Chapter 28 Nuclear Chemistry

Radioactivity Chemical reactions occur in order for atoms to form stable electron configurations. Nuclear reactions occur because unstable isotopes are attempting to gain stability. Nuclear reactions are unaffected by: Temperature, Pressure or Catalysts They also cannot be: Sped up, Slowed down or Stopped

Radioactivity French chemist Henri Becquerel Famous associates: Accidental discovery in 1896 Famous associates: Marie Curie and Pierre Curie Discovered rays emitted by uranium ore. They called the process of giving off the rays radioactivity, and the rays themselves were called radiation. Nobel Prize 1903 for the discovery.

Radioactivity What makes something radioactive? Radiation comes from the unstable nucleus of a radioisotope. What makes an unstable nucleus? The proportion of neutrons to protons. Relative size of the nucleus.

Types of Radiation 3 types of radiation emitted: Alpha, Beta and Gamma Alpha radiation consists of a He nuclei being emitted from a radioactive source. Beta radiation consists of an electron that is released by a neutron. Gamma radiation is high-energy electromagnetic radiation given off during the break down of a radioisotope.

Alpha Radiation 23892U  23490Th + 42He A He nuclei is emitted from a radioisotope. 23892U  23490Th + 42He In general not very dangerous, alpha particles can be stopped by a thin sheet of paper or even your skin. Dangerous if ingested.

Alpha Radiation

Beta Radiation Created when a neutron decomposes into a proton and electron, the electron is the beta particle or when a proton decomposes into a positron and a neutron and the positron is the beta particle. 146C  147N + 0-1e (beta minus) 2211Na  2210Ne + 0+1e (beta plus) More dangerous than alpha radiation. But… can be stopped by aluminum foil or a thin piece of wood.

Beta Radiation

Gamma Radiation Often emitted with alpha or beta radiation. Extremely high energy. 23892U  23490Th + 42He + g 23490Th  23491Pa + 0-1e + g Gamma rays have no mass and no electrical charge and are extremely penetrating, need thick concrete or lead

Gamma Radiation

Stability Atoms with an atomic number of about 20 or below are stable: Neutron to proton ratio = 1 Atoms with atomic numbers greater than 20 are stable: Neutron to proton ratio = 1.2 - 1.5 Too many neutrons and you get beta emission. Too few neutrons and an electron is captured or a positron is released from a proton and converted to a neutron. Too much mass (Atomic # > 83) and you get alpha emission.

Half-Life Since every radioisotope decays, then every radioisotope has a half-life. The half-life is the amount of time that it takes half the nuclei of the radioisotope to decay.

Half-Life Nitrogen-13 emits beta radiation and decays to carbon-13 with a half-life of 10 min. Assume a starting mass of 2.00 grams of 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 Fission Fission is when a nucleus of an atom splits into 2 smaller atoms. This can be accomplished when the nuclei of unstable isotopes are bombarded with neutrons. Nuclear Bombs!

Nuclear Fission

James Chadwick(1891-1974)

Neutron Bombardment James Chadwick discovered the neutron in 1932 by bombarding 9Be with alpha particles Chadwick tried to fire neutrons at various heavy elements hoping the neutrons would penetrate inside the nuclei of the atoms However hard he fired the neutrons he could not manage to get them to get inside the nucleus Why? Particles were moving too fast.

Enrico Fermi (1901-1954) Element 100 (Fm) = Fermium

Neutron Bombardment In 1934 Enrico Fermi, an Italian physicist working in a secluded villa in Rome, solved the problem of why neutrons were not entering the nuclei of atoms The neutrons were going TOO FAST Using water from the fish pond outside the villa Fermi managed to slow a beam of neutrons sufficiently so that they could enter inside the nuclei of atoms …and thus change the nature of the nucleus - and history!

Neutron Bombardment Enrico Fermi tried to bombard Uranium with SLOW neutrons hoping to form a new element 23892U +10n  23993Np + 0-1e but 239Np did not form ! (it wasn’t detected) Analysis of the uranium after bombardment revealed the presence of small amounts of 92Kr and 141Ba isotopes What happened?

Lise Meitner Element 109 (Mt) = Meitnerium

Fission In 1939 Lise Meitner an Austrian chemist in Sweden also working on neutron bombardment experiments proposed that the Barium formed as a result of the neutrons splitting the Uranium atoms into two fragments. She proposed the term “fission” to describe this process.

Uranium Isotopes Naturally occurring Uranium contains two major isotopes Uranium-238 (99.3%) Uranium-235 (0.7%) As it turns out the only isotope of Uranium that undergoes fission is Uranium-235

235U Fission 23592U + 10n  23692U* and 10-14 seconds later... 23692U*  9236Kr + 14156Ba + 3 10n + ENERGY 50 possible sets of fission products (sum of atomic numbers = 92) 3 neutrons released for ONE 23592U each neutron can split another 23592U CHAIN REACTION POSSIBLE If amount of 23592U is sufficient (CRITICAL MASS) then the number of neutrons generated is high enough to result in a nuclear explosion )

Where does all this energy come from?

You might recognize this equation… E = mc2

E = mc2 Albert Einstein E = Energy (joules) m = mass (kg) c = speed of light c = 3 x 108 m/s

Nuclear Fusion Fusion occurs when nuclei combine to form a larger nucleus. In the sun 4 H nuclei and 2 Beta particles form a He nucleus. Many problems with fusion on Earth: Need to reach high temperatures How to contain the energy

Nuclear Fusion The Future…