12. MOLECULAR ORBITAL THEORY Flashcards
1
Q
- What does molecular Orbital Theory focus on?
A
- it focuses on the chemical bonds formed by placing electrons in molecular orbitals
- it looks at the electron arrangement in molecules
2
Q
- What do Molecular Orbitals cover?
A
- they cover the entire molecules
3
Q
- What do Molecular orbitals allow for?
A
- excited electronic states
- types of bonds (single, double, triple) to be determined
- allow for magnetic properties of the molecules to be determined
(does it go towards or away from the magnetic field)
4
Q
- What happens when 2 1s atomic orbitals are joined together mathematically?
A
- they form two molecular orbitals
5
Q
- What type will these two Molecular Orbitals be?
Where will they be located?
A
- they will be of sigma (σ) type
- one will be at lower energy
- one will be at higher energy
6
Q
- Which molecular orbital is at higher energy?
How is this indicated?
A
- the anti-bonding orbital will be at higher energy
- it is indicated by an asterisk
7
Q
- Which molecular orbital will be at lower energy?
A
- the bonding orbital
8
Q
- Where is the electron density concentrated between two bonding molecular orbitals?
What does this promote?
A
- it is between the two nuclei
- it forms an ellipsoid electron cloud
- this promotes bond formation
9
Q
- What happens with the electron density in anti-bonding Molecular Orbitals?
What does this promote?
A
- it is pushed away from the centre
- this promotes bond destruction
10
Q
- Where do we place the two Hydrogen electrons in a hydrogen molecule?
A
- we place 2 electrons starting with the lowest bonding energy
11
Q
- What exists between the two electron densities in the anti-bonding molecular orbital that pushes them away from the middle?
A
- a node
12
Q
- Why does helium prefer to stay not bonded (Monatomic)?
A
- the average energy of the electrons is higher when the helium is diatomic
- this makes it more unstable
- there is no energetic benefit to helium being bonded
13
Q
- How can Bond Order be Calculated?
A
- Bond Order= 1/2 (NB- NAB)
- NB= the number of bonding electrons
- NAB= the number of anti bonding electrons
14
Q
- What do the following values of Bonding orders mean:
14.1: BO= 0
14.2 BO= 1
14.3: BO= 2
14.4: BO= 3
14.5: BO= 1/2
A
14.1: there is no bond formed
: this means that there is no molecule
: the element exists as individual atoms
14.2: there is a single bond formed
14.3: there is a double bond formed
14.4: there is a triple bond formed
14.5: it is a half integer
15
Q
- How would you put the electrons of the Hydrogen molecule into an excited electron state?
A
- in Hydrogen’s ground electron state:
- it has a bond order of 1 - to put the Hydrogen electrons into an excited state:
- you would promote an electron to the anti-bonding
molecular orbital
- you separate the two bonded electrons by doing this
- the bond order now becomes zero
- the molecule splits up
16
Q
- What would happen if we were to promote a hydrogen electron to the σ₂s Molecular Orbital?
A
- we would get a different excited state
- ti would have a Bond Order of 1
- this means that the molecule would still exist
17
Q
- What is the Bond Order for the H2+ Molecular Ion?
A
- Number of electrons:
2-1 = 1 - there is only one electron in the H2+
- it is placed in the σ1s molecular orbital
- this is a bonding orbital
- Bond Order: 1/2 (1-0)
: this is equal to a half
18
Q
- What does a “half-bond” mean?
A
- the covalent bond has approximately half the strength of a single covalent bond
- ions produce half integer bonds
19
Q
- What happens when we combine two 2s atomic orbitals?
A
- we get two molecular orbitals:
- a σ2s Bonding orbital
- a σ2s﹡Anti-Bonding Orbital
20
Q
- How would we bond two lithium atoms?
A
- lithium has 3 electrons
- two lithium atoms have 6 electrons
- we start by:
- placing the electrons in the molecular orbitals from
lower to higher energy - we put 2 electrons in the σ1s bonding orbital
- we put two electrons in the σ1s﹡ anti-bonding orbital
- the last two go in the σ2s bonding orbital
- placing the electrons in the molecular orbitals from
- Bond Order= 1/2 (4-2)
= 1
= one single bond
21
Q
- What happens when we combine 2p atomic orbitals?
A
- when the 2p (dumbbell shaped) atomic orbitals come together head on:
- they form sigma type Molecular Orbitals
- σ2s and σ2s﹡
22
Q
- What happens when we combine 2p atomic orbitals that are parallel to each other?
A
- they form pi-type Molecular Orbitals
- two π2p bonding molecular orbitals will be formed
- two π2p﹡anti-bonding molecular orbitals will be
formed
23
Q
- How many molecular orbitals would we have if six 2p atomic orbitals were combined?
A
- we would have six
24
Q
- Do Molecular Orbital Configurations for Diatomic Molecules show valence shell (n=1) electrons?
A
- no
- they also do not show the σ1s and the σ1s﹡ molecular orbitals
25
Q
- Does the Hund’s rule apply to Molecular orbitals?
A
- it does
- when two electrons are to be placed in orbitals of equal energy
- they need to be placed spin up
- and in separate orbitals
- this is the case for B2 and O2
26
Q
- What is the Molecular Orbitals diagram for the
O2 2+ molecular ion?
A
27
Q
- What is the bond order for the O2 2+ ion?
A
Bond Order= 1/2 (NB - NAB)
= 1/2 (8-2)
= 3
- there is a triple covalent bond present
28
Q
- What produces stronger bonds in Molecular Orbitals?
A
- removing electrons from the anti-bonding orbitals
- adding electrons to the bonding orbitals
29
Q
- Define Paramagnetic.
A
- being attracted to a magnetic field
30
Q
- Define Diamagnetic.
A
- being weakly repelled by a magnetic field
31
Q
- What is a Paramagnetic molecule?
A
- it is a molecule with one or more unpaired electrons
32
Q
- What is a Diamagnetic molecule?
A
- it is a molecule where all the electrons are paired up
33
Q
- Read through this summary.
Does everything make sense?
A
- yes
