Glossary

The glossary explains the technical terms that are used in the project "The structure of substances".

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

  Aliphatic compounds

Aliphatic compounds (short form, aliphatics) are organic substances in which the carbon atoms are arranged in a chain. The chain can be straight or branched and may carry substituents. In contrast, in cyclic compounds (this includes the aromatics) rings of carbon atoms occur. Examples of aliphatic compounds are pentane, or acetic acid.

  Allotropy

In some elements, molecules of different sizes form. Examples are "normal" oxygen O2 and ozone O3 or the various fullerenes (C60, C70 and others).

This phenomenon is called allotropy, the individual "forms" (e.g. O2 and O3) are called allotropes of the element.

Related topic: polymorphism

  Anisotropy

Anisotropy is the phenomenon whereby physical properties in a crystal are dependent on direction. Some examples are:

  • Hardness: graphite
  • Electrical conductivity: graphite
  • Thermal conductivity
  • Refraction of light

  CAS number

The Chemical Abstracts Service of the American Chemical Society assigns a number to every substance, the CAS number. This number is now being used more and more for the clear identification of substances.

Conformers

The conformations of a molecule that have minimum energy are called conformers. In the case of cyclohexane, for example, the chair and twist-boat forms, which correspond to the energy minima, are conformers, while the high-energy bath form is not.

Coordination number

The coordination number of an atom (in a crystal) is the number of its nearest neighbouring atoms.

  Dimer

A dimer is a molecule that is created from 2 similar molecules (or formula units, in the case of ionic compounds) by a reaction.

Halogens

The elements in the 7th main group of the periodic table (fluorine, chlorine, bromine, iodine, and astatine) are called halogens. The name comes from the Greek words "hals" (salt) and "gennan" (generate), and therefore means salt-formers. Compounds of halogens and metals are always salts.

High-purity metals

Metals produced on an industrial scale are usually not all that pure. This degree of purity (e.g. 95 - 99%) is sufficient for many technical applications. Higher purities can be achieved in several ways:

  • Electrolytic refining, e.g. with copper
  • By utilising transport processes, e.g. with nickel, aluminium and silicon

    Ligand

In a complex compound, a central atom is surrounded by several groups of atoms. These are called the ligands of the complex.

  Metastable

Of all the polymorphs of a substance, at any given temperature and pressure one of them has the lowest energy content. It is called the thermodynamically most stable polymorph of this substance. All the others are said to be metastable. The metastable polymorphs change (albeit often at an extremely low rate) into the thermodynamically most stable polymorph.

Noble gas configuration

When the valence shell of an atom contains exactly 8 electrons, we say that it has the noble gas configuration. This is because the noble gases have eight valence electrons (the exception is helium, where the valence shell is fully populated with just two electrons).

  Octet ruleAtoms and ions attempt to fill their valence shell (i.e. their outermost electron shell) with 8 electrons.

For the elements in the second period (lithium to neon), the octet rule applies strictly. That is, these elements can never have more than 8 valence electrons.

For the elements in the third and further periods, the rule applies only to a limited extent. The valence shell often takes on 8 electrons, because this configuration is stable, but it may also absorb more electrons.

  Polar

A molecule is called polar if it is uncharged when regarded as a whole, but still has a non-uniform charge distribution. In the water molecule, for example, the oxygen atom has a small negative charge, while the hydrogen atoms have small positive charges. The water molecule is thus a little negatively charged at "one end", a little positively charged at the "other end".

Polar molecules have some notable properties: for example, they are usually highly reactive, can form hydrogen bonds and are deflected in an electric field.

Polyhedron

A polyhedron (Greek "many-sided object") is a body that is bounded by flat surfaces. In regular polyhedrons, these surfaces are all equal. Examples of regular polyhedrons are tetrahedrons, cubes and octahedrons. Irregular polyhedrons include trigonal bipyramids, square pyramids, or a hexagonal column, for example.

  Polymorphism

Many substances can occur in crystals with different internal structures. Examples are carbon (diamond, graphite) and zinc sulphide (sphalerite, wurtzite).

This phenomenon is called polymorphism, and the individual "forms" (e.g. diamond and graphite) are called polymorphs of the substance.

Related topic: allotropy

  Pyrolysis

Pyrolysis is a generic term for chemical processes in which substances are decomposed by the effect of heat (temperatures generally above 1000 °C) in the absence of air. This does not mean incineration, but the creation of intermediate products that cannot be obtained in large quantities in any other way.

Example: the pyrolysis of methane to ethyne (acetylene gas).

    Rotational barrier

The rotational barrier of a molecule is the energy difference between the lowest-energy and the highest-energy conformations of the molecule.

  Shear forces

Shear forces are forces that act on an object from opposite directions. An example.

Speed of electrons in metallic conductors

Electrons in metallic conductors do not move at roughly the speed of light, but rather slowly, in fact. The speed depends on the cross-sectional area of the conductor, on its material, and on the magnitude of the current. For example, in a copper wire with a cross-section of 1 mm2 and a current of 1 A, it is only about 30 cm per hour. Any movement of the electrons propagates at approximately the speed of light, however, so that all the electrons in an electrical conductor start to move almost simultaneously.

  Thermodynamically stable

Of all the polymorphs of a substance, at any given temperature and pressure one of them has the lowest energy content. It is called the thermodynamically most stable polymorph of this substance. All the others are said to be metastable. The metastable polymorphs change (albeit often at an extremely low rate) into the thermodynamically most stable polymorph.

Trimer

A trimer is a molecule that is created from 3 similar molecules (or formula units, in the case of ionic compounds) by a reaction.

  Unit cell

In crystals, the atoms or ions have a regular spatial arrangement. We select the smallest possible segment of crystal (just a few atoms) from which, by joining such segments together in all 3 directions in space, we can construct the whole crystal. This segment is called the unit cell. (Almost) every substance has a different unit cell.

          Valence electrons and valence shell

The valence shell is the outermost electron shell of an atom. The electrons in this shell are called valence electrons. Only the valence electrons determine the chemical behaviour of the atom.

Viscosity

When a liquid flows, individual molecules of the liquid are displaced in relation to their neighbours. If large cohesive forces are encountered, the liquid flows slowly and viscously, and it has a high internal friction. The viscosity is a measure of this internal friction.

For example, glycerine or olive oil has a much higher viscosity than water.

VRML

VRML (Virtual Reality Modelling Language) is a programming language with which one can describe three-dimensional scenes (called worlds). The worlds of the project "The structure of substances" do not contain people, roads and buildings, but molecules.

VRML browser

A VRML browser is a program for exploring VRML worlds. Users see the VRML world as if they were walking or flying through it. VRML browsers are usually plug-ins (add-on programs) for the popular Internet browsers such as Microsoft Internet Explorer or Mozilla Firefox.

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