Encyclopedia of Renewable Energy. James G. Speight

Any oxygen that enters the system during the operation forms thiosulfates in the solution, destroying its effectiveness. Special precautions are taken to prevent air entering the system at pump packing, or in the solution storage vessels, etc. This difficulty limits the use of the process to oxygen-free gases.

      Alkenes

An alkene (olefin) is an unsaturated chemical compound containing at least one carbon-to-carbon double bond (Table A-18). The simplest acyclic alkenes, with only one double bond and no other functional groups, form a homologous series of hydrocarbons with the general formula C_nH_2n. The simplest alkene is ethylene (C₂H₄), which has the International Union of Pure and Applied Chemistry (IUPAC) name ethene. Alkenes are also called olefins (an archaic synonym, widely used in the petrochemical industry).

      The physical properties of alkenes are comparable with those of alkanes (aliphatic hydrocarbons). The physical state depends on molecular mass (gases from ethylene to butene - liquids from pentene onwards). The simplest alkenes, ethylene, propylene (propene), and butylene (butene) are gases. Linear alkenes of approximately 5 to 16 carbons are liquids, and higher molecular weight alkenes are waxy solids. Alkenes are more reactive than alkanes 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.

Table A-18 Physical properties of selected olefins.

Olefin	Melting point, °C	Boiling point, °C	Density, g/ml @20 °C
Ethylene	-169	-102
Propylene	-158	-48
1-Butene		-6.5
1-Pentene		30
1-Hexene	-138	63.5	0.643
1-Heptene	-119	93	0.675
1-Octene	-104	122.5	0.698
1-Nonene		146	0.716
1-Decene	-87	171	0.731
Cis-2-butene	-139	4	0.743
Trans-2-butene	-106	1
Isobutylene	-141	-7
Cis-2-pentene	-151	37
2-Methyl-2-butene	-123	39	0.655
2,3-Dimethyl-2-butene	-74	73	0.660
Cyclopentene	-93	46	0.705
Cyclohexene	-104	83	0.774
1,3-Cyclopentadiene	-85	42	0.810
1,3-Cyclohexadiene	-49	87	0.798
			0.847

      Alkenes serve as a feedstock for the petrochemical industry because they can participate in a wide variety of reactions.

      The simplest acyclic alkenes, with only one double bond and no other functional groups, form a homologous series of hydrocarbons with the general formula C_nH_2n.

      The physical properties of alkene derivatives are comparable with those of alkane derivatives (aliphatic hydrocarbon derivatives). The physical state depends on molecular mass (gases from ethylene to butene and liquids from pentene onwards). The simplest alkenes, ethylene, propylene (propene), and butylene (butene) are gases. Linear alkenes of approximately 5 to 16 carbons are liquids, and higher molecular weight alkenes are waxy solids.

      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

      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 +2e, but, as a nuclear equation, describes a nuclear reaction