Chapter 28Chapter 28Nuclear ChemistryNuclear Chemistry

RadioactivityRadioactivity Chemical reactions occur in order for Chemical reactions occur in order for

atoms to form stable electron atoms to form stable electron configurations.configurations.

Nuclear reactions occur because Nuclear reactions occur because unstable isotopes are attempting to unstable isotopes are attempting to gain stability.gain stability.

Nuclear reactions are unaffected by:Nuclear reactions are unaffected by:– Temperature, Pressure or Catalysts

They also cannot be:They also cannot be:– Sped up, Slowed down or Stopped

RadioactivityRadioactivity French chemist Henri BecquerelFrench chemist Henri Becquerel

– Accidental discovery in 1896 Famous associates:Famous associates:

– Marie Curie and Pierre Curie Discovered rays emitted by uranium Discovered rays emitted by uranium

ore.ore. They called the process of giving off They called the process of giving off

the rays radioactivity, and the rays the rays radioactivity, and the rays themselves were called radiation.themselves were called radiation.

Nobel Prize 1903 for the discovery.Nobel Prize 1903 for the discovery.

RadioactivityRadioactivity What makes something What makes something

radioactive?radioactive?– Radiation comes from the unstable

nucleus of a radioisotope. What makes an unstable What makes an unstable

nucleus?nucleus?– The proportion of neutrons to protons.

– Relative size of the nucleus.

Types of RadiationTypes of Radiation 3 types of radiation emitted:3 types of radiation emitted:

– Alpha, Beta and Gamma Alpha radiation consists of a He nuclei Alpha radiation consists of a He nuclei

being emitted from a radioactive source.being emitted from a radioactive source. Beta radiation consists of an electron Beta radiation consists of an electron

that is released by a neutron.that is released by a neutron. Gamma radiation is high-energy Gamma radiation is high-energy

electromagnetic radiation given off electromagnetic radiation given off during the break down of a radioisotope.during the break down of a radioisotope.

Alpha RadiationAlpha Radiation A He nuclei is emitted from a A He nuclei is emitted from a

radioisotope.radioisotope.

2382389292U U 234234

9090Th + Th + 4422He He

In general not very dangerous, In general not very dangerous, alpha particles can be stopped by alpha particles can be stopped by a thin sheet of paper or even your a thin sheet of paper or even your skin.skin.

Dangerous if ingested.Dangerous if ingested.

Alpha RadiationAlpha Radiation

Beta RadiationBeta Radiation Created when a neutron decomposes Created when a neutron decomposes

into a proton and electron, the into a proton and electron, the electron is the beta particle or when electron is the beta particle or when a proton decomposes into a positron a proton decomposes into a positron and a neutron and the positron is the and a neutron and the positron is the beta particle.beta particle.

141466C C 1414

77N + N + 00-1-1e (beta minus)e (beta minus)

22221111Na Na 2222

1010Ne + Ne + 00+1+1e (beta plus)e (beta plus)

More dangerous than alpha radiation.More dangerous than alpha radiation. But… can be stopped by aluminum But… can be stopped by aluminum

foil or a thin piece of wood.foil or a thin piece of wood.

Beta RadiationBeta Radiation

Gamma RadiationGamma Radiation Often emitted with alpha or beta Often emitted with alpha or beta

radiation.radiation. Extremely high energy.Extremely high energy.

2382389292U U 234234

9090Th + Th + 4422He + He +

2342349090Th Th 234234

9191Pa + Pa + 00-1-1e + e +

Gamma rays have no mass and no Gamma rays have no mass and no electrical charge and are extremely electrical charge and are extremely penetrating, need thick concrete or penetrating, need thick concrete or leadlead

Gamma RadiationGamma Radiation

StabilityStability Atoms with an atomic number of about 20 or Atoms with an atomic number of about 20 or

below are stable: Neutron to proton ratio = 1below are stable: Neutron to proton ratio = 1 Atoms with atomic numbers greater than 20 Atoms with atomic numbers greater than 20

are stable: Neutron to proton ratio = 1.2 - 1.5are stable: Neutron to proton ratio = 1.2 - 1.5 Too many neutrons and you get beta Too many neutrons and you get beta

emission.emission. Too few neutrons and an electron is captured Too few neutrons and an electron is captured

or a positron is released from a proton and or a positron is released from a proton and converted to a neutron.converted to a neutron.

Too much mass (Atomic # > 83) and you get Too much mass (Atomic # > 83) and you get alpha emission.alpha emission.

Half-LifeHalf-Life Since every Since every

radioisotope radioisotope decays, then decays, then every radioisotope every radioisotope has a half-life.has a half-life.

The half-life is the The half-life is the amount of time amount of time that it takes half that it takes half the nuclei of the the nuclei of the radioisotope to radioisotope to decay.decay.

Half-LifeHalf-Life Nitrogen-13 emits beta radiation Nitrogen-13 emits beta radiation

and decays to carbon-13 with a and decays to carbon-13 with a half-life of 10 min. Assume a half-life of 10 min. Assume a starting mass of 2.00 grams of starting mass of 2.00 grams of Nitrogen-13.Nitrogen-13.– A) How long is 3 half-lives? (30 min.)

– B) How many grams of the isotope will still be present at the end of three half-lives? (0.25g)

Nuclear FissionNuclear Fission Fission is when a nucleus of an Fission is when a nucleus of an

atom splits into 2 smaller atom splits into 2 smaller atoms. atoms.

This can be accomplished This can be accomplished when the nuclei of unstable when the nuclei of unstable isotopes are bombarded with isotopes are bombarded with neutrons.neutrons.

Nuclear Bombs!Nuclear Bombs!

Nuclear FissionNuclear Fission

James Chadwick(1891-1974)James Chadwick(1891-1974)

Neutron BombardmentNeutron Bombardment James Chadwick discovered the neutron James Chadwick discovered the neutron

in 1932 by bombarding in 1932 by bombarding 99Be with alpha Be with alpha particles particles

Chadwick tried to fire neutrons at various Chadwick tried to fire neutrons at various heavy elements hoping the neutrons heavy elements hoping the neutrons would penetrate inside the nuclei of the would penetrate inside the nuclei of the atomsatoms

However hard he fired the neutrons he However hard he fired the neutrons he could not manage to get them to get could not manage to get them to get inside the nucleusinside the nucleus

Why?Why? Particles were moving too fast.Particles were moving too fast.

Enrico Fermi (1901-1954)Enrico Fermi (1901-1954)

Element 100 (Fm) = Fermium

In 1934 Enrico Fermi, an Italian physicist In 1934 Enrico Fermi, an Italian physicist working in a secluded villa in Rome, working in a secluded villa in Rome, solved the problem of why neutrons were solved the problem of why neutrons were not entering the nuclei of atomsnot entering the nuclei of atoms

The neutrons were going TOO FASTThe neutrons were going TOO FAST Using water from the fish pond outside the Using water from the fish pond outside the

villa Fermi managed to slow a beam of villa Fermi managed to slow a beam of neutrons sufficiently so that they could neutrons sufficiently so that they could enter inside the nuclei of atomsenter inside the nuclei of atoms

……and thus change the nature of the and thus change the nature of the nucleus - and history!nucleus - and history!

Neutron BombardmentNeutron Bombardment

Neutron BombardmentNeutron Bombardment Enrico Fermi tried to bombard Enrico Fermi tried to bombard

Uranium with SLOW neutrons hoping Uranium with SLOW neutrons hoping to form a new elementto form a new element

2382389292U +U +11

00n n 2392399393Np + Np + 00

-1-1ee but but 239239Np did not form ! (it wasn’t Np did not form ! (it wasn’t

detected)detected) Analysis of the uranium after Analysis of the uranium after

bombardment revealed the presence bombardment revealed the presence of small amounts of of small amounts of 9292Kr and Kr and 141141Ba Ba isotopesisotopes

What happened?What happened?

Lise MeitnerLise Meitner

Element 109 (Mt) = Meitnerium

FissionFission In 1939 Lise Meitner an Austrian In 1939 Lise Meitner an Austrian

chemist in Sweden also working on chemist in Sweden also working on neutron bombardment experiments neutron bombardment experiments proposed that the Barium formed as proposed that the Barium formed as a result of the neutrons splitting the a result of the neutrons splitting the Uranium atoms into two fragments.Uranium atoms into two fragments.

She proposed the term “She proposed the term “fissionfission” to ” to describe this process.describe this process.

Uranium IsotopesUranium Isotopes Naturally occurring Uranium Naturally occurring Uranium

contains two major isotopescontains two major isotopes Uranium-238 (99.3%)Uranium-238 (99.3%) Uranium-235 (0.7%)Uranium-235 (0.7%) As it turns out the only isotope of As it turns out the only isotope of

Uranium that undergoes fission is Uranium that undergoes fission is Uranium-235Uranium-235

235235U FissionU Fission 235235

9292U + U + 1100n n 236236

9292UU**

and 10and 10-14-14 seconds later... seconds later... 236236

9292UU** 92923636Kr + Kr + 141141

5656Ba + Ba + 3 3 1100nn + + ENERGYENERGY

50 possible sets of fission products (sum of 50 possible sets of fission products (sum of atomic numbers = 92)atomic numbers = 92)

3 neutrons released for ONE 3 neutrons released for ONE 2352359292UU

each neutron can split another each neutron can split another 2352359292UU

CHAIN REACTIONCHAIN REACTION POSSIBLE POSSIBLE If amount of If amount of 235235

9292U is sufficient (U is sufficient (CRITICAL CRITICAL MASSMASS) then the number of neutrons generated ) then the number of neutrons generated is high enough to result in a is high enough to result in a nuclear nuclear explosionexplosion ) )

Where does all this energy come from?

You might recognize this You might recognize this equation…equation…

E = mc2

Albert EinsteinAlbert Einstein

E = mcE = mc22

E = Energy E = Energy (joules)(joules)

m = mass (kg)m = mass (kg)

c = speed of light c = speed of light

c = 3 x 10c = 3 x 1088 m/s m/s

Nuclear FusionNuclear Fusion Fusion occurs when nuclei combine Fusion occurs when nuclei combine

to form a larger nucleus.to form a larger nucleus. In the sun 4 H nuclei and 2 Beta In the sun 4 H nuclei and 2 Beta

particles form a He nucleus.particles form a He nucleus. Many problems with fusion on Many problems with fusion on

Earth:Earth:– Need to reach high temperatures– How to contain the energy

Nuclear FusionNuclear Fusion

The Future…The Future…