11. BONDING THEORIES Flashcards
1
Q
- What is the Valence Bond Theory?
A
- it is the theory that states that:
- chemical bonds are formed by the overlap of atomic
orbitals on adjacent atoms - the atoms are brought very close together
- and are then overlapped
2
Q
- What is the Molecular Orbital Theory?
A
- it is the theory that states:
- chemical bonds are formed by placing electrons in
molecular orbitals - analogues of atomic orbitals for molecules
- it is the joining of atomic orbitals
- chemical bonds are formed by placing electrons in
ATOMIC ORBITALS:
- the way that the electron cloud is distributed around the
nucleus
3
Q
- Explain the Valence bond theory of joining H2 ?
A
- one H atom has a spin up electron
- the other has a spin down electron
- the two atoms overlap due to the attractive charges pulling them together
- there are electrons present in the shared region of the two overlapping atoms
- they bond covalently
- this produces an H2 molecule
NB: an unpaired electron is required for a chemical bond to form
4
Q
- What does a bigger overlap of the two atoms result in?
A
- it results in a stronger bond between the two atoms
5
Q
- What happens if there are two unpaired, spin up electrons in the atoms that want to chemically bond?
A
- as these two atoms move closer to each other
- the spin up electron of the one atom will have to change its direction
- it will turn into a spin down electron
- the two can now co-exist in the same shared region
6
Q
- Does Methane have hybrid (blended) atomic orbitals?
A
- it does
7
Q
- Explain how Carbon can bond with hydrogen to form Methane?
A
- According to the Valence Bond theory, unpaired electrons in the two orbitals that will overlap are needed to form a covalent bond
- Carbon in its ground has two unpaired spin up electrons
- this means that it can form only two covalent bonds
- to bond with H4, it needs 4 unpaired electrons
- we can promote a 2s electron into the empty 2p orbital
- this will require us to use energy
- this produces 4 unpaired electrons
8
Q
- What is Promotion?
A
- it is when we move an electron out of a lower energy orbital into a higher energy orbital
- we do this to produce unpaired electrons that are ready to form covalent bonds
9
Q
- Are all the orbitals in CH4 the same after we have promoted an electron?
A
- no, they are not
- there is one 2s electron and three 2p electrons
- the 2s orbital is smaller
- the bonds are not all identical
- we need to blend the one 2s orbital with the three 2p orbitals
- this would produce a sp3 orbital
10
Q
- What process makes the bonds/orbitals identical?
A
- hybridization
- this is also known as blending
- we blend together the electron density of different atomic orbitals
- this forms an equal number of hybrid orbitals
11
Q
- What shape is the 2p orbital?
A
-it is a dumbbell shape
- it is equal on both ends
12
Q
- What shape is the sp3 orbital (the hybrid orbital)?
A
- it is a tetrahedral shape
- it has one big lobe and one small lobe
- the big lobe is involved in the overlapping
- and the formation of the covalent bond
13
Q
- Which is bigger, the big lobe in the sp3 orbital or in the 2p orbital?
A
- the big lobe of an sp3 orbital is bigger in size
- it is bigger than the lobe of an atomic 2p orbital
14
Q
- What does a bigger orbital size lead to?
A
- it leads to a greater overlap
- and a formation of a stronger bond
15
Q
- What is released when a chemical bond is formed?
A
- energy is released
16
Q
- When is energy used up?
A
- when bonds are being broken
17
Q
- How is the energy that is used to promote returned?
A
- the formation of the stronger bonds with the sp3 hybrid orbitals
makes up for the energy that was used to promote
18
Q
- How do we arrange the four sp3 orbitals in a CH4 molecule?
A
- we arrange them to a common centre
(the atomic nucleus) - the big lobes are placed as far apart from each other as possible
- this gives a basic tetrahedral shape
- all bonds now have the same length and strength
- the bond angle is 109.5°
19
Q
- What are the two problems that need to be solved when it comes to bonding that involved hybrid atomic orbitals?
A
- all the bonds need to be identical
- a tetrahedral geometry needs to be achieved
20
Q
- How many unpaired electrons does ammonia (NH3) have in its ground state?
A
- it has a valence shell configuration of 2s3 2p3
- this means that it has 3 unpaired electrons
- it can form three covalent bonds
- this means that promotion is not necessary
- as N has the correct number of unpaired electrons to bond with three hydrogen atoms
21
Q
- Should you still hybridize if there is no need for promotion?
A
- yes
- hybridization ensures that all the bonds in the molecule are identical
- it ensures tetrahedral geometry is achieved
- it is the preferred option
22
Q
- What are the two benefits of Hybridization in the case of Ammonia (NH3)?
A
- the sp3 lobes are bigger than the 2p lobes
- this leads to the formation of a strong bond
- this releases much energy
- there is a need to undergo hybridization to explain the experimentally observed molecular geometry of NH3
- the NH3 molecule has a trigonal pyramidal geometry
- this is a subset of the tetrahedral electron pair geometry
23
Q
- What is a sigma (σ) bond?
A
- it is a cylindrically symmetrical bond
- if you were to slice the sigma bond in any way
- the exposed surface would look like a perfect circle
24
Q
- Which types of bonds are all sigma bonds?
A
- all single covalent bonds
- this is where orbitals come in to each other head on
- and overlap
25
Q
- What are Pi (π) bonds?
A
- they are involved in double and triple covalent bonds
26
Q
- What does a double bond consist of?
A
- it consists of one sigma bond and one pi bond
27
Q
- What does a triple bond consist of?
A
- one sigma bond
- 2 pi bonds
28
Q
- What does a PI bond look like?
A
- it is made up of two p orbitals (dumbbells) that are parallel to each other
- these two orbitals over lap
- OR
- it looks like two charged electron clouds above and below a plane
29
Q
- What bonds does Ethylene (Ethene) have?
A
- it has 5 sigma bonds
- it has one pi bond
30
Q
- How does Ethylene (C2H4) form?
A
- the C atoms undergo promotion
- they are then hybridized to form sp2 orbitals
- one pure atomic 2p orbital is kept unhybridized
- this is necessary to form a pi bond on both C atoms
31
Q
- What is the configuration of each C atom in Ethylene?
A
C 1s2 (2sp2) 2p1
- the sp2 orbitals form the three sigma chemical bonds
- two sigma bonds with the H atoms
- one sigma bond between the two C atoms
- the 2p orbitals join together to form the pi bond
32
Q
- What kind of bonds does Acetylene have?
A
- it has a triple bond holding the two carbon atoms together
- it has a single bond holding the carbon atom and the hydrogen atom together
- it has another single bond holding the carbon and the hydrogen atom together
- it has 3 sigma bonds
- it has 2 pi bonds
33
Q
- How does Acetylene come to acquire these bonds?
A
- the C atoms undergo promotion
- they then undergo sp hybridization
- two pure atomic 2p orbitals are kept unhybridized
- this is necessary to have on both C atoms in order to form the two pi bonds
34
Q
- What is the electronic configuration of each C atom in Acetylene?
A
- C 1s2 (2sp)2 2p2
- the sp orbitals form three sigma chemical bonds
- two between the H and the C atoms
- one between the Carbon atoms
- the 2p orbitals join to form the 2 pi bonds
35
Q
- Read this summary.
Does everything make sense?
A
- yes
36
Q
- Read through this summary.
Does everything make sense?
A
- yes
