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MED 102- GENERAL CHEMISTRY > 11. BONDING THEORIES > Flashcards

11. BONDING THEORIES Flashcards

1
Q
  1. 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
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2
Q
  1. 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

ATOMIC ORBITALS:
- the way that the electron cloud is distributed around the
nucleus

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3
Q
  1. 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

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4
Q
  1. What does a bigger overlap of the two atoms result in?
A
  • it results in a stronger bond between the two atoms
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5
Q
  1. 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
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6
Q
  1. Does Methane have hybrid (blended) atomic orbitals?
A
  • it does
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7
Q
  1. 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
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8
Q
  1. 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
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9
Q
  1. 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
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10
Q
  1. 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
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11
Q
  1. What shape is the 2p orbital?
A

-it is a dumbbell shape
- it is equal on both ends

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12
Q
  1. 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
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13
Q
  1. 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
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14
Q
  1. What does a bigger orbital size lead to?
A
  • it leads to a greater overlap
  • and a formation of a stronger bond
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15
Q
  1. What is released when a chemical bond is formed?
A
  • energy is released
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16
Q
  1. When is energy used up?
A
  • when bonds are being broken
17
Q
  1. 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
  1. 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
  1. 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
  1. 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
  1. 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
  1. 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
  1. 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
  1. 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
  1. What are Pi (π) bonds?
A
  • they are involved in double and triple covalent bonds
26
Q
  1. What does a double bond consist of?
A
  • it consists of one sigma bond and one pi bond
27
Q
  1. What does a triple bond consist of?
A
  • one sigma bond
  • 2 pi bonds
28
Q
  1. 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
  1. What bonds does Ethylene (Ethene) have?
A
  • it has 5 sigma bonds
  • it has one pi bond
30
Q
  1. 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
  1. 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
  1. 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
  1. 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
  1. 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
  1. Read this summary.
    Does everything make sense?
A
  • yes
36
Q
  1. Read through this summary.
    Does everything make sense?
A
  • yes

MED 102- GENERAL CHEMISTRY (27 decks)

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