Topic 2 Valence Bond & Topic 2: Molecular Orbital Theory: Homonuclear Diatomics

Question 1

How to tell the electronegativity of an atomic pair.
This shows how much … and … character is has.
The closer to … the more … character.

Answer 1

Δχ = χx – χY
covalent
ionic
0
ionic

Question 2

Lewis Theory

Answer 2

Method of describing the arrangement of valence electrons in molecules.
Uses dots and crosses to represent the number of valence electrons associated with the nuclei.
If possible, all electrons in a molecule should appear in pairs.
Single electrons signify a radical species.

Question 3

Why can it be energetically favourable to promote electrons to a higher energy orbitals

Answer 3

Bond formation gains energy for the system (releases energy)

Question 4

Hybrids formed for:
s + p
s + 2p
s+ 3p

Answer 4

2 sp
3 sp2
4 sp3

Question 5

Molecular orbital theory

Answer 5

Current approach to describe bonding in molecules and predict trends such as bond length and dissociation energy differences, magnetism and chemical reactivity (not geometry)
Atoms have AO’s, some with electrons and some without, so
molecules must have MO’s
Electrons go into Molecular Orbitals and these are filled from the lowest energy state progressively upwards

Question 6

MO theory has a … as an electron in
an Atomic Orbital A can be described by a … (the mathematical function that replaces the classical concept of trajectory andwhose square defines probability of position), A,
so too can an electron in a Molecular Orbital of amolecule AB by a wavefunction AB

Answer 6

quantum mechanical approach
wavefunction

Question 7

How to Determine Molecular Wavefunctions?

Answer 7

Solve Schrödinger’s equation for
exactly 1 electron system like H2+,
More complex, use the Linear Combination of Atomic Orbitals
(LCAO) approach

Question 8

The LCAO Approach Interaction criteria:

Answer 8

1. the energies of the atomic orbitals must be similar
2. the symmetries of the atomic orbitals must be compatible
3. the overlap between the atomic orbitals must be significant

Question 9

Place the atomic nuclei of the molecule in their …
Consider the Atomic Orbitals and determine ways that they can interact to form Molecular
Orbitals through …
The total number of MO’s to be created is equal
to the number of AO’s in the basis set
If 2 AO’s combine to form a new …, they will also form a new …
Antibonding MO’s have a … between the nuclei.
When all MO’s have been constructed they can be ranked in energy to create the … which is then filled with …

Answer 9

equilibrium (lowest energy) positions
simple linear combinations via addition and subtraction
bonding MO
antibonding MO
nodal plane (change in phase)
MO Energy Level Diagram
electrons

Question 10

Aufbau Principle

Answer 10

Orbitals of lowest energy are filled first

Question 11

Pauli Exclusion Principle

Answer 11

no two electrons in the same atom can have identical values for all four of their quantum numbers

Question 12

Hund’s Rule

Answer 12

if there are several orbitals of the same energy (degenerate) then the electrons will arrange themselves so that there is the maximum number
of unpaired electrons

Question 13

The number of AO’s and MO’s is …

Answer 13

the same

Question 14

Bonding MO’s leads to … electron density between nuclei while antibonding MO’s leads to …

Answer 14

more
less

Question 15

The MO probability function for (H2+) =
 is wave function

Answer 15

2(H2+)

Question 16

1 =
1* =
 is wave function

Answer 16

N(c1A + c2B)
N(c1A - c2B)
N and N are normalisation constants to ensure the probability of finding an electron in the orbital over all space = 1
c1 and c2 are overlap coefficients that reflect the “parentage” of the MO from the AO’s

Question 17

Orbitals that are symmetric about the internuclear axis are labelled as …
Orbitals that are asymmetric about the internuclear axis are labelled as …
Antibonding orbitals are labelled with a …, have a … between the
nuclei and the plane is orthogonal to the internuclear axis

Answer 17

sigma
pi
*
nodal plane (plane of zero probability)

Question 18

bond order =
Stable molecular have a bond order …
Bond order is proportional to … and …

Answer 18

(number of electron in bonding MO - anti bonding MO) / 2
>0
bond dissociation energy
1/bond length

Question 19

s-s overlap forms

Answer 19

bonding orbital sigma
anti bonding orbital sigma*

Question 20

s-p overlap forms

Answer 20

s + head on p (e.g. pz) :
bonding orbital pi
anti bonding orbital pi*

when s + side on p (e.g. px) :
non-bonding

Question 21

p-p overlap forms

Answer 21

When head on (e.g. pz + pz) :
bonding orbital sigma
anti bonding orbital sigma*

When side on (e.g px + px/py + py) :
bonding orbital pi
anti bonding orbital pi*

When no overlap (e.g. px + pz) :
non-bonding

Question 22

Both sigma/pi and sigma */pi * orbitals have … while only … has …

Answer 22

internuclear axis
sigma/pi
nodal plane perpendicular to internuclear axis

Question 23

MO energy ranking

Answer 23

sigma 1s
sigma* 1s
sigma 2s
sigma* 2s
sigma 2pz
pi 2px and pi 2py
pi* 2px and pi* 2py
sigma* 2pz