Question

what is the point group of BF4- and what are the principle rotation axis ..?

Answer 1

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»Ex: S4 axis indicates the following. operations: S4 = rotation by 90° then σ

»Three C2 axes containing each B-F bond lie in the plane of the molecule perpendicular to the three-fold axis. The rotation axis of highest order (i.e., C3) is called the principal axis of rotation. When the principal Cn axis has n even, then it contains a C2 operation associated with this axis.

»For example, the XeF4 molecule belongs to the D4h point group. the XeF4 contains one C4 rotation axis, one C2 rotation axis, and four C2 perpendicular rotation axis, 2σv planes, 2σd planes and 1σh plane, those composed the character table of the D4h Point group.

»For example, H2O has 4 symmetry elements E, C2, and 2 σv, as do CH2Cl2 and cis-Cr(CO)4(PH3)2. ... If a molecule is linear and has no inversion center, then its point group is C∞v.