Symmetry elements

Question 1

Why are we interested in symmetry?

Answer 1

• Governs properties such as polarity, chirality and colour

- Assigning molecular vibrations

Question 2

When is a molecule symmetric?

Answer 2

If there is a transformation that moves the molecule, but it is indistinguishable from the original molecule

Question 3

What is a symmetry operation?

Answer 3

Movement

Question 4

What is a symmetry element?

Answer 4

Point, line or plane or a combination

Question 5

What are the 5 symmetrical elements that we consider?

Answer 5

Identify 
Rotation about a proper axis 
Reflection in a plane 
Inversion through a centre 
Combined rotation-reflection , an improper rotation

Question 6

What is identity element?

Answer 6

E

Present in all molecules as it leaves the molecule unchanged

Question 7

What is rotation about an n-fold axis of symmetry (Cn) ?

Answer 7

A molecule thats rotated about an axis and generates an identical appearance n times in a complete rotation 360 degrees

Question 8

What happens when a molecule contains more than one type of Cn axis with the same value of n?

Answer 8

They are distinguished by primes, C2’, C2’’

Question 9

What is the principle axis?

Answer 9

When a molecule has more than one Cn axis with different n, the axis of highest order

Question 10

How many operations does Cn provide?

Answer 10

n - including E

Question 11

What operation is C₄² equal to?

Answer 11

C₂

Question 12

What is σ?

Answer 12

Reflection in a mirror plane

Question 13

What are the three types of mirror plane?

Answer 13

Horizontal mirror plane
Vertical mirror plane
Dihedral mirror plane

Question 14

What is σh?

Answer 14

Horizontal mirror plane

Question 15

What is σv?

Answer 15

Vertical mirror plane

Question 16

What is σd?

Answer 16

Dihedral mirror plane

Question 17

What is horizontal mirror plane ?

Answer 17

Perpenditular to the principle axis of rotation

Question 18

What is vertical mirror plane?

Answer 18

Contains the principle axis of rotation

Question 19

What is the dihedral mirror plane?

Answer 19

Bisects two C2 axis of symmetry and contains the principle axis

Question 20

What is i?

Answer 20

Inversion through a centre of symmetry

Question 21

When does a molecule have an inversion centre?

Answer 21

Has pairs of atoms that a diametrically opposite to each other

Question 22

What are some examples of molecules with inversion centres?

Answer 22

CO₂
SF₆
Staggered ethane

Question 23

Can tetrahedral molecules have a centre of inversion?

Answer 23

No

Question 24

Does water have a centre of inversion?

Answer 24

No

Question 25

What is an improper rotation?

Answer 25

Rotation of the molecule through a certain angle around an axis followed by a reflection in the plane perpendicular to that axis

Question 26

What is Sn?

Answer 26

Improper axis of rotation

Question 27

What does n in Sn refer to?

Answer 27

Order of rotation

Question 28

What does an S₁ improper axis correspond to?

Answer 28

Reflection (σ)

Question 29

What does an S₂ improper axis correspond to?

Answer 29

Inversion (i)