[Winga]

The principal axis of XeF4 is C4 axis, but I am not quite understand that it also coincides with a C2 axis.

Does it mean that the XeF4 is a rectangular base rather than square base?

[lemonoman]

All C4 axes will have a coincidental C2 axis...only because a C4 axis implies that rotations of 90°, 180°, 270°, and 360° result in no change in the structure's orientation.  To be a C2 axis, this only has to be satisfied by rotations of 180° and 360° - so a C4 is a special kind of C2.

Like you said, a rectangle has a C2, and a square has a C4 - but the square also has a C2 - after all, a square is just a special kind of rectangle.  A C4 is a special kind of C2.

And P.S., a C4 also has a coincidental C1 .
And P.P.S., A C8, you may have guessed, has a coincidental C4, C2, and C1.

Does it mean that the XeF4 is a rectangular base rather than square base?

Not quite...and I hope it is clear after what I've said...if it's not, let us know...I've been off of my game for a while

[Winga]

I see.

Thank you!

[AWK]

C4 has 4 C2 perpendicular to it.

[lemonoman]

C4 has 4 C2 perpendicular to it.

I'm sure you already know this, AWK, but just to clarify for other readers, this is only true for certain structures.  The presence of a C4 axis does NOT imply four C2 axes perpendicular to it ... it just happens to be true for XeF4.  It is entirely possible to have a C4 axis with NO perpendicular axes of symmetry.

[AWK]

C4 has 4 C2 perpendicular to it.

I'm sure you already know this, AWK, but just to clarify for other readers, this is only true for certain structures.  The presence of a C4 axis does NOT imply four C2 axes perpendicular to it ... it just happens to be true for XeF4.  It is entirely possible to have a C4 axis with NO perpendicular axes of symmetry.

We just discuss abot flat squared structure. I did not say that this always is valid. And this statement concerns only an idealized structure.

[lemonoman]

We just discuss abot flat squared structure. I did not say that this always is valid. And this statement concerns only an idealized structure.

Agreed.  I said what I did just to clarify, in case someone decides to generalize a rule...we wouldn't want anyone to mislead themselves!