The consistent combination of proper or improper symmetry axes which leaves one point fixed is called a point group.
The simplest combinations are those characterized by the presence of only one axis which may be a proper axis or an inversion axis, or simultaneously proper and improper axis.
One proper symmetry axis point group:
One inversion axis point group:
One proper and improper axis point group:
/, /, /, /, /, /, ...
Let us see the arrangement of the set of symmetrically equivalent points for the various cases:
/ : is a trivial axis, thus /.
/ /m where m is perpendicular to the axis.
/ , since = contains .
/ /m where m is perpendicular to the axis.
/
//m where m is perpendicular to the axis.
If two rotation symmetry axes L1
and L2 coexist and intersect in O,
the first axis making the object in P symmetrically equivalent to
the object in Q,
the second axis making the object in Q equivalent to
the object in R,
a third axis L3 must exist making the object in R
equivalent to the object in P.
If L1 is a proper axis while L2
is an inversion one, then L3 will be an inversion
axis.
The only allowed combinations are
n22 , 233, 432, 532
where n may assume any integral value. The angles between the axes are as follows:
222 | 90(2 2) | 90(2 2) | 90(2 2) |
322 | 90(2 3) | 90(2 3) | 60(2 2) |
422 | 90(2 4) | 90(2 4) | 45(2 2) |
622 | 90(2 6) | 90(2 6) | 30(2 2) |
233 | 54 44'08"(2 3) | 54 44'08"(2 3) | 70 31'44"(3 3) |
432 | 35 15'52"(2 3) | 45(2 4) | 54 44'08"(4 3) |
New point groups may be found by allowing symmetry axes to be inversion axes.