chem 105 unit 3
total sigma + pi bond order equation
( sigma BO + pi BO) / (# of bonds)
constructive interference
- The interference that occurs when two waves combine to make a wave with a larger amplitude - A + A = 2A
destructive interference
- The interference that occurs when two waves combine to make a wave with a smaller amplitude - A - (-A_) = 0
molecular orbital theory
- a theory of the electronic structure of molecules in terms of molecular orbitals, which may spread over several atoms or the entire molecule - Bonding electron pairs may be delocalized over the entire molecule; no need for resonance structures
Sigma bond (σ bond)
- a type of sp2 hybridization in which the electron pair is shared in an area centered on a line running between the atoms - Lobes of bonding orbital point toward each other...head-on collision - has NO planar node that contains the bond axis
pi bond
- forms, when parallel orbitals overlap and share electrons...sideways overlap - has one planar node that contains the bond axis
Major difference of MOT and LEM
- how they handle delocalized molecules - bonding electron pairs may be delocalized over the entire molecule - MOT uses correlation diagrams - no need for resonance structures in MOT
orthogonal
- of or involving right angles; at right angles. - the sigma and pi frameworks are orthogonal aka always perpendicular
Polygon Rule and number of energy levels
- pi MO energy levels scale with the position of the vertices, *with one vertex pointing down* - 6
total combine bond order equation
- you add the bond orders for the individual sigma and pi bond orders
steric number
-number of atoms bonded to central atom + number of lone pairs on the central atom -gives basic geometry but not structure name
3 rules of molecular geometry
1. steric number 2..Repulsion order rule 4.Bent's rule
pi bond order equation
1/2 x (pi electrons - anti-bonding electrons) OR 1/2 x (bonding - anti bonding)
sp^3 hybrid orbitals
109 - set of four sp^3 hybrid orbitals, no pure p orbitals remain
sp^2 Hybrid orbitals
120 - set of three sp^2 hybrid orbitals, 1 pure p orbitals remains
sp hybrid orbitals
180 - set of two sp hybrid orbitals, 2 pure p orbitals remain
how many degrees per lone pair
2-3 degrees
SN 2,3,4,5,6
2-Linear 3-Trigonal Planar 4-Tetrahedral 5-Trigonal Bipyramid 6-Octahedral
in cyclical pi MO what energy levels are anti bonding
4,5, and 6
Sawhorse or Seesaw
90-120
Calculate the MOT (molecular - orbital - theory) bond order
= ½(# electrons in bonding MO's - # electrons in anti-bonding MO's)
3 failures of LEM
Failure #1: Needs the use of resonance to describe the delocalization of electrons (not all electrons always reside between 2 atoms). Failure #2: Molecules are known to exist in excited states. LEM cannot describe or account for these. aka only ground-state bonding Failure #3: LEM fails to predict certain molecular properties. EX - O2; LEM says all electrons are paired but that's not true
____ only describes ground-state bonding
LEM
Use _____ for sigma framework, use ______ for pi framework
LEM; MOT
Repulsion order rule
LP-LP > LP-BP > BP-BP - bc lone pairs (LP) occupy more room around a central atom than bonding pairs (BP) -slightly affects bonding angles
Second-order diagram is used for
Li2 - N2 and their ions
MOT stands for
Molecular Orbital Theory
First-order diagram is used for
O2 -Ne2 and their ions
Lewis structure with VSEPR can predict _______
Shape of molecules
localized electrons
The electron pairs are localized between 2 specific atoms
delocalized electrons
The electrons are spread over more two atoms
LUMO
The lowest unoccupied molecular orbital; when a molecule in the ground state is excited, an electron may jump from a HOMO into a LUMO.
linear pi MOs in order of increasing energy and number of nodes perpendicular to bonding axis
pi bonding MO = 0 pi non-bonding MO = 1 pi anti-bonding MO = 2
Covalent bonds are formed by the _______ of electron waves on adjacent atoms
constructive interference
CN
corrdination number
Atomic orbitals on ________ atoms are "mixed" to form molecular orbitals that extend over the molecule
different
number of nodes and _______ are directly correlate
energy level
HOMO
highest occupied molecular orbital
How to tell if an atom is stable
if it has a bond order larger than 0
the electrons are stabilized by / attracted to the _______
local nuclei
LEM
localized electron model
axial position
location in a trigonal bipyramidal geometry in which there is another atom at a 180° angle and the equatorial positions are at a 90° angle
ignore ____ when naming geometry
lone pairs
MO
molecular orbitals
homonuclear diatomic molecules
molecules composed of two like nuclei
Bent's Rule
more electronegative atoms (smaller atoms) prefer axial positions - less electronegative atoms (larger atoms and lone pairs) prefer equatorial positions
Molecular orbitals for homonuclear diatomic are ______
nonpolar
Procedure for determining hybridization around an atom (3 steps)
o Draw Lewis structure o Determine shape (i.e., need only the basic geometry) using VSEPR (or experimental geometry) and thereby determine the bond angles o Pick type of hybrid orbitals that produce these bo
equatorial position
one of the three positions in a trigonal bipyramidal geometry with 120° angles between them; the axial positions are located at a 90° angle
single bond in LEM
one sigma component
double bond in LEM
one sigma component and one pi component
triple bond in LEM
one sigma component and two pi component
probability of finding an electrons in is highest in region _______ which is the _______
overlap ; bond
hydrogen atoms cannot participate in ______ bonding
pi
lone pairs in resonance are in the _____ system not the _____ system
pi; sigma
bonding lobe typically has a positive or negative amplitude?
positive
Hybridization is used to
produce equivalent bonding orbitals to give the correct bond angles pointing in the correct direction
only lone pairs in _____ system contribute to the SN, and participate in hybridization
sigma
only lone pairs in the ______ system contribute to SN
sigma
SN
steric number
the small lobe in hybridization
tail lobe
what lobes overlap?
tail lobes
shape of a bonding lobe
tear drop
Hybridization is _____
the mixing of pure 's' and 'p' atomic orbitals on an atom to form hybrid atomic orbitals on that same atom
superposition principle and 2 types
the wave that results when two or more waves overlap - wave amplitudes are additive - Destructive Interference - constructive interference
coordination number
total number of bonded atoms around central atom
paramagnetic
unpaired electrons - spin doesn't = 0
sigma framework
uses hybrid orbitals and 1s atomic orbital for hydrogen atoms
pi framework
uses pure p atomic orbital
in LEM model you can only use the ______ once to make a bond
valence electron
for SN 6 put large substituents on opposite _____
vertices
when do you adda start to a pi orbital
when the net bond order is anti-bonding
by convention the molecular axis lies along _____ axis
z
another name for LEM
Valence Bond Theory
VSEPR theory
Valence-shell electron-pair repulsion theory; because electron pairs repel, molecules adjust their shapes so that valence electron pairs are as far apart as possible
diamagnetic
all electrons are paired - spin = 0
in the region of overlap ____ is the highest
amplitude
an electron orbital is the same as _____
an electron wave
the less electronegative atom contributes more to the _________ MO
anti-bonding
MOT mixes
atomic orbitals on different atoms that may extend over the entire atom
LEM mixes
atomic orbitals on the same atoms (hybridization)
in pi bonds why is are double bonds rigidly co-planar / have a rotational restriction?
because they don't want to give up any overlap
where is the radial node in hybridized diagrams
between tail and bond lobes
how do you show that 2 atoms are participating in a pi bond
bond arch beween 2 atoms
the more electronegative atom contributes more to the _________ MO
bonding
the large lobe in hybridization
bonding lobe
