In: Chemistry
If an object is at point x,y,z (Cartesian coordinates) and its symmetry mate is related by a 21 screw axis, what is the location in Cartesian coordinates for the symmetry mate?
Hello!!
A screw axis is a symmetry operation in which rotation is
followed by translation i.e. you first rotate the object by an
angle
and then translate it in a perpendicular direction by a distance
1/a. Assume the screw axis to be na axis, then here,
= 360/n and the translation is to be made in the perpendicular
direction by 1/a. You can consider it to be absolutely similar to a
screw (hence getting its name).
Here, for 21 screw axis, the object has to be rotated by an angle of 360/2= 180o followed by a translation of 1/2.
Thus, in your case, for a 21 screw axis, if we consider c-axis as our axis of rotation, the object will change its coordinates from (x,y,z) to (-x,-y,z) upon a 180o rotation because for 180o rotation, the cartesian coordinates remains the same for the direction of the axis and they are inverted for the other 2-axis. This will further be followed by a translation of 1/2 along c-axis.
Finally,
Feel free to ask any of your queries in the comments.