symmetry (probert) Flashcards
Define composition
what elements are present and in what amount
define connectivity
which atoms are bonded together
define stereochemistry
qualitative geometrical information - giving some indication of shape
define symmetry
equivalence or inequivalence of different atoms and bonds
define quantitative geometry
the exact relative positions of atoms which leads to the calculations of bond lengths, bond angles, torsion angles etc
how to convert picometre to meters (pm - m)
1pm = 10^-12 m
how do you convert nanometers to meters
1nm = 10^-9 m
how to convert angstrom to meters (A-m)
1A = 10^10.
what theory predicts the shapes of molecules and what does it stand for
VSEPR
Valence
Shell
Electron
Pair
Repulsion
what are the basis and assumptions of VSEPR
- Atoms in molecules are linked through covalent bonds involving shared electrons
- More than one bonding pair of electrons may bind any two atoms together
- Any remaining valence electrons not utilized in bonding are classified as lone pairs
- Electron pairs do not like to be near each other i.e. they repel, the repulsion strength is dependent on the distance from the central atom. This means that the order of repulsion strengths is: lone pair : lone pair > lone pair : bonding pair > bonding pair : bonding pair
- Lone pairs are held closer to nucleus than bonding pair, bonding pair is between the bond takes up a larger angular range
how to use VSEPR
Step 1 pick central atom
Step 2 Draw a Lewis structure for the molecule
Step 3 Count the number electrons in the valence shell of the central atom
Step 4 Chose an appropriate shape that places the electron pairs as far apart as possible
Step 5 Assign the bonding pairs and lone pairs of electrons to the appropriate vertices of these geometries by taking account of different spatial requirements
name and bond angle 2 bonded pairs
linear 180
name and bond angle 3 bonded pairs
trigonal planar 120
name and bond angle 4 bonded pairs
tetrahedral 109.5
name and bond angle 5 bonded pairs
trigonal bipyramidal
angles of 90,120 and 120 between directions
name and bond angle 6 bonded pairs
octahedral
90 and 120
name and bond angle 7 bonded pairs
pentagonal bipyramidal
angles of 72 90 and 180
name and bond angle 2 bonded pairs 1 lone pair
bent/angular
less than 120
name and bond angle 3 bonded pairs 1 lone pairs
trigonal pyramidal
107
name and bond angle 2 bonded pairs 2 lone pairs
bent/ angular
104.5
name and bond angle 4 bonded pairs 1 lone pair
seesaw
less than 120 and 90
name and bond angle 3 bonded pairs 2 lone pairs
T shapes
less than 180 and 90
name and bond angle 2 bonded pairs 3 lone pairs
linear
180
name and bond angle 5 bonded pairs 1 lone pair
square pyramidal
less than 90
name and bond angle 4 bonded pairs 2 lone pairs
square planar 90
name and bond angle 3 bonded pairs 3 lone pairs
t shaped
90 degrees
define symmetry element
A point, line or plane through which the symmetry operation takes place i.e. the rotation axis about which you rotate an object
define symmetry operations
The movement (or lack of) that returns the object to an indistinguishable position to the original one. This usually does not mean that nothing moves and the actual atoms in places may change, but the object looks the same.
what are the 5 symmetry operations
1) identity E (don’t do anything – this is required by maths to complete the ‘group’)
2)proper rotation about an axis Cn
3) reflection in a plane sigma
4) inversion through a point (i)
5) improper rotation about an axis (sn)
what is identity E
This is the ONLY operation that is present in ALL molecules (and indeed objects). This operation leaves the molecule completely unchanged and therefore indistinguishable from the original position.
what is proper rotation
This is an operation that is present if the symmetry element of a rotation axis can be identified, such that the molecule may be rotated about that axis by a number of degrees ( 360/ 𝑛 ) and leave the molecule indistinguishable from the original position
1) A Cn has an order of rotation of n
2) When a molecule possesses more than one Cn axis where the values of n differ, the Cn with the highest n is the principal axis, and in turn defines the z-direction (up)
3) The C1 operation is the same operation as E (the identity)
what is reflection
This is an operation that is present if the symmetry element of a mirror plane can be identified such that if the molecule is reflected in the plane the resulting orientation will be indistinguishable from the original
There are 3 types of mirror planes
1) Horizontal mirror planes
2) Verticle mirror planes
3) Dihedral mirror planes
what is inversion
Inversion symmetry is that possessed by molecules where the atoms can be paired along common vectors through the centre of the molecule. These pairs can be described as being diametrically opposite. The inversion operation acts through the inversion centre (the symmetry element), if a molecule possesses this element ALL symmetry elements MUST pass through this point.
what are improper rotations
rotation then perpendicular reflection
This is the only non-intuitive symmetry operation that molecules possess. It is actually a combination of 2 operations one after the other. An improper rotation consists of a proper rotation followed by a reflection perpendicular to the rotation. Just as with the proper rotation axes the n defines the order of the rotation. There are 2 special cases, S1 = σ and S2 = I
define point group
a list of the ways in which the orientation of a crystal can be changed without seeming to change the position of its atoms
define symmetry element
a symmetry element is a geometrical entity about which a symmetry operation is performed
such as a point, axis or plane
