symmetry (probert)

Question 1

Define composition

Answer 1

what elements are present and in what amount

Question 2

define connectivity

Answer 2

which atoms are bonded together

Question 3

define stereochemistry

Answer 3

qualitative geometrical information - giving some indication of shape

Question 4

define symmetry

Answer 4

equivalence or inequivalence of different atoms and bonds

Question 5

define quantitative geometry

Answer 5

the exact relative positions of atoms which leads to the calculations of bond lengths, bond angles, torsion angles etc

Question 6

how to convert picometre to meters (pm - m)

Answer 6

1pm = 10^-12 m

Question 7

how do you convert nanometers to meters

Answer 7

1nm = 10^-9 m

Question 8

how to convert angstrom to meters (A-m)

Answer 8

1A = 10^10.

Question 9

what theory predicts the shapes of molecules and what does it stand for

Answer 9

VSEPR
Valence
Shell
Electron
Pair
Repulsion

Question 9

what are the basis and assumptions of VSEPR

Answer 9

• 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

Question 10

how to use VSEPR

Answer 10

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

Question 11

name and bond angle 2 bonded pairs

Answer 11

linear 180

Question 12

name and bond angle 3 bonded pairs

Answer 12

trigonal planar 120

Question 13

name and bond angle 4 bonded pairs

Answer 13

tetrahedral 109.5

Question 14

name and bond angle 5 bonded pairs

Answer 14

trigonal bipyramidal
angles of 90,120 and 120 between directions

Question 15

name and bond angle 6 bonded pairs

Answer 15

octahedral
90 and 120

Question 16

name and bond angle 7 bonded pairs

Answer 16

pentagonal bipyramidal
angles of 72 90 and 180

Question 17

name and bond angle 2 bonded pairs 1 lone pair

Answer 17

bent/angular
less than 120

Question 18

name and bond angle 3 bonded pairs 1 lone pairs

Answer 18

trigonal pyramidal
107

Question 19

name and bond angle 2 bonded pairs 2 lone pairs

Answer 19

bent/ angular
104.5

Question 20

name and bond angle 4 bonded pairs 1 lone pair

Answer 20

seesaw
less than 120 and 90

Question 21

name and bond angle 3 bonded pairs 2 lone pairs

Answer 21

T shapes
less than 180 and 90

Question 22

name and bond angle 2 bonded pairs 3 lone pairs

Answer 22

linear
180

Question 23

name and bond angle 5 bonded pairs 1 lone pair

Answer 23

square pyramidal
less than 90

Question 24

name and bond angle 4 bonded pairs 2 lone pairs

Answer 24

square planar 90

Question 25

name and bond angle 3 bonded pairs 3 lone pairs

Answer 25

t shaped
90 degrees

Question 26

define symmetry element

Answer 26

A point, line or plane through which the symmetry operation takes place i.e. the rotation axis about which you rotate an object

Question 27

define symmetry operations

Answer 27

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.

Question 28

what are the 5 symmetry operations

Answer 28

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)

Question 29

what is identity E

Answer 29

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.

Question 30

what is proper rotation

Answer 30

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)

Question 31

what is reflection

Answer 31

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

Question 32

what is inversion

Answer 32

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.

Question 33

what are improper rotations

Answer 33

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

Question 34

define point group

Answer 34

a list of the ways in which the orientation of a crystal can be changed without seeming to change the position of its atoms

Question 35

define symmetry element

Answer 35

a symmetry element is a geometrical entity about which a symmetry operation is performed
such as a point, axis or plane