Nuclear Chemistry. Neutron-Proton Ratios. Radioactive Decay ATOMIC COMPOSITION

Transcription

1 Nuclear Chemistry ATOMIC COMPOSITION Protons + electrical charge mass =.676 x - g relative mass =.7 atomic mass units (amu) Electrons negative electrical charge relative mass =.5 amu Neutrons no electrical charge mass =.9 amu The Nucleus Isotopes Remember that the nucleus is comprised of the two nucleons, protons and neutrons. The number of protons is the atomic number. The number of protons and neutrons together is effectively the mass of the atom, described by a mass number. Atoms of the same element (same Z) but different mass number (A). Boron- ( B) has 5 p and 5 n: 5B Boron- ( B) has 5 p and 6 n: 5B B B Isotopes of atoms that have an unstable nucleus will undergo radioactive decay and is called a radioactive nuclide. Radioactive Decay One of the pieces of evidence for the fact that atoms are made of smaller particles came from the work of Marie Curie (876-9). She discovered radioactive decay, the spontaneous disintegration of a nucleus into a slightly lighter nucleus by the emission of particles, electromagnetic radiation, or both. Neutron-Proton Ratios Any element with more than one proton (i.e., anything but hydrogen) will have repulsions between the protons in the nucleus. A strong nuclear force helps keep the nucleus from flying apart.

2 Neutron-Proton Ratios Neutrons play a key role stabilizing the nucleus. Therefore, the ratio of neutrons to protons is an important factor. For smaller nuclei (mass number ) stable nuclei have a neutron-to-proton ratio close to :. Neutron-Proton Ratios As nuclei get larger, it takes a greater number of neutrons to stabilize the nucleus. The shaded region in the figure shows what nuclides would be stable, the so-called belt of stability. Nuclear Reactions Ernest Rutherford found Ra forms Rn gas by emitting a particle now known as an alpha particle. Rutherford et. al. (9) proposed radioactive decay is the result of an unstable radioactive nuclide trying to increase stability by emitting particles or radiation to gain a more favorable neutron to proton ratio resulting in the formation of an isotope of a different element Nuclear Reactions Radioactive decay is the process by which the unstable nucleus of one isotope releases energy by emitting particles and/or radiation to form a more stable isotope. If the radioactive decay results in a change in the identity an element to an isotope of another element by a change in the number of its protons, the decay is called a transmutation. Transmutations occur naturally by alpha emission, beta emission, positron emission, and electron capture (usually accompanied by gamma emission). Types of Radioactive Decay Alpha Decay: Alpha emission Loss of an -particle (a helium nucleus) Note that mass number goes down by and atomic number goes down by. 8 9U 9Th +

3 Types of Radioactive Decay Beta Decay: Loss of a - particle (a high energy electron) or e Beta emission Note that mass number (A) is unchanged and atomic number (Z) goes up by. ow does this happen 5I 5Xe + e Types of Radioactive Decay Positron Emission: Positrons have the same mass as electrons, but have an opposite charge. Positrons are emitted as a proton is converted to a neutron in the absence of an electron. Observe the positron emission of polonium-7. β Po β 8 7 Positron emission 7 8Po β A 7 - = 7 Z 8 - = 8 The result of the positron emission of polonium-7 is the formation of bismuth- 7. Where do you think the positron mass may have come from 8Po β Bi Types of Radioactive Decay Electron Capture (K-Capture): Electron capture involves the capture of an inner shell electron by a proton in the nucleus resulting in the formation of a neutron. Observe the electron capture of beryllium-7. 7 Be -e Electron Capture 7 Be -e A 7 - = 7 Z - = We see that the electron capture by beryllium-7 results in the formation of lithium Be -e Li

4 Types of Radioactive Decay Gamma Emission: Penetrating Ability Loss of a -ray (high-energy radiation that almost always accompanies the loss of a nuclear particle) This is why exposure to radioactive materials is harmful!! Rate of Radioactive Decay Every radioisotope has a characteristic rate of decay measured by its half-life, or the time required for onehalf of the nuclei to decay to products. Rate of Radioactive Decay Radioactive Series Large radioactive nuclei cannot stabilize by undergoing only one nuclear transformation. They undergo a series of decays until they form a stable nuclide called a decay series. In a decay series, a parent nuclide decays into several daughter nuclides until a stable nuclide is reached. Artificial Transmutations Nuclear transformations can be induced by accelerating a particle and colliding it with the nuclide. Artificial transmutations have been used to produce the transuranium elements, or those with more than 9 protons in their nuclei.

5 Two important consequences of the trend in radioactive nuclide stability:. avy nuclei gain stability and therefore give off energy if they are fragmented into mid-sized nuclei (nuclear fission).. Greater amounts of energy can be released if very light nuclei are combined to give more massive nuclei (nuclear fusion). Process responsible for nuclear power and nuclear weapons Bombardment of the radioactive nuclide with a neutron starts the process. Neutrons released in the transmutation strike other nuclei, causing their decay and the production of more neutrons. This process continues in what we call a nuclear chain reaction. If there are not enough radioactive nuclides in the path of the ejected neutrons, the chain reaction will die out. Therefore, there must be a certain minimum amount of fissionable material present for the chain reaction to be sustained: Critical Mass. In nuclear reactors the heat generated by the reaction is used to produce steam that turns a turbine connected to a generator. To formation of more stable, heavier nuclides from the combination of low-mass nuclides Responsible for the energy generated by the sun. -e Nuclear Fusion - e Fusion would be a superior method of generating power. The good news is that the products of the reaction are not radioactive. The bad news is that in order to achieve fusion, the material must be in the plasma state at several million kelvins. 5

6 Tokamak apparati like the one shown below show promise for carrying out these reactions. They use magnetic fields to heat the material. 6