chapter 10 Flashcards
what is VSEPR? what does it predict?
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
what determines the shape for a compound with one central atom?
• For a compound with one central atom the shape is
determined by the number of electron groups around the
central atom
what are electron groups counted as?
Electron groups are counted as a lone pair or a
bonding group - single/double/triple bonds all count
as 1 bonding group
what is electron geometry?
the geometric arrangement of
electron groups around the central atom
what dictates the type of electron geometry?
The type of electron geometry is dictated by the number
of electron groups around the central atom (2-6 groups)
what is molecular geometry?
the geometric arrangement of
the bonding groups around the central atom
what does molecular geometry describe? determined by?
• This geometry describes the actual 3D shape of the
molecule
• This geometry is determined by the distribution of lone
pairs and bonding groups
what happens with two electron groups?
no lone pairs
linear geometry (electron &
molecular)
what is linear geometry determined by? max repulsion?
!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
what happens with 3 electron groups? how can they get max separation? why do distortions occur?
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
what happens with 4 electron groups? how far apart are the atoms?
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
what does the four electron groups look like? why isn’t it a square?
!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
describe 5 electron groups
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
describe 6 electron groups (max)
Octahedral
• In this arrangement all 6 atoms are in equivalent
positions, no equatorial or axial positions
• All atoms are 90 degrees from each other
what happens when an electron group is replaced by a lone pair?
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
what is electron geometry an arrangement of?
arrangement of electron groups
what is molecular geometry an arrangement of?
arrangement of atoms
(with lone pairs) which geometry changes? stays the same?
The electron geometry stays the same, but the
molecular geometry changes
what does electron geometry influence?
• The electron geometry influences the molecular
geometry as we see with the bond angles
why do lone pairs occupy more space? what are bonding electrons attracted to?
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
hierarchy (distortion of bond angles, only referring to bond angle between atoms not lone pairs )
lone pair-lone pair> lone pair-bonding pair> bonding
pair-bonding pair
what happens with 3 electron groups with 1 lone pair? ADD ANGLES (less than)
• 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
max number of lone pairs for a tetrahedral electron geometry? direction of distortions? repulsion between lone pairs?
(less than 109)
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
what happens with 1 lone pair for 5 electron groups? ADD ANGLES (less than)
1 Lone pair can either occupy axial or equatorial position
• More repulsions when in the axial position
what happens with 2 lone pair for 5 electron groups? ADD ANGLES (less than)
2 lone pairs also occupy the equatorial position (120
degrees apart)
what happens with 3 lone pair for 5 electron groups? ADD ANGLES (less than)
With 3 lone pairs they all occupy equatorial positions
and the molecule is linear
what does 1 lone pair for a 6 electron group result in? 2? ADD ANGLES (less than)
1 lone pair results in square pyramidal shape
• 2 lone pairs result in square planar with both lone pairs
in the axial position
review drawing, table in textbook
ok
what does polarity depend on?
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
what are dipole moments?
• Dipole moments are vector quantities and can be added
up
4 steps to determine polarity
- Draw a Lewis structure for the molecule
- Determine the molecular geometry
- Determine whether a molecule contains polar bonds
- Determine whether the sum of the polar bonds cancel
out
VSPER theory summary
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
what is the basic principle of valence bond theory?
a covalent bond forms when two orbitals of two atoms overlap and a pair of electrons occupies the overlap region
2 central themes of VB theory?
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
what is the energy of a vb reaction usually like?
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
why is there hybridization?
The simplistic method cannot explain bonding in all
types of molecule• Orbitals in a molecule are not the same as orbitals in an
atom
what are orbitals? (hybridization)
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
what are the new orbitals? what is the process of mixing them called?
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
how do hybrid orbitals minimize the energy of molecules?
Orbitals are still localized on individual atoms, but with
different shapes and energies
• Hybrid orbitals minimize the energy of molecules by
maximizing orbital overlap
2 rules of hybrid orbitals
- Number of standard atomic orbitals added together
always equals the number of hybrid orbitals formed - Particular combination of standard atomic orbitals
added together determine the shapes and energies of
the hybrid orbitals formed
what is the shape of sp3? what is it a hybrid of?
Tetrahedral shape of CH4 can be explained if we
hybridize the 2s and 2p orbitals
are the new orbitals degenerate? describe carbons sp3
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
LOOK AT PICS FOR ORBITAL HYBRIDIZATION OF CARBON AND FORMATION OF METHANE
OK
what combines to form sp2 hybridization? shape?
One s and 2 p orbitals combine to form sp2 hybridization • Orbitals have trigonal planar geometry
describe sp hybridization and shape
One s and one p orbital, leaves two unhybridized p orbitals • Orbitals have linear geometry
what is sp3d used for? what does it need for row 3+?
look at diagrams
Used for molecules with 5 electron groups • Need empty d-orbitals, for non-metals in row 3 and greater
quantum number of arsenic unhybridized and hybridized
4s 4p (empty d)
4sp3d ( one in each of 5 boxes)
when is sp3d2 used? requires?
Molecules with 6-electron groups • Also requires empty d orbitals • Use non-metals in row 3 and greater
hybridization of sulfur
3s 3p (empty d) becomes 3sp3d2 (one in each of 6 boxes)
look at hybridization scheme chart
ok
what does vb explain
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
how does vb theory explain molecular shape? practice hybridization questions
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
what are sigma bonds? where is the highest electron density
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
how are pi bonds formed?
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
what bond is formed from pi bonds?
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
pi bonding in acetylene
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
vb theory bonding summary
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
5 steps to predict hybridization and bonding scheme
- Start by drawing the Lewis structure
- Use VSEPR Theory to predict the electron group
geometry around each central atom - Select the hybridization scheme that matches the
electron group geometry - Sketch the atomic and hybrid orbitals on the atoms
in the molecule, showing overlap of the appropriate
orbitals - Label the bonds as σ or π
what are the limitations of vb theory?
• 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
steric number and electron geometry
Electron regions (steric number) Electronic geometry 2 linear 3 trigonal planar 4 tetrahedral 5 trigonal bipyramidal 6 octahedral
how is the steric number found?
covalent bonds + lone pairs on electron
bond angles
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∘
