Molecular Shapes, Symmetry, and Point Groups Flashcards
How do you calculate formal charge?
Number of electrons atom should have - n of lone pair electrons - 1/2 n of electrons in a bond
What are the shapes for 2-7 electrons pairs based on VSEPR? What are the assumptions for VSEPR?
2: Linear
3: Trigonal planar
4: Tetrahedral
5: Trigonal Bipyramidal
6: Octahedral
7: Pentagonal Bipyramidal
The geometry of a molecule depends only upon electron-electron interactions
With electron pairs repelling as far as possible to occupy positions which minimise repulsion the most
What are the refinements to the VSEPR model?
Lone pairs repel more than bonding pairs
Triple>Double>Single bonds in terms of repulsion
The more electronegative the terminal atoms (i.e not centre), the further away the electron pairs are from the centre, reducing electron pair repulsion
What is the shape of XeF₆? Why? And SeCl6 2-?
Distorted octahedral (not pentagonal bipyramidal as predicted)
As a hypervalent molecule with very diffuse orbitals, the lone pair is delocalised, and so distorted octahedral
As before, but the lone pair largely occupies an s orbital which is not directional so does not largely contribute to repulsion
What are the shapes of 5 bonding pairs, with varying lone pairs?
5 bp : trigonal bipyramidal
4 bp 1lp: ‘ ‘ seesaw but lp equatorial
3 bp 2lp: ‘ ‘ t shape both equatorial
2bp 3lp: ‘ ‘ linear
What are the shapes of 6 bonding pairs, with varying lone pairs?
What is the shape of radicals? What’s Bent’s rule?
Generally assume counts as a lone pair, but less repulsion so larger bond angles with the others
Bent’s rule: electronegative species prefer orbitals with greater s character, applied to why CF3 is pyramidal vs CH3 trigonal
What are the main symmetry elements and what they mean?
E= whole of space
Cn= Rotation axis, where rotation degree= 360 / n
σ = mirror planes, σH/V relative to the principal axis
i= centre of inversion
Sn= improper axis
What is the principal axis? What is an improper axis?
Principal: axis with the highest Cn
Improper: if a rotation followed by reflection in the plane perpendicular gives the same molecule, with Sn representing the rotation number
e.g with the molecule below
What is a centre of inversion?
From the point of inversion, look at 180 degrees on either side is the same from all points
Only very symmetrical molecules: octahedral, square planar, some linear
How do lone pairs affect symmetry?
Lone pairs are not counted
e.g c infinity symmetry for linear 5 electron pairs
How are mirror planes names?
σv= vertical= mirror plane in the same as principal axis
σv= horizontal= mirror plane perpendicular to the principal axis
What is the general route for determining a point group?
Linear: C infinity v or D infinity h
Very symmetric: Td, Od, Ih
Cn: perpendicular C2: H , d, n
Cn: no perpendicular: H, V, S2, Cn
No Cn: mirror plane : Cs
inversion, or C1
What main shapes are associated with the D point group?
Square planar
Pentagonal bipyramidal
Some octahedral
What is the point group of chlorobromomethane?
Cs
There is a horizontal mirror plane in the plane of the board through the bromine and chlorine
