Lecture 23 © slg CHM 151 TOPICS: 1. Molecular Shapes 2. Polyatomic ion Shapes 3. Introduction to Bond Polarity
Dec 21, 2015
Lecture 23 © slg CHM 151
TOPICS:
1. Molecular Shapes2. Polyatomic ion Shapes3. Introduction to Bond Polarity
Molecular and Polyatomic Ion Shapes
Once a Lewis structure is drawn, the three -dimensional geometry of the species can easilybe determined by utilizing the “valence shell electronpair repulsion theory” called “VSEPR”:
“VSEPR” theory is based on the tendency of negativelycharged regions to repel each other and align as farapart as possible, resulting in predictable shapes forany covalently bonded species.
To utilize “VSEPR”, the number of regions of electron density around the central atom in the species iscounted.
Count as “one region”:
• Single Bonds
• Unshared Pairs
• Multiple bonds between same two atoms
Basic Shapes predicted by VSEPR:
Two regions: Three Regions:
A
120o
trigonal planar
A
180o
linear
Bond Angles
Geometry
Four Regions: Five Regions:
Six Regions:
A
trigonal-bipyramidal
90o
120o
A
90o
octahedral
A
tetrahedral
109.5o
Before we begin, some guidelines about forming double and triple bonds in Lewis structures:
C, N, O, S form double and triple bonds and never showincomplete octets (less than 8 e’s)
Metals, metalloids, and halogens do not as a rule formmultiple bonds. Compounds containing these elementswill often show an incomplete octet around the centralatom.
Type One: Two Regions Examples: BeCl2, CO2, NO2
+, HCN
Be Cl 2 BeCl ClBe 22Cl 14
16 e's/2= 8 prs
BeCl Cl (octet violator)
Number of regions around CENTRAL ATOM: 2
BeCl Cl shape : LINEARbond angles: 180o
CO 2 CO O C 42O 12
16 e's/2= 8 prs
CO O
Number of regions around CENTRAL ATOM: 2
shape : LINEARbond angles: 180o
CO O
CO O
NO 2+
NO O N 52O 12+1 -1e
16 e's/2= 8 prs
NO O
Number of regions around CENTRAL ATOM: 2
shape : LINEARbond angles: 180o
NO O
NO O+
HCN CH N H 1 C 4 N 5
10 e's/2= 5 prs
CH N
Number of regions around CENTRAL ATOM: 2
shape : LINEARbond angles: 180o
CH N
CH N
NO 3-
NO O
N 53O 181- 1
24 e's/2=12 prs
O
NO O
O
NO O
O-
(three re s onance s truc tures )
Type Two: Three Regions NO3
-, NO2-, CH2O
Number of regions around CENTRAL ATOM: 3
shape : TRIGONAL PLANARbond angles: 120o
NO O
-
NO O
-
Black orbitalindicates pair ofunshared e’s
NOTE: “molecular geometry” (bonds only): BENT
CH 2O CH H
C 4O 62H 2
12 e's/2=6 prs
Number of regions around CENTRAL ATOM: 3
shape : TRIGONAL PLANARbond angles: 120o
O
CH H
O
CH H
O
CH H
O
NH 3 NH H
N 53H 3
8e's/2=4 prs
Number of regions around CENTRAL ATOM: 4
shape : TETRAHEDRALbond angles: < 109.5o
NH H
HNH H
H
H
Note: molecular geometry, trigonal pyramid
As is turns out, unshared pairs of electrons around the central atom are not held in place between two atoms as bonded pairs are.
They tend to occupy more space and to be somewhatmore “repulsive” than bonded pairs.
When grouped with bonded pairs to tiny atoms like H,they tend to distort the bond angles, pushing thebonded pairs closer together.
The bond angles in ammonia are closer to 107o.
H2O OH H
O 62H 2
8e's/2=4 prs
Number of regions around CENTRAL ATOM: 4
shape : TETRAHEDRALbond angles: < 109.5o
OH
OH H
H
(~105o)
Note: molecular geometry: BENT
NH 4+
NH H
N 54H 41+ -1
8e's/2=4 prs
Number of regions around CENTRAL ATOM: 4
shape : TETRAHEDRALbond angles: 109.5o
NH H
H
H
H
H +
Number of regions around CENTRAL ATOM: 3
shape : TRIGONAL PLANARbond angles: 120o
CO O
O2-
CO32- 4 + 18 + 2 e 's = 24 e 's , 12 pairs
Number of regions around CENTRAL ATOM: 4
shape : TETRAHEDRALbond angles: 109.5o
SiCl Cl
Cl
Cl
Si Cl4 4 + 28 e's = 32 e's, 16 pairs
Number of regions around CENTRAL ATOM: 5
shape : Trigonal Bipyramidalbond angles: 120o, 90o
PF
F
F
FF
F PF
F
F
F
Bond anglesin triangle,120o
Bond angles,each “axial” F,90o from trigonalplane
ClF 3 Cl F
Cl 73F 21
28e's/2=14 prs
Number of regions around CENTRAL ATOM: 5
shape : Trigonal Bipyramidalbond angles: 90o
F
F
Cl F
F
F
Note: “T-shaped”; unshared pairs always trigonal planar
IF 2-
I F
I 72F 141- 1
22e's/2=11 prs
Number of regions around CENTRAL ATOM: 5
shape : Trigonal Bipyramidalbond angles: 180o
F
I
F
F
-
-
Note: “linear” molecular geometry
Note: “Seesaw” molecular geometry
SF 4 S F
S 64F 28
34e's/2=17 prs
Number of regions around CENTRAL ATOM: 5
shape : Trigonal Bipyramidalbond angles: 90o, 120o
F
S
F
F
F
F
F
F
Number of regions around CENTRAL ATOM: 6
shape : OCTAHEDRALbond angles: 90o
IF
F F
FF
Note: molecular geometry “square pyramidal”
Xe F4 XeF
Xe 84F 28
36e's/2=18 prs
Number of regions around CENTRAL ATOM: 6
shape : OCTAHEDRALbond angles: 90o
F
F F
XeF
F F
F
Note: “Square planar”
ICl4+
Number of regions around CENTRAL ATOM: 5
shape : Trigonal Bipyramidalbond angles: 90o, 120o
I
Cl
Cl
Cl
Cl
7 + 28 -1 e's = 34 e's, 17 prs
+
Number of regions around CENTRAL ATOM: 5
shape : Trigonal Bipyramidalbond angles: 90o
Xe
F
F
XeO F2 8 + 6 + 14 e's = 28 e's, 14 prs
O
Number of regions around CENTRAL ATOM: 6
shape : OCTAHEDRALbond angles: 90o
ICl
Cl Cl
Cl
ICl4- 7 + 28 +1 e's = 36 e's, 18 prs
-
To see relevance of “shape work”, let’s turn nextto bond and molecular polarity. To help examinethis topic we turn back to the property of“electronegativity”:
Unit 5, Lecture 24, next week!!
ELECTRONEGATIVITY
The trends in ionization energies and electron affinities can be thought of as summarized in a single property called “electronegativity” (en or X).
Electronegativity is a unit-less set of assigned values on a scale of 0 --> 4 describing the ability of an atom to attract electrons to itself.
The values reaches a maximum at fluorine, with an X =4.Nonmetals have the largest values, metals the lowest.Noble gases have no assigned X value.
The electronegativity values are quite useful inevaluating bond type and what we will term“bond polarity,” which arises when electrons areshared unevenly.
In summation:
Metals: larger size, lower ionization potential, lower electron affinity, and lower electronegativity; tend to form positive ions
Non-Metals: smaller size, higher ionization potential,higher electron affinity, higher electronegativity; function as anions in ionic compounds.
We have classified bonds “ionic” and “covalent”,depending on whether electron pairs are shared orelectrons are completely transferred from one atom to another.
In actuality, there is no sharp dividing line between thetwo types but rather a continuum:
Evenly shared electrons
Unevenly shared electrons
Transferredelectrons
To determine where a bond lies in this “continuum”, it is useful to consider the difference in electronegativitybetween the two atoms making up the bond:
When the difference is less than 0.5, sharing is fairlyeven and electrons are not much closer to one atom than the other.
When the difference is between 0.5 and about 1.5, theelectrons are closer to the more electronegative atom and partial charge buildup, polarization, develops.
When electronegativity difference is greater than1.5 or so, ionic bonding becomes the more likelytype and valence electrons are transferred to themore electronegative atom.
So, we need to consider a third more specializedtype of bond, “the polar covalent bond:”
This type of bond will be the important factor tobe considered when we look at molecular polarity, which arises from molecular shape and bondpolarity.
The polar molecular in turn will exhibit different solubilities and boiling points than non polar molecules. The relationship between these iswhat we will next examine.