Nuclear Chemistry

Nuclear vs. Chemical Chemical reactions involve atoms binding through sharing or giving electrons. The formation of these bonds either absorb (endothermic) or release (exothermic) energy. Nuclear reactions involve changes in the mass of the nuclei of an atom. Enormous amounts of energy can be produced through these chain reactions. (E=mc2)

Nucleus of an Atom Made up of protons and neutrons Atomic number is the number of protons Atomic Mass Number is the number of protons and neutrons. Mass of proton = 1.6726 X 10-27 kg Mass of neutron = 1.6749 X 10-27 kg Protons and neutrons are made up of quarks and gluons.

Isotopes Atoms that contain varying number of neutrons are called isotopes. Isotopes all behave the same way since they all have the same number of protons and electrons. Isotopes are written with the atomic symbol and the mass. U-235 and U-238 are two isotopes of uranium (atomic number 92)

Stability of Isotopes Some isotopes are stable, while others are not. All stable isotopes have an atomic number of 82 or less. Unstable isotopes are termed radioisotopes. In radioisotopes, changes occur in the nucleus resulting in radioactive decay. Stability is related to the ratio of protons to neutrons.

Role of Protons and Neutrons Protons are positively charged, and thus, repel each other. Neutrons have 2 roles: They increase the distance between the protons in the nucleus. They exert strong forces on each other and the protons.

Radioactivity There are three basic types of radioactivity. Refered to as ionizing since they knock electrons loose from other atoms. Alpha particles  helium – 4 nuclei Can be stopped by a piece of paper Short and curved paths Beta particles  electrons Can be stopped by a sheet of aluminum Straight and thin paths Gamma radiation  electromagnetic radiation Short wavelengths and more energetic Can be stopped by a lead shield More penetrating than Xrays

Radioactive Decay Example of Alpha Decay: Example of Beta Decay:

Measuring Stability of Radioisotopes Nuclear decay is a random process. Studies have shown that over time, a percent of the nuclei will decay. The amount of time required for half of the radioisotope nuclei to decay is called its half-life. Final mass = initial mass X (1/2)(number of half -lives) The longer the half-life, the more stable the radioisotope is.

Mass Defect The measured mass of the nucleus of an isotope is not quite the same as the sum of the masses of its neutrons and protons. The energy involved in a nuclear reaction is based on the difference between the masses of the nuclei of reactants and products. The difference represents the binding energy that holds the nucleus together.

Nuclear Fission Consider the following reaction: The mass of U-235 and the mass of the one neutron is more than the sum of Ba-141, Kr-92, and 3 neutrons. The energy released will be quite large when many nuclei undergo fission. Nuclear Reactor

Nuclear Fusion Consider the following reaction: Reaction occurs in the Sun. Two nuclei fuse together in high temperatures, producing He-4, 1 neutron, and large amounts of energy. Again, the reactant’s masses are greater than the product’s masses.

Final Notes The energy released from a fission reaction is about a million times the amount of energy released in exothermic chemical reactions. The energy released from a fusion reaction is 3 to 4 times greater than this. Because high temperatures are required for a fusion reaction, a fission reaction is needed to create it. This resulted in the Hydrogen bomb.