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chapter 10 Flashcards

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1
Q

what is VSEPR? what does it predict?

A

Valence shell electron pair repulsion - predicts the 3D
geometry of a molecule
• We need the correct Lewis structure of compounds
what shape minimizes the repulsion

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2
Q

what determines the shape for a compound with one central atom?

A

• For a compound with one central atom the shape is
determined by the number of electron groups around the
central atom

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3
Q

what are electron groups counted as?

A

Electron groups are counted as a lone pair or a
bonding group - single/double/triple bonds all count
as 1 bonding group

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4
Q

what is electron geometry?

A

the geometric arrangement of

electron groups around the central atom

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5
Q

what dictates the type of electron geometry?

A

The type of electron geometry is dictated by the number

of electron groups around the central atom (2-6 groups)

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6
Q

what is molecular geometry?

A

the geometric arrangement of

the bonding groups around the central atom

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7
Q

what does molecular geometry describe? determined by?

A

• This geometry describes the actual 3D shape of the
molecule
• This geometry is determined by the distribution of lone
pairs and bonding groups

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8
Q

what happens with two electron groups?

A

no lone pairs
linear geometry (electron &
molecular)

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9
Q

what is linear geometry determined by? max repulsion?

A

!determined by repulsion between the two groups
!can get maximum repulsion at 180 degrees between
the two
!molecules with single bonds and no extra electron
groups is rare, more common are molecules with
double or triple bonds

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10
Q

what happens with 3 electron groups? how can they get max separation? why do distortions occur?

A

Trigonal Planar
Can get maximum separation between the three groups
if they are 120 degrees apart
Even with no lone pairs on the central atom, distortions
can occur when the peripheral atoms contain lone pairs

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11
Q

what happens with 4 electron groups? how far apart are the atoms?

A

Tetrahedral
• Harder to visualize because the 3D shape is not easily
represented in 2D
• A tetrahedral geometry has atoms 109.5 degrees apart
from each other

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12
Q

what does the four electron groups look like? why isn’t it a square?

A

!Can be represented by an equilateral triangle/pyramid
(tetrahedron)
!We might assume that it would form a square planar
geometry, but then the atoms only have 90 degrees
separation from each other

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13
Q

describe 5 electron groups

A

Trigonal Bipyramidal
• Three atoms are in the equatorial position and two are in
the axial position
• The axial atoms are 90 degrees apart from the equatorial
groups
• The equatorial groups are 120 degrees apart from each
other

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14
Q

describe 6 electron groups (max)

A

Octahedral
• In this arrangement all 6 atoms are in equivalent
positions, no equatorial or axial positions
• All atoms are 90 degrees from each other

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15
Q

what happens when an electron group is replaced by a lone pair?

A

When an electron group is replaced by a lone pair on the
central atom the typical geometries are distorted
• Lone pairs have more repulsion than electron groups

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16
Q

what is electron geometry an arrangement of?

A

arrangement of electron groups

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17
Q

what is molecular geometry an arrangement of?

A

arrangement of atoms

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18
Q

(with lone pairs) which geometry changes? stays the same?

A

The electron geometry stays the same, but the

molecular geometry changes

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19
Q

what does electron geometry influence?

A

• The electron geometry influences the molecular

geometry as we see with the bond angles

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20
Q

why do lone pairs occupy more space? what are bonding electrons attracted to?

A

lone pairs occupy more space than bonding electrons
because they are attracted to only one nucleus
• Bonding electrons are simultaneously attracted to both
nuclei involved in the bond

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21
Q

hierarchy (distortion of bond angles, only referring to bond angle between atoms not lone pairs )

A

lone pair-lone pair> lone pair-bonding pair> bonding

pair-bonding pair

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22
Q

what happens with 3 electron groups with 1 lone pair? ADD ANGLES (less than)

A

• Electron geometry is trigonal planar, but lone pair takes
up more space
• Resulting molecular geometry is bent, with the angle
between the two bonding groups <120º away from each
other

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23
Q

max number of lone pairs for a tetrahedral electron geometry? direction of distortions? repulsion between lone pairs?
(less than 109)

A

Can have a maximum of 2 lone pairs around the central
atom
• Distortions away from tetrahedral geometry become
more pronounced as the number of lone pairs increase
• The lone pairs have more repulsion and push the
bonding groups closer together

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24
Q 
what happens with 1 lone pair for 5 electron groups?
ADD ANGLES (less than)
A

1 Lone pair can either occupy axial or equatorial position

• More repulsions when in the axial position

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25
Q 
what happens with 2 lone pair for 5 electron groups?
ADD ANGLES (less than)
A

2 lone pairs also occupy the equatorial position (120

degrees apart)

26
Q 
what happens with 3 lone pair for 5 electron groups?
ADD ANGLES (less than)
A

With 3 lone pairs they all occupy equatorial positions

and the molecule is linear

27
Q 
what does 1 lone pair for a 6 electron group result in? 2?
ADD ANGLES (less than)
A

1 lone pair results in square pyramidal shape
• 2 lone pairs result in square planar with both lone pairs
in the axial position

28
Q

review drawing, table in textbook

A

ok

29
Q

what does polarity depend on?

A

We know that bonds can be polar, depending on the
electronegativity difference between atoms
Depending on the molecular geometry the entire
molecule can be polar

30
Q

what are dipole moments?

A

• Dipole moments are vector quantities and can be added

up

31
Q

4 steps to determine polarity

A
  1. Draw a Lewis structure for the molecule
  2. Determine the molecular geometry
  3. Determine whether a molecule contains polar bonds
  4. Determine whether the sum of the polar bonds cancel
    out
32
Q

VSPER theory summary

A

The geometry of a molecule is determined by the number
of electron groups on the central atom
• The number of electron groups can be determined from
the Lewis structure of the molecule
• Each of following count as a single electron group: a lone
pair, a single bond, a double bond, a triple bond, or a
single electron
• Geometry of the electron groups is determined by
repulsion
!lone pair-lone pair> lone pair-bonding pair> bonding pair bonding pair
• Bond angles vary from idealized angles because double
and triple bonds (and lone pairs) occupy more space

33
Q

what is the basic principle of valence bond theory?

A

a covalent bond forms when two orbitals of two atoms overlap and a pair of electrons occupies the overlap region

34
Q

2 central themes of VB theory?

A 
The space formed by the
overlapping orbitals has a
maximum capacity of two
electrons that have opposite
spin
• The greater the orbital overlap,
the stronger the bond
35
Q

what is the energy of a vb reaction usually like?

A

The energy of interaction is usually negative (stabilizing)
when the interacting atomic orbitals contain a total of
two electrons that can spin-pair
• Commonly two half-filled orbitals that then spin-pair

36
Q

why is there hybridization?

A

The simplistic method cannot explain bonding in all
types of molecule• Orbitals in a molecule are not the same as orbitals in an
atom

37
Q

what are orbitals? (hybridization)

A

mathematical functions
that describe where an electron is likely to be
• If we combine these functions, we derive a new set of
equations which can describe the orbitals in bonding
atoms

38
Q

what are the new orbitals? what is the process of mixing them called?

A

These new orbitals are a mixture of the existing atomic
orbitals
• The process of mixing atomic orbitals to form new
orbitals is called hybridization
!Results in hybridized orbitals

39
Q

how do hybrid orbitals minimize the energy of molecules?

A

Orbitals are still localized on individual atoms, but with
different shapes and energies
• Hybrid orbitals minimize the energy of molecules by
maximizing orbital overlap

40
Q

2 rules of hybrid orbitals

A
  1. Number of standard atomic orbitals added together
    always equals the number of hybrid orbitals formed
  2. Particular combination of standard atomic orbitals
    added together determine the shapes and energies of
    the hybrid orbitals formed
41
Q

what is the shape of sp3? what is it a hybrid of?

A

Tetrahedral shape of CH4 can be explained if we

hybridize the 2s and 2p orbitals

42
Q

are the new orbitals degenerate? describe carbons sp3

A

These new orbitals are degenerate (same energy)
• Carbon’s 4 sp3 orbitals can bond with 4 H atoms
• Ammonia (NH3) also has sp3 hybridized orbitals

43
Q

LOOK AT PICS FOR ORBITAL HYBRIDIZATION OF CARBON AND FORMATION OF METHANE

A

OK

44
Q

what combines to form sp2 hybridization? shape?

A 
One s and 2 p
orbitals combine
to form sp2
hybridization
• Orbitals have
trigonal planar
geometry
45
Q

describe sp hybridization and shape

A 
One s and one p
orbital, leaves
two unhybridized
p orbitals
• Orbitals have
linear geometry
46
Q

what is sp3d used for? what does it need for row 3+?

look at diagrams

A 
Used for molecules with 5
electron groups
• Need empty d-orbitals, for
non-metals in row 3 and
greater
47
Q

quantum number of arsenic unhybridized and hybridized

A

4s 4p (empty d)

4sp3d ( one in each of 5 boxes)

48
Q

when is sp3d2 used? requires?

A 
Molecules with 6-electron
groups
• Also requires empty d orbitals
• Use non-metals in row 3
and greater
49
Q

hybridization of sulfur

A

3s 3p (empty d) becomes 3sp3d2 (one in each of 6 boxes)

50
Q

look at hybridization scheme chart

A

ok

51
Q

what does vb explain

A

VB Theory explains that a covalent bond forms when
two atomic orbitals overlap and two electrons with
paired (opposite) spins spend more time in the
overlapped regions

52
Q

how does vb theory explain molecular shape? practice hybridization questions

A

To explain molecular shape, the theory proposes that,
during bonding, atomic orbitals mix to form hybrid
orbitals with a different shape and direction. This
process gives rise to greater orbital overlap, and thus
stronger bonds
• Based on the observed molecular shape, the type of
hybrid orbital accounts for the shape

53
Q

what are sigma bonds? where is the highest electron density

A

Single bonds are formed from end-to-end overlap of
orbitals
• Sigma bonds have their highest electron density along
the bond axis and is shaped liked an ellipse
• All single bonds are σ bonds

54
Q

how are pi bonds formed?

A 
The end-to-end
overlap of sp2
hybridized orbitals
between O and C
forms a sigma
bond
• Side to side
overlap of
unhybridized
orbitals forms a pi
(π) bond
55
Q

what bond is formed from pi bonds?

A

A triple bond is formed from one sigma-bond of
overlapping sp orbitals, and two pi bonds from two
side-by-side unhybridized p-orbitals
• Acetylene has a carbon carbon triple bond

56
Q

pi bonding in acetylene

A 
In acetylene, both carbons are sp hybridized
Unhybridized atom
2s 2p
↑↓ ↑ ↑
sp hybridized atom
2sp
↑ ↑ ↑ ↑
2p
Two unpaired electrons
are in unhybridized p orbitals
Lewis structure of acetylene
A triple bond consists
of a σ-bond between
sp hybridized orbitals
and two π-bonds
between unhybridized
p-orbitals
57
Q

vb theory bonding summary

A 
End-to-end overlap of
orbitals forms a sigma (σ)
bond (from hybridized sp/
sp2/sp3, or unhybridized)
• The side to side overlap of
half-filled p-orbitals forms a
pi (π) bond
• A double bond always consists of a σ-bond and a πbond
• A triple bond always consists of a σ-bond and two πbond
58
Q

5 steps to predict hybridization and bonding scheme

A
  1. Start by drawing the Lewis structure
  2. Use VSEPR Theory to predict the electron group
    geometry around each central atom
  3. Select the hybridization scheme that matches the
    electron group geometry
  4. Sketch the atomic and hybrid orbitals on the atoms
    in the molecule, showing overlap of the appropriate
    orbitals
  5. Label the bonds as σ or π
59
Q

what are the limitations of vb theory?

A

• Predicts some behaviours better than Lewis Theory
• There are still some properties it cannot predict
!magnetic behaviour of O2
• VB theory predicts that electrons are localized inside of
the orbitals
! It doesn’t account for electron delocalization

60
Q

steric number and electron geometry

A 
Electron regions
(steric number)	Electronic geometry
2	linear
3	trigonal planar
4	tetrahedral
5	trigonal bipyramidal
6	octahedral
61
Q

how is the steric number found?

A

covalent bonds + lone pairs on electron

62
Q

bond angles

A

Electron regions
(steric number) Electronic geometry Bond angles
2 linear 180∘
3 trigonal planar 120∘
4 tetrahedral 109.5∘
5 trigonal bipyramidal 90∘, 120∘, and 180∘
6 octahedral 90∘ and 180∘

chem 101 (13 decks)

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