point group

point group
Crystall.
a class of crystals determined by a combination of their symmetry elements, all crystals left unchanged by a given set of symmetry elements being placed in the same class. Also called symmetry class.
[1900-05]

* * *

also called  Crystal Class,  

      in crystallography, listing of the ways in which the orientation of a crystal can be changed without seeming to change the positions of its atoms. These changes of orientation must involve just the point operations of rotation about an axis, reflection in a plane, inversion about a centre, or sequential rotation and inversion. Only 32 distinct combinations of these point operations are possible, as demonstrated by a German mineralogist, Johann F.C. Hessel, in 1830. Each possible combination is called a point group, or crystal class. A crystal can be assigned to one of these point groups on the basis of its external shape, or morphology. The addition of translational changes will yield a total of 230 possible combinations; these are called space groups (space group).

* * *


Universalium. 2010.

Игры ⚽ Нужна курсовая?

Look at other dictionaries:

  • Point group — The Bauhinia blakeana flower on the Hong Kong flag has C5 symmetry; the star on each petal has D5 symmetry. In geometry, a point group is a group of geometric symmetries (isometries) that keep at least one point fixed. Point groups can exist in a …   Wikipedia

  • point group — taškinė grupė statusas T sritis chemija apibrėžtis Baigtinio skaičiaus taškų išsidėstymas apie ypatingas pozicijas gardelėje, kuriam būdingas tam tikras simetrijos elementų rinkinys. atitikmenys: angl. point group rus. точечная группа …   Chemijos terminų aiškinamasis žodynas

  • point group — taškinė grupė statusas T sritis fizika atitikmenys: angl. point group vok. Punktgruppe, f rus. точечная группа, f pranc. groupe ponctuel, m …   Fizikos terminų žodynas

  • point group — noun a group of isometries leaving a fixed point …   Wiktionary

  • point group — Crystall. a class of crystals determined by a combination of their symmetry elements, all crystals left unchanged by a given set of symmetry elements being placed in the same class. Also called symmetry class. [1900 05] …   Useful english dictionary

  • Crystallographic point group — In crystallography, a crystallographic point group is a set of symmetry operations, like rotations or reflections, that leave a central point fixed while moving other directions and faces of the crystal to the positions of features of the same… …   Wikipedia

  • Elk Point Group — Stratigraphic range: Middle Devonian …   Wikipedia

  • Six Point Group — The Six Point Group was a British feminist campaign group founded by Lady Rhondda in 1921 to press for changes in the law of the United Kingdom in six areas.AimsThe six original specific aims were: # Satisfactory legislation on child assault; #… …   Wikipedia

  • Point groups in three dimensions — In geometry, a point group in three dimensions is an isometry group in three dimensions that leaves the origin fixed, or correspondingly, an isometry group of a sphere. It is a subgroup of the orthogonal group O(3), the group of all isometries… …   Wikipedia

  • Point groups in two dimensions — In geometry, a point group in two dimensions is an isometry group in two dimensions that leaves the origin fixed, or correspondingly, an isometry group of a circle. It is a subgroup of the orthogonal group O(2), the group of all isometries which… …   Wikipedia

Share the article and excerpts

Direct link
… Do a right-click on the link above
and select “Copy Link”