Download Important Concepts from Chapter 9 • DRAWING LEWIS ELECTRON

Important Concepts from Chapter 9
• DRAWING LEWIS ELECTRON DOT STRUCTURES
• octet rule
• resonance structures
• bond polarity (ionic, polar covalent and covalent bonds)
• ionic vs. covalent compounds
• electronegativity
• dipole vectors
• calculating formal charges and partial charges
• bond order
• bond lengths
• VSEPR and predicting shapes of molecules
• polarity of molecules
CHAPTER 10 – BONDING AND MOLECULAR STRUCTURE: ORBITAL
HYBRIDIZATION AND MOLECULAR ORBITALS
The electrons in an atom are located in the _____________ orbitals. Similarly when
atoms combine to form molecules, molecular orbitals (MO) are used to describe the
electrons in molecules. There are two theories available to explain the formation of
chemical bonds based on orbitals: the valence bond (VB) theory and the molecular
orbital (MO) theory
What happens when two atoms
come together to form a bond?
An energy level diagram shows that
a bond forms when two atomic
orbitals overlap to make a stable
molecule.
The idea that _____________
________________________
________________________ is the
basis for the VB theory.
The covalent bond that arises from the overlap of two atomic orbitals, one each from the
two atoms, is called a __________ _________. The electron density of a σ bond is
greatest along the axis of the bond.
In H2 molecule, a σ bond is formed by the overlap of ________ 1s orbitals of the
hydrogen atoms.
In HF molecule, a σ bond is formed by the overlap of the hydrogen ____ and the fluorine
_______ orbitals.
In F2 molecule, a σ bond is formed by the overlap of _______ _____ orbitals of the
fluorine atoms.
Hybridization of atomic orbitals
The Lewis dot structure of boron trifluoride, BF3, shows _________ B⎯F
covalent bonds. The VSEPR theory predicts a ____________ __________ configuration
for BF3 with a ________ angle.
How to account for three bonds with 120° angle using one spherical s orbital and three p
orbitals (that are 90° to each other)? The boron atom in its ground state has only 1
unpaired electron, not the three needed to form three covalent bonds.
Orbital hybridization theory explains bonding in molecules with more than two atoms. A
new set of hybrid orbitals can be created by mixing the s, p, and d orbitals on an atom.
1. The number of hybrid orbitals is always the __________ as the number of atomic
orbitals mixed to create them
2. The hybrid orbitals are more directed from the central atom toward the terminal atoms,
than are the unhybridized atomic orbitals, leading to better overlap and a strong bond
between the central and terminal atoms.
_____ hybrid orbital is required for each σ bond or lone electron pair on the central atom.
BF3 and sp2 hybridization
In BF3, there are three σ bonds and hence we need 3
hybrid orbitals. One s orbital and two p orbitals can be mixed to
form three _______ hybrid orbitals and they are separated by a
120° angle. The one p orbital which is empty does not take part
in hybridization.
2s
2p
An orbital (2p) from each F overlaps with one of the sp2 hybrid
orbitals to form three σ bonds. The energy of a sp2 orbital is
closer to the p energy because it has more ‘p character’ than ‘s character’
CF4 and sp3 hybridization
CF4 is _________________ molecule with four C⎯F σ
bonds and a 109.5° angle at the central atom. C has 4 valence
electrons (________). To form 4 σ bonds, we mix ______ ___
and ________ ___ orbital to form _______ ____ hybrid
orbitals and they are separated by a 109.5° angle. Each C⎯F
bond is formed by the overlap of one sp3 orbital of C with a
______ of fluorine. Note that the energy of an sp3 orbital is
very close to the energy of the p orbital.
2s
2p
BeF2 and sp hybridization
BeF2 is _______________ molecule with two Be⎯F σ
2s
bonds and a 180° angle at the central atom. Be has 2 valence
electrons in the 2s orbital. To form 2 σ bonds, we mix _______
and ________ orbital to form two sp hybrid orbitals and they
are separated by a 180° angle. The 2 unoccupied p orbitals do
not participate in hybridization. The energy of a sp orbital is
midway between the energy of an s and a p orbital. Each Be⎯F
bond is formed by the overlap of one sp orbital of Be with a 2p of fluorine.
2p
The maximum numbers of hybrid orbitals that can be created from the s and p orbitals are
_________. For atoms with coordination number higher than ________, s, p and d
orbitals are involved in hybridization.
Hybrid orbitals and VSEPR
Electron pair
geometry
Linear
Bond angle
# bonds
hybridization
Example
180
2
Two electron pairs
sp
BeF2
120
3
Trigonal planar
Three electron pairs
sp2
BF3
Tetrahedral
Four electron pairs
109.5
4
sp
3
CF4
Trigonal bipyramidal
Five electron pairs
axial – 90
equatorial –120
5
90
6
dsp3 or sp3d
PF5
Octahedral
Six electron pairs
d2sp3 or sp3d2
SF6
Identify the hybridization of the central atom in the following compounds
(a) BH4-
(b) OSF4
(c) XeO64-
Multiple bonds
According to VB theory, two orbitals on adjacent atoms overlap to form a bond. Hence
formation of double bond requires ________ sets of overlapping orbitals and for triple
bond, overlap of _________ sets of atomic orbitals are required.
Double bonds
The Lewis structure of ethene, CH2CH2:
Each carbon atom is bonded to 2 H atoms by a single bond and to the other carbon atom
by a double bond. Hence each C atom has__________ ________ geometry with ______
bond angle.
Carbon has ____ valence electrons; to form 3 bonds we mix one s and two p orbitals to
form three sp2 orbitals of equal energy in the molecular plane and there is ____ electron
present in the unhybridized p orbital, perpendicular to the molecular plane.
Coplanar (flat) ethene molecule
top view
Three sp2 orbitals of the C overlap with orbitals of 2 H and 1 C to form three σ bonds.
The two unhybridized p orbitals, one on each C, can overlap to form a second bond
between the carbons. The overlap does not occur on the C⎯C axis, instead they overlap
______________ and the electron pair occupies an orbital with electron density above
and below the molecular plane. The bond formed by sideways overlap of the p atomic
orbitals is called a _____ bond.
The π bond requires that all the six atoms of ethene lie in one plane (coplanar). π bond
always forces the molecule to be coplanar. The C=C double bond in ethene consists of a
σ and a π bond. Double bonds always consist of one σ and one π bond.
Triple bond
Consider ethyne, CHCH (C2H2)
The molecular geometry is linear with a 180° bond angle. Each carbon atom is bonded to
an H and to the other C by an σ bond. To form 2 bonds, we mix one s and one p orbitals,
to get two sp hybrid orbitals. The two unhybridized p orbitals on each atom overlap to
form 2 π bonds. These π bonds are perpendicular to the molecular axis and perpendicular
to each other. Triple bonds always consist of one σ and two π bond.
Cis-trans isomerism: A consequence of π bonding
Free rotation (rotation of one end of a molecule relative to the other end) of
molecules can occur around the axis of a single (σ) bond. For molecules containing π
bonds like ethene, twisting one end of the molecule distorts the molecule away from
planarity and the p orbitals, forming the π bond, would rotate out of alignment. Hence the
p orbitals would no longer overlap and the π bond will be broken. It requires very large
amount of energy to break π bonds, that rotation around C=C does not occur at room
temperature. Due to restricted rotation, isomers occur for many compounds containing
C=C bond.
cis-1,2-dichloroethene
trans-1,2-dichloroethene
Isomers are compounds that have same _____________ but different ______________.
The two isomers of 1,2-dichloroethene differs only in the orientation of the groups
attached to the carbons of the double bond.