Everything about Point Groups totally explained
In
mathematics, a
point group is a
group of geometric
symmetries (
isometries) leaving a point fixed.
Overview
Point groups can exist in a
Euclidean space of any dimension. A discrete
point group in 2D is sometimes called a
rosette group, and is used to describe the symmetries of an ornament. The
3D point groups are heavily used in chemistry, especially to describe the symmetries of a
molecule and of
orbitals forming
covalent bonds, and in this context they're also called
molecular point groups.
There are infinitely many discrete point groups in each number of dimensions. However, the
crystallographic restriction theorem demonstrates that only a finite number are compatible with
translational symmetry. In 1D there are 2, in 2D 10, and in 3D 32 such groups, called
crystallographic point groups.
In two dimensions
Point groups in 2D fall into two distinct families, according to whether they consist of
rotations only, or include
reflections. The
cyclic groups, C
n (abstract group type Z
n), consist of rotations by 360°/
n, and all integer multiples. For example, a
swastika has
symmetry group C
4, consisting of rotations by 0°, 90°, 180°, and 270°. The symmetry group of a
square belongs to the family of
dihedral groups, D
n (abstract group type Dih
n), including as many reflections as rotations. The infinite rotational symmetry of the circle implies reflection symmetry as well, but formally the
circle group S
1 is distinct from Dih(S
1) because the latter explicitly includes the reflections.
An infinite group need not be continuous; for example, we've a group of all integer multiples of rotation by 360°/√2, which doesn't include rotation by 180°. Depending on its application,
homogeneity up to an arbitrarily fine level of detail in a
transverse direction may be considered equivalent to full homogeneity in that direction, in which case these symmetry groups can be ignored.
C
n and D
n for
n = 1, 2, 3, 4, and 6 can be combined with translational symmetry, sometimes in more than one way. Thus these 10 groups give rise to 17
wallpaper groups.
In three dimensions
More complex symmetries arise in 3D, see
point groups in three dimensions.
Generalization
In any dimension
d, the continuous group of all possible fixed point isometries is the
orthogonal group, denoted by O(
d); and its continuous subgroup of all possible rotations is the
special orthogonal group, denoted by SO(
d). This isn't
Schönflies notation, but the conventional names from
Lie group theory.
Further Information
Get more info on 'Point Groups'.
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