[chita]

Hi, I do not understand the difference between ammonia and C3v point group

Ammonia has 1 C3 and 3 σv planes

C3v point group has 2 C3 and 3 σv

Why?

Thanks

[Corribus]

You are confusing the symmetry element (the C3 axis) with the symmetry operation (rotation about the C3 axis).

C3v group has one rotation axis (a C3 axis). But you can rotate about it either once or twice (performing it three times returns you to your starting position, which is equivalent to the identity element). This is why the character table has a "2" in front of the C3 element - it represents the number of different operations about the principle axis, not that there are two C3 rotation axes.

[chita]

Thank you