ElectronegativityElectronegativity, symbolized as χ, is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its atomic number and the distance at which its valence electrons reside from the charged nucleus. The higher the associated electronegativity, the more an atom or a substituent group attracts electrons.
Valence electronIn chemistry and physics, a valence electron is an electron in the outer shell associated with an atom, and that can participate in the formation of a chemical bond if the outer shell is not closed. In a single covalent bond, a shared pair forms with both atoms in the bond each contributing one valence electron. The presence of valence electrons can determine the element's chemical properties, such as its valence—whether it may bond with other elements and, if so, how readily and with how many.
Hypervalent moleculeIn chemistry, a hypervalent molecule (the phenomenon is sometimes colloquially known as expanded octet) is a molecule that contains one or more main group elements apparently bearing more than eight electrons in their valence shells. Phosphorus pentachloride (), sulfur hexafluoride (), chlorine trifluoride (), the chlorite () ion, and the triiodide () ion are examples of hypervalent molecules. Hypervalent molecules were first formally defined by Jeremy I. Musher in 1969 as molecules having central atoms of group 15–18 in any valence other than the lowest (i.
Critical point (thermodynamics)In thermodynamics, a critical point (or critical state) is the end point of a phase equilibrium curve. One example is the liquid–vapor critical point, the end point of the pressure–temperature curve that designates conditions under which a liquid and its vapor can coexist. At higher temperatures, the gas cannot be liquefied by pressure alone. At the critical point, defined by a critical temperature Tc and a critical pressure pc, phase boundaries vanish.
Carbon tetrachlorideCarbon tetrachloride, also known by many other names (such as carbon tet for short and tetrachloromethane, also recognised by the IUPAC) is a chemical compound with the chemical formula CCl4. It is a non-flammable, colourless liquid with a "sweet" chloroform-like smell that can be detected at low levels. It was formerly widely used in fire extinguishers, as a precursor to refrigerants and as a cleaning agent, but has since been phased out because of environmental and safety concerns.
FreonFreon (ˈfriːɒn ) is a registered trademark of the Chemours Company and generic descriptor for a number of halocarbon products. They are stable, nonflammable, low toxicity gases or liquids which have generally been used as refrigerants and as aerosol propellants. These include chlorofluorocarbons and hydrofluorocarbons, both of which cause ozone depletion (although the latter much less so) and contribute to global warming.
Dimerization (chemistry)In chemistry, dimerization refers to the process of joining two molecules or ions by bonds. The resulting bonds can be either strong or weak. Many symmetrical chemical species are described as dimers, even when the monomer is unknown or highly unstable. The term homodimer is used when the two subunits are identical (e.g. A–A) and heterodimer when they are not (e.g. A–B). The reverse of dimerization is often called dissociation. When two oppositely charged ions associate into dimers, they are referred to as Bjerrum pairs, after Niels Bjerrum.
HydroxyapatiteHydroxyapatite (HA), is a naturally occurring mineral form of calcium apatite with the formula , but it is usually written to denote that the crystal unit cell comprises two entities. Hydroxylapatite is the hydroxyl endmember of the complex apatite group. The ion can be replaced by fluoride, chloride, producing fluorapatite or chlorapatite. It crystallizes in the hexagonal crystal system. Pure hydroxylapatite powder is white. Naturally occurring apatites can, however, also have brown, yellow, or green colorations, comparable to the discolorations of dental fluorosis.
Perfluorooctanoic acidPerfluorooctanoic acid (PFOA; conjugate base perfluorooctanoate; also known colloquially as C8, for its 8 carbon chain structure) is a perfluorinated carboxylic acid produced and used worldwide as an industrial surfactant in chemical processes and as a material feedstock. PFOA is considered a surfactant, or fluorosurfactant, due to its chemical structure, which consists of a perfluorinated, n-heptyl "tail group" and a carboxylate "head group". The head group can be described as hydrophilic while the fluorocarbon tail is both hydrophobic and lipophobic.
Group (periodic table)In chemistry, a group (also known as a family) is a column of elements in the periodic table of the chemical elements. There are 18 numbered groups in the periodic table; the 14 f-block columns, between groups 2 and 3, are not numbered. The elements in a group have similar physical or chemical characteristics of the outermost electron shells of their atoms (i.e., the same core charge), because most chemical properties are dominated by the orbital location of the outermost electron.
