Diatomics

Question 1

What is meant by paramagnetic?

Answer 1

This is a molecule that contains unpaired electrons

Question 2

What is meant by diamagnetic?

Answer 2

These are molecules that have all paired electrons

Question 3

What is an isoelectronic species?

Answer 3

These are species that have the same number of electrons or the same electronic configuration

Question 4

What MOs do these AOs form? 
S+S
Pz+pz
Px+px 
Py+py

Answer 4

S forms sigma
Pz forms sigma
(Sigma is end on overlap)
Px forms pi 
Py forms pi 
(Pi is side on overlap)

Question 5

What are parity label?

Answer 5

These refer to the symmetry of the orbital with respect to the mathematic operation of inversion
Refers to the symmetry of orbital phase

Question 6

What is inversion?

Answer 6

When an orbital is inverted, every point at coordinates (xyz) is mapped on to a point at coordinated (-x, -y, -z)

Question 7

What is a g type orbital ?

Answer 7

This is an orbital that looks identical after inversion
+->+
Symmetric with respect to inversion

Question 8

What is a u type orbital?

Answer 8

This is an orbital that keeps the same shape but changes its phase
+->-
Antisymmetric with respect to inversion

Question 9

How does symmetry determine which AOs can interact?

Answer 9

Only AOs with the same symmetry around the internuclear can interact

Question 10

Why could S react with pz but not px or py

Answer 10

This is because orbitals with the same symmetry can interact
Both S and pz are cylindrically symmetrical around the internuclear axis which means they could interact
Px and py have the wrong symmetry to interact

Question 11

How do you calculate overlap integral S?

Answer 11

S = f¥A¥B dt

S=0 means wrong symmetry

Question 12

What is a heteronuclear molecule ?

Answer 12

This describes a molecule composed of different atoms

Question 13

What is valence shell ionisation energy?

Answer 13

This is the energy that is required to ionise an electron for a given orbital

Question 14

How do heteronuclear MD differ from homonuclear?

Answer 14

The AOs will be at different energies and so sigma bonding and antibonding are unequally shared

Question 15

How can you determined AO energies for heteronuclear molecules

Answer 15

1) look up AO IE to see energy of AO
2) more electronegative atoms will hold onto electrons more tightly which means their HUMOS will be lower in energy
The atoms with the higher zeff means their orbitals will be lower in energy as they are held closer to nucleus

Question 16

How can you determine the sharing of sigma bonding and antibonding orbitals between the two atoms of a heteronuclear diatomic?

Answer 16

The AO that has closer energy to the particular MO will contribute more to it

Question 17

Describe the energy positions of the sigma and sigma antibonding orbitals relative to the AOs of a heteronuclear diatomic

Answer 17

The sigma antibonding lies closest to the highest energy AO
The sigma bonding lies closest to the lowest energy AO
The AO orbital that the MO is closest to will contribute more to it

Question 18

How can you mathematically represent the % of AO in the MO

Answer 18

C is the coefficient and tells you about the % of AO in the MO

¥bonding= Cx¥x + Cy¥y
¥antibonding= Cx¥x - Cy¥y
( this shows x is more cantered)

Question 19

How do yo build an MO diagram for a heteronuclear diatomic?

Answer 19

1) list valence orbitals
2) atom with greatest zeff, orbitals lie lower in energy
3) identify AOs that can overlap
4) MOs on inside, AOs on outside
Sigma bonding MO lies closest to lowest AO, antibonding lies closest to highest energy AO
5) add electrons

Question 20

Why will orbitals not overlap?

Answer 20

Not a good match in energy
Different sizes
Poor overlap and weak bonds

Question 21

Describe the trend in 2s orbitals in energy

Answer 21

2s orbitals decrease in energy as you go along the group

Question 22

Why will a pz orbital overlap with an S orbital

Answer 22

If the 2s orbital on the other atom is too low in energy, it won’t interact
This means the 2s will interact with the pz as it as more similar energy
Correct shape

Question 23

Why will 1s and 2s not interact?

Answer 23

Although they have the correct shape and symmetry, the energy gap is too large to interact

Question 24

Why is the 1s orbital high in energy?

Answer 24

It has a small zeff so not held as close to the nucleus
This means it will interact with pz rather than 2s as they are more similar in energy as the 2s will on another atom will be a lot lower in energy

Question 25

What MOs do orbitals that don’t interact form?

Answer 25

They form non bonding MOs which do not contribute to bond order
This is if the AOs have a mismatch in symmetry or too different energies

Question 26

Describe the energy of non bonding MOs relative to the AOs

Answer 26

They have the same energy