James G. Speight - Encyclopedia of Renewable Energy

Alkene derivatives are more reactive than alkane derivatives due to the presence of a carbon-carbon pi-bond (double bond). The majority of the reactions of alkenes involve the rupture of this pi bond, forming new single bonds.

Alpha decay (α-decay) is the radioactive emission of an α-particle which is the nucleus of helium-4 ( 4He), consisting of two protons and two neutrons which is a stable nucleus as it is doubly magic. The daughter nucleus has two protons and four nucleons fewer than the parent nucleus. In general, alpha decay leads to the ground state of the daughter nucleus so that the emitted particle carries away as much energy as possible.

By way of definition, a nucleon is either a proton or a neutron considered in its role as a component of an atomic nucleus. The number of nucleons in a nucleus defines the mass number (the nucleon number) of an isotope. In fact, protons and neutrons are best known in their role as nucleons, i.e., as the components of atomic nuclei, but they also exist as free particles. Free neutrons are unstable, with a half-life of approximately 13 minutes, but they have important applications. Protons not bound to other nucleons are the nuclei of hydrogen atoms when bound with an electron or – if not bound to anything – are ions or cosmic rays. Both the proton and the neutron are composite particles insofar as each is composed of smaller parts and, thus, neither the proton nor the neutron is an elementary particle.

A pertinent example from the area of nuclear energy is the decay of uranium-238 ( 238U) to thorium-234 ( 234Th). An alpha particle (also called alpha rays or alpha radiation) has a charge of +2 e , but, as a nuclear equation, describes a nuclear reaction without considering the electrons and consists of two protons and two neutrons bound together into a particle identical to a helium-4 ( 4He) nucleus and are typically the product of the alpha decay process. The symbol for the alpha particle is α or α 2+, and because alpha particles are identical to helium nuclei, they are also sometimes written as He 2+or 4 2He 2+which indicates a helium ion with a +2 charge (missing two electrons). Once the ion gains electrons from its environment, the alpha particle becomes a normal (electrically neutral) helium atom 4 2He.

Alpha decay typically occurs in the heaviest nuclides, and, theoretically, it can occur only in nuclei somewhat heavier (higher atomic number) than nickel atomic (number = element 28), where the overall binding energy per nucleon is no longer a minimum and the nuclides are therefore unstable toward spontaneous fission-type processes. In practice, this mode of decay has only been observed in nuclides considerably heavier than nickel, with the lightest known alpha emitters being the lightest isotopes (mass numbers 104 to 109) of tellurium (atomic number = element 52).

When an atom emits an alpha particle in alpha decay, the mass number of the atom decreases by four due to the loss of the four nucleons in the alpha particle. The atomic number of the atom decreases by two, as a result of the loss of two protons, and the atom becomes a new element. Also, unlike other types of decay, alpha decay as a process must have a minimum-size atomic nucleus that can support it. The process may leave the nucleus in an excited state after which the emission of a gamma ray then removes the excess energy.

See also: Beta Decay, Gamma Decay, Nuclear Energy.

An alpha particle (also termed alpha radiation or alpha rays) is a helium nucleus stripped of its orbital electrons and which is emitted from a radioactive atom with a velocity of about 1/20 that of the speed of light and with energies ranging from 4 to 9 MeV. The alpha particle was the first nuclear radiation to be discovered; beta particles and gamma rays were identified soon thereafter.

Alpha particles cause ionizations in matter when they are deflected by the positive charge of a nucleus and pull the orbital electrons (attracted by the alpha’s positive charge) along with them. Alpha particles also cause excitation along their path by pulling inner orbital electrons to outer orbits. Energy is then given off by the atom as fluorescent radiation (low energy x-rays) when the electrons drop back down to the inner orbital vacancies.

Because of its relatively large mass (2 neutrons and 2 protons), high electrical charge (2+) and low velocity, the specific ionization of an alpha particle is high and, as a result, it creates many ion pairs in a very short path length. Because of this, it loses all of its energy in a short distance. The range in air is only several centimeters even for the most energetic alpha particles.

Alpha particles are highly ionizing because of the double positive charge, large mass (compared to a beta particle), and because they are relatively slow. They can cause multiple ionizations within a short distance which gives alpha particles the potential to do much more biological damage for the same amount of deposited energy. Alpha particles cannot penetrate the normal layer of dead cells on the outside of our skin but can damage the cornea of the eye. Alpha-particle radiation is normally only a safety concern if the radioactive decay occurs from an atom that is already inside the body or a cell. Alpha-particle emitters are particularly dangerous if inhaled, ingested, or if they enter a wound. Once inside the body, surrounded by living tissue, damage will be to the local area in which the alpha emitter is deposited. Thus, alpha emitters are an internal hazard and intake to the body must be prevented.

See also: Beta Decay, Beta Particle, Gamma Decay, Nuclear Energy.

Alternate fuels (renewable fuels, advanced fuels, synthetic fuels which include liquid and gaseous fuels, as well as clean solid fuels) are any materials or substances that can be used as fuels, other than conventional fuels. Alternate fuels are derived from resources such as coal, oil shale, or tar sands, and various forms of biomass – in the last case the erm that is more appropriate is renewable fuels. Renewable fuels are making headway into the fuel balance by reducing dependence on imported oil. Often, renewable fuels produce less pollution than crude oil-derived gasoline or crude oil-derived diesel fuel.

Examples of renewable fuels and renewable energy sources are: (i) biodiesel, which is diesel fuel derived from vegetable oils and animal fats. It usually produces less air pollutants than crude oil-based diesel, (ii) biogas which is produced from the anaerobic digestion of diverse organic waste sources using various methods, (iii) biomass liquids: produced by biomass-to-gas-to-liquid conversion and by pyrolysis processes which involve the conversion of biomass, such as wood and agricultural residues, (iv) ethanol or other alcohols produced domestically from corn and other crops and produces less greenhouse gas emissions than conventional fuels, and (v) hydrogen which is produced from biomass, (vi) nuclear power, or (vii) other sources such as solar energy, tidal energy, and wind energy and hydroelectric power.

Biodiesel from plant sources is similar to diesel, but has differences that include higher cetane rating (45 to 60 compared to 45 to 50 for crude oil-derived diesel fuel), and it acts as a cleaning agent to get rid of dirt and deposits. As with alcohols and gasoline engines, taking advantage of the high cetane number of biodiesel potentially overcomes the energy deficit compared to ordinary number 2 diesel.

See also: Biodiesel, Biogas, Hydrogen, Synthetic Fuel.

Gaseous fuels are those fuels that are in the gaseous state under ambient conditions. In some circumstances, the definition of gaseous fuels may also include the low-boiling hydrocarbons such as pentane but such fuels are considered to be liquid fuels.