Chemistry

NUCLEAR CHEMISTRY

The Nucleus
A. Introduction
1. Atomic nuclei are made of protons and neutrons which are collectively called nucleons.
2. In nuclear chemistry, an atom is referred to as a nuclide, and is identified by the number of protons and neutrons in its nucleus.
3. Nuclides can be represented in two ways.

B. Mass Defect and Nuclear Stability
1. The measured mass of an atom is less than the calculated mass.
2. The mass defect is the difference between the mass of an atom and the sum of the masses of its protons, neutrons and electrons.
3. The mass defect is caused by the conversion of mass to energy upon formation of the nucleus

C. Nucleons and Nuclear Stability
1. The stable nuclei cluster over a range of neutron-proton ratios is referred to as the band of stability
a. For atoms having low atomic numbers, the most stable nuclei are those with a neutron-proton ratio of approximately 1:1.
b. At higher atomic numbers, the stable neutron-proton ratio increases to about 1.5:1. More neutrons are required to increase teh nuclear force and stabilize the nucleus
c. Beyond the atomic number 83, bismuth, the repulsive force of the protons is so great that no stable nuclides exist.
2. Stable nuclei tend to have even numbers of nucleons. This indicates that the stability of a nucleus is greatest when nucleons, like electrons are paired.

D. Nuclear Reactions
1. Unstable nuclei undergo spontaneous changes that change their number of protons and neutrons. In this process they give off large amounts of energy and increase their stability.
2. A nuclear reaction is a reaction that affects the nucleus of an atom. In equations representing nuclear reactions, the total of the atomic number must be equal on both sides of the equation.
3. When the atomic number changes, the identity of teh element changes.
4. A transmutation is a change in the identity of a nucleus as a result of a change in teh number of its protons.

E. Radioactive Decay
1. Radioactive decay is the spontaneous disintegration of a nucleus into a slightly lighter nucleus, accompanied by emission of particles, electromagnetic radiation, or both.
2. Alpha emission is restricted almost entirely to very heavy nuclei, which reduces its penetrating power (more mass=>less penetrating power).
3. Elements above the band of stability are unstable because they have too many neutrons. To decrease the number of neutrons, a neutron can be converted into a proton and an electron.
4. A beta particle is an electron emitted from the nucleus during some kinds of radioactive decay.
5. The atomic number increases by 1 and the mass number stayas the same.
6. Gamma rays are high electromagnetic waves emitted from a nucleus as it changes from an excited state to a ground energy state
7. According to the nuclear shell model, gamma rays are produced when nuclear particles undergo transitions in nuclear-energy levels. Gamma emission usually occurs immediately following other types of decay, which leave the nucleus in an excited state.

F. Half-Life
1. No two radioactive isotopes decay at the same rate.
2. Half-life is the time required for half the atoms of a radioactive nuclide to decay.
a. More stable nuclides decay slowly and have longer half-lives.
b. Less stable nuclides decay more quickly and have shorter half-lives.

G. Nuclear Radiation
1. Different types of nuclear radiation have different penetrating abilities. Nuclear radiation includes alpha particles, beta particles, and gamma rays.
a. Alpha particles have a range of only a few centimeters in air and have a low penetrating ability due to their large mass and charge. They cannot penetrate skin. However, they can cause damage if ingested or inhaled.
b. Beta particles travel at speeds close to the speed of light and have a penetrating ability about 100 times greater than that of alpha particles. They have a range of a few meters in the air.
c. Gamma rays have the greatest penetrating ability. Protection from gamma rays requires shielding with thick layers of lead or concrete, or both.

H. Nuclear Fission and Nuclear Fusion
1. In nuclear fission, a very heavy nucleus splits into more stable nuclei of intermediate mass. This process releases enormous amounts of energy.
a. The mass of teh products is less tahn the mass of the reactants. The missing mass is converted to energy.
2. A chain reaction is a reactio nin which the material that starts the reaction is also one of the products and can start another reaction.
a. The minimum amount of nuclide that provides the number of neutrons needed to sustain a chain reaction is called the critical mass.
3. Nuclear reactors are devices that use controlled fission chain reactions to produce energy or radioactive nuclides.
4. In nuclear fusion, light mass nuclei combine to form a heavier, more stable nucleus.
a. Nuclear fusion releases even more energy per gram of fuel than nuclear fission.
5. Uncontrolled fusion reactions of hydrogen are the source of energy for the hydrogen bomb. A fission reaction is used to provid eht heat and pressure necessary to trigger the fusion of nuclei.