lab

Nuclear Power Plant

There are two main confinement approaches:The energy from the reaction drives a steam cycle to produce electricity

Nuclear Power produces no greenhouse gas emissions; each year U.S. nuclear plants prevent atmospheric emissions totaling:•5.1 million tons of sulfur dioxide•2.4 million tons of nitrogen oxide•164 million tons of carbon

Nuclear power in 1999 was the cheapest source of electricity costing 1.83 c/kWh compared to 2.04 c/kWh from coal

D

T

D-T Fusion4He3.52 MeV

Neutron14.1 MeV

Uses of radioisotopes

• Medicine• Medical imaging –

trace amounts of short half life isotopes can be ingested and the path of the isotope traced by the radiation given off

• cancer treatment – radiation kills cancerous cells more easily than healthy cells

• Sterilisation – γ – rays can be used to kill germs and hence sterilise food and plastic equipment

• Industry – used to trace blockages in pipes, or to test the thickness of materials (by putting a source on one side of the material and detector on the other)

• Carbon dating• Once a living organism dies, it is no

longer taking in any Carbon.• C14 is radioactive, and decays over time.• By measuring the activity of C14 in an

object and comparing it with the amount of C14 which was present initially you can estimate when the organism died

Smoke detectors

• a radioactive source ionises the air between two electrodes. Thus current flows between them

• If smoke particles enter this space they stick to the ions and the current is reduced.

• This reduced current triggers the alarm