CHAPTER 25 Nuclear Chemistry

Key Terms Radioactivity- the process by which nuclei emit particles and rays Radiation- the penetrating rays and particles emitted by a radioactive source Radioisotope- an isotope that has an unstable nucleus and undergoes radioactive decay

History French chemists Antoine Henri Becquerel; Marie and Pierre Curie

Chemical Nuclear Atoms tend to attain stable electron configurations by losing or sharing electrons The nuclei of unstable isotopes gain stability by undergoing changes- which emit large amounts of energy Not affected by: temp, pressure, or catalysts Cannot be sped up, slowed down or turned off

Discovery of nuclear reactions Disproved Dalton’s assumption that atoms are indivisible.

Why does this happen? Unstable nuclei Bad proton: neutron ratio An unstable nucleus releases energy by emitting radiation during the process of radioactive decay.

When this happens Unstable radioisotopes of one element transferred into stable isotopes of another element.  TRANSMUTATION Radioactive decay is spontaneous  does not require any input of energy.

Types of Radiation Alpha (α) Beta (β) Gamma (γ)

Alpha Radiation helium nuclei emitted from a radioactive source 2 p + and 2 n o ; double positive charge 4 2 He OR α

Alpha Radiation- Equations  Atomic number decreases by 2  Mass number decreases by 4

Alpha Particles Do not travel far/not very penetrating because they are so large Stopped by a piece of paper or skin Dangerous when ingested

Beta Radiation A neutron breaks apart into a proton, which remains in the nucleus, and a fast moving electron, which is released.

Beta Radiation- Equations  Atomic number increases by 1  Mass number remains the same

Beta Particles (β) More penetrating- can pass through paper but are stopped by aluminum foil or thin pieces of wood.

Gamma Radiation (γ) A high energy photon Electromagnetic radiation (wave-like) Nuclei often emit gamma rays along with α or β particles during radioactive decay Does not change mass or atomic numbers

Gamma Radiation

Gamma Rays Very penetrating Stopped by lead shields

Electron Capture “inverse beta decay”  electron in an atom's inner shell drawn into the nucleus it combines with a proton, forming a neutron and a neutrino. The neutrino is ejected Atomic # , mass # doesn’t change

A POSITRON Is a particle with the mass of an electron but a positive charge During positron emission, a proton changes to a neutron and positron (which is emitted) Atomic # , mass # doesn’t change

Radiation Comparison

Modes of radioactive decay Alpha Decay (α)  2 protons, 2 neutrons 4 2 He nucleus Beta Particle (β - )  Electron emitted from the nucleus 0 -1 e Positron Particle (β + )  Mass of an electron but positive charge 0 +1 e Gamma Radiation (γ)  High energy radiation (higher than x-ray)  No mass and no charge

The symbols used in nuclear chemistry can be found on Reference Table O

Stability and Decay More than 1500 nuclei are known Only 264 are stable

All nuclei that have An atomic number >83 are radioactive- too many p + and n o

NOTE: If all the masses in a nuclear reaction were measured accurately enough, you would find that mass is not exactly conserved. An extremely small quantity of mass is converted into energy released in radioactive decay

Half-life (t 1/2 ) The time required for one-half of the nuclei of a radioisotope sample to decay to products The half-life of a radioactive nuclide cannot be changed

Uranium U-238 decays through a complex series of radioactive isotopes to the stable isotope Pb- 206 t 1/2 = 4.5 x 10 9 years- possible to date rocks as old as the solar system

Carbon Dating C-14: t 1/2 = 5,715 years

Exactly how much time must elapse before 16 grams of potassium-42 decays, leaving 2 grams of the original isotope? 1. 8 x 12.4 hours 2. 2 x 12.4 hours 3. 3 x 12.4 hours 4. 4 x 12.4 hours

Transmutation The conversion of an atom of one element to an atom of another element can occur by radioactive decay (natural) or when particles bombard the nucleus of an atom (artificial)

Elements with atomic numbers Greater than 92- transuranium elements None occur in nature Synthesized in nuclear reactors and nuclear accelerators

Nuclear Fission Fission- the splitting of a nucleus into smaller fragments U-235 and Po-239 are the only fissionable isotopes chain reaction Can release enormous amounts of energy:  1 kg U > explosion of 20,000 tons of dynamite

Nuclear Fission

Nuclear Reactors controlled fission=useful energy Neutron moderation- slows down neutrons; reactor fuel captures them to continue the chain reaction Neutron absorption- decreases the number of slow-moving neutrons Control rods- used to absorb neutrons

Nuclear Reactors

Nuclear Fusion nuclei combine to produce a nucleus of greater mass In solar fusion, hydrogen nuclei (protons) fuse to make helium nuclei Fusion reactions release much more energy than fission reactions Problems with achieving the high temperature necessary for reactions

Nuclear Fusion

There are benefits and risks associated with fission and fusion reactions Benefits to making electricity with nuclear fission  A small amount of fuel makes a large amount of electricity  Not dependent on foreign oil  Using fission instead of burning fossil fuels does not pollute the air  Cheap electricity

There are benefits and risks associated with fission and fusion reactions Risks to making electricity using nuclear fission  Exposure to radioactive material can cause cancer, mutations or death  Transportation and storage of fissionable material is dangerous  Nuclear accidents  Disposal of nuclear waste  Thermal pollution

Nuclear Waste Water cools the spent rods, and also acts as a radiation shield to reduce the radiation levels

Radiation in your life

Ionizing Radiation Is radiation with enough energy to knock electrons off some atoms of the bombarded substance to produce ions Radiation cannot be seen, heard, felt of smelled

Devices such as Geiger counters, scintillation counters and film badges are commonly used to detect radiation

Geiger Counters: gas-filled metal tube used to detect the presence of beta radiation Scintillation Counter: device that uses a coated phosphor surface to detect ionizing radiation

Radioisotopes can be used To diagnose medical problems, and in some cases, treat diseases  I-131: thyroid  Co-60: cancer Irradiated food  Gamma rays