Chemistry 140
Chapter 10
“Radioactivity and Nuclear Processes
”
The Composition and Structure of the Nucleus
In your study of atomic structure you investigated experiments which revealed the general composition of the atomic nucleus.
Recall that all nuclei except for ordinary hydrogen (protium) contain two fundamental particles-protons and neutrons.
Nuclei of protium contain only a proton. The protons and neutrons are referred to as
nucleons.
Nuclear Force The nucleons are packed tightly together. Because
of this close packing, positive charges repel each other.
An attractive force is thus required to hold the positively charged protons in the tiny nucleus.
The force that keeps the nucleons together is called the strong nuclear force. This force is the powerful short-range force of attraction between, respectively, proton and proton, proton and neutron, and neutron and neutron.
The strong nuclear force affects only the nucleons, and is distinct from the electrostatic force or gravitational force.
Mass Defect and Nuclear Binding Energy
On the atomic mass scale, the isotope of carbon with six protons and six neutrons in its nucleus is defined as having an atomic mass of exactly 12 u.
On this scale, a He nucleus has a mass of 4.0015 u. The mass of a proton is 1.0073u and the mass of a neutron is 1.0087 u.
A He nucleus contains two protons and two neutrons. You might expect its mass to be the combined mass of these four particles - 4.0320 u [2(l.0073 u) + 2(l.0087 u = 4.0320 u].
Note, however, that there is a difference of 0.0305 u between the measured mass (4.0015 u) and the calculated mass, 4.0320 u, of a He nucleus.
Mass Defect and Nuclear Binding Energy
This difference in mass is called the nuclear mass defect.
The nuclear mass defect is the difference between the mass of a nucleus and the sum of the masses of its constituent particles.
The mass defect is converted into energy units by using Einstein's equation, E = mc2.
This energy is generally referred to as the nuclear binding energy, which is the energy released when a nucleus is formed from its constituent particles.
This energy must be supplied to a nucleus to separate it into its constituent particles.
Naturally Occurring Radioactive Nuclides
Because of their radioactivity, radioactive nuclides have several unusual properties. Some of these properties are:
1. They affect the light-sensitive emulsion on a photographic film.
2. They produce an electric charge in the surrounding air.
3. They produce fluorescence with certain other compounds.
4. Their radiations have special physiological effects. 5. They undergo radioactive decay. Half-life is the length of time during which half of a given
number of atoms of a radioactive nuclide decays.
Nature of radiation The radiation given off by radioactive nuclides
can be separated into three different kinds of particles and rays.
1. The (alpha) particles are helium nuclei. Their mass is nearly four times that of a protium atom. They have a charge of +2 and move at speeds that are approximately one-tenth the speed of light. Because of their relatively low speed, they have low penetrating ability. A thin sheet of aluminum foil or a sheet of paper stops them. They burn flesh and ionize air easily, however.
Nuclear Symbol - 2
4He0
0
Nature of radiation 2. The (beta) particles are electrons. They
travel at speeds close to the speed of light, with penetrating ability about 100 times greater than that of alpha particles.
Nuclear Symbol
1
1H
-1
0e
Nature of radiation 3. The - (gamma) rays are high-energy
electromagnetic waves. They are the same kind of radiation as visible light, but are of much shorter wavelength and higher frequency. Gamma rays are produced when nuclear particles undergo transitions in nuclear-energy levels. They are the most penetrating of the radiations given off by radioactive nuclides. Alpha and beta particles are seldom, if ever, given off simultaneously from the same nucleus. Gamma rays, however, are often produced along with either alpha or beta particles.
Nuclear Symbol- 0
0
Nuclear Equations nuclear equation - an equation in which
only nuclei are represented Nuclear symbols must be used in nuclear
equations. You will need to know the symbol of each of the following particles.– alpha electron– beta proton– gamma neutron– positron deuteron
Sample Equations
Write the nuclear equation for each of the following reactions.– alpha decay of U-238– beta decay of Th-234– alpha decay of Rn-222– beta decay of Pa-234
Assignment: Page 704, 1-4
Types of Nuclear Reactions Because of the difference in stability of different nuclei, there are
four types of nuclear reactions. In each type, a small amount of the mass of the reactants is converted into energy, forming products of greater stability.
1. Radioactive decay refers to the emission of an alpha particle, a beta particle, or gamma radiation and the formation of a slightly lighter and more stable nucleus.
2. A nucleus is bombarded with alpha particles, protons, deuterons (deuterium nuclei), neutrons, or other particles. The unstable nucleus emits a proton or a neutron and becomes more stable in a process called nuclear disintegration.
3. Fission refers to the process in which a very heavy nucleus splits to form medium-weight nuclei.
4. Fusion refers to the process in which lightweight nuclei combine to form heavier, more stable nuclei
Artificial Radiation transuranium elements - elements with more
than 92 protons nuclear bombardment - striking the nucleus
with another particle at a very high speed When you see the word bombardment, this
means a particle must be added to the reactant side of a nuclear equation.
Nuclear Bombardment Equations
Write the equation for the bombardment of U-238 by a neutron resulting in the formation of Np-239 and the release of another particle.
Write the equation for the deuteron bombardment of Pu-239 resulting in beta emission and formation of another element.
