Transcript Covalent Bonding - Wappingers Central School District

Covalent Bonding
Electrons are shared!
• Occurs between nonmetal atoms with similar
electronegativities
• Localized Electron model: A molecule is a
group of atoms bound together by sharing
pairs of electrons using the atomic orbitals of
the free atoms.
– Bonding pairs in-between atoms
– Nonbonding pairs localized on one specific atom
Types of Formulas
• Molecular formulas: show which elements &
how many atoms
– H2O, C6H12O6
• Structural Formulas: above plus show
“connectivity.” Show how the atoms are
arranged and the type of bonding (use -, =, or
)
• Lewis diagrams: above plus show all the
nonbonding valence electrons
Types of Covalent Substances
• Nonpolar covalent (no separation of charge)
• Polar covalent (separation of charge resulting
from large differences in electronegativity)
• Network covalent: giant molecule or lattice
• Coordinate covalent: common in polyatomic
ions with H
Lewis Structures
• Diagrams showing all the valence electrons of
the molecule or polyatomic ion. Shows how
the atoms are connected, the types of bonds,
and the lone pairs.
Why Lewis
structures are
important!
Procedure for Writing Lewis Structures
1. Count up the number of valence electrons & jot # down.
2. Draw skeleton (gets easier with experience, use bonding
capacities as guide lines)
3. Compare, in the skeleton, the total number of electrons still
needed for each atom to have an octet / duet to the # of
electrons remaining for distribution after drawing skeleton.
a)
b)
If deficient by 2, add one bond to skeleton, deficient by 4, add two bonds to
skeleton, deficient by 6, add three bonds to skeleton (exceptions: Be, B, oddelectron molecules)
If excess electrons, they may be placed on the CENTRAL atom if the the central
atom is from row 3 or below in the periodic table (has d orbitals)
4. When the numbers match, distribute electrons from outside
in.
5. Perform the two validity checks
Multiple Bonds
• When drawing the Lewis structure of a
molecule or ion, if every atom can’t get an
octet using single bonds, then invoke multiple
bonding.
Resonance
• Occurs when more than one valid Lewis
structure can be written for a species
– Atoms arranged the same way
– Electrons distributed differently
• A “fix” to the LE model
Molecular Shape & VSEPR
• Use the Lewis structure to determine the
shape of many small molecules.
• Count up the electron domains on the central
atom and the number of atoms bonded to the
central atom
• Compare these two numbers (each can range
from 2 to 6) to get molecular geometry.
What are electron domains in the
Lewis Structure?
• An electron domain in the Lewis structure is any
region of electron density.
• Each of the following counts as ONE electron
domain.
–
–
–
–
–
Single bond
Double bond
Triple bond
Lone Pair
Single electron on an atom in an odd-electron species.
Table for Molecular Geometry
# of electron
domains on central
atom
# of atoms bonded
to central atom
Shape
Examples
2
2
Linear
CO2, CS2, BeF2
3
3
Trigonal PLANAR
BH3, BF3
3
2
Bent, bond angle =
120
SO2, O3
4
4
Tetrahedral, bond
angles = 109
CH4, CF4, CH2F2
4
3
Trigonal Pyramid,
bond angles = 107
NH3, NF3, PH3
4
2
Bent, bond angles =
105
H2O, H2S, H2Se, OF2
Table for Molecular Geometry
# of electron
domains on central
atom
# of atoms bonded
to central atom
Shape
Examples
5
5
Trigonal BIPYRAMID
PF5, AsF5
5
4
See-Saw
TeF4
5
3
T-shape
ICl3
5
2
Linear
KrF2
6
6
Octahedral
SF6, SCl6, SeF6
6
5
Square Pyramid
IF5
6
4
Square Planar
KrF4
Diatomic Molecules
• Always linear: 2 points make a line!
• For molecules with 3 or more atoms, use
Lewis structure.
Page 16
4 domains on central O
2 atoms bonded to central O
(4, 2) is bent with a 105 bond angle
< ---- >
< ---- >
Note that in all 3 resonance structures for CO2, the
central C atom has 2 electron domains & 2 atoms
bonded to it. (2, 2) is linear. Bond angle = 180
Page 16
Central N atom has 4 electron
domains & 3 atoms bonded to
it. (4, 3) is trigonal pyramid
with a 107 bond angle.
Central C atom has 4 electron
domains and 4 atoms bonded
to it. (4, 4) is tetrahedral
with a 109 bond angle.
Page 16
Central S atom has 3 electron domains and 3
bonded atoms. (3, 3) is trigonal planar.
Page 16
Starting to get some larger
molecules. So think in terms of
local geometry rather than global
geometry.
Around the C: (4, 4) which is
tetrahedral.
Around the O: (4, 2) which is
bent.
Page 16
Central S atom has 4 electron
domains and 2 atoms bonded to
it. (4, 2) is Bent with a 105
bond angle.
Hint: recognize that S is in the same column
of the periodic table as O, so this molecule
should have the same shape as H2O.
Page 16
The central atom is C with 4
electron domains & 4 atoms
bonded to it. (4, 4) is
tetrahedral with a 109 bond
angle.
Using Lewis Structure to find
Molecular shape
• What is the procedure?
• What do we mean by electron domains?
What factors influence bond energy?
• Type of bond: single vs. double vs. triple.
• Bond Length
– Size of the atoms involved in the bond.
• Polarity of the bond.
Comparison of Bond Strengths for Nonpolar Bonds
Type of Bond
Bond Length
(pm)
Avg. Bond
Energy (kJ/mol)
C-C
C=C
CC
N-N
N=N
NN
154
134
120
145
125
110
346
612
835
163
418
945
Bond Strength: triple > double > single
Note: the higher the bond energy, the shorter the bond.
Comparison of Bond Strengths for
Nonpolar Bonds
• As bond order increases, there is more
electron density between the two nuclei,
increasing the attractive forces between the
electrons & nuclei. ( A deeper potential well.)
• A triple bond is shorter & stronger than a
single bond!
Bond length increases as atomic size
increases
Type of Bond
H-F
H-Cl
H-Br
H-I
Bond Length
(pm)
92
127
142
161
Avg. Bond
Energy (kJ/mol)
565
427
363
295
As atomic radii increase, nuclei are farther away
from each other. Attractive forces between electrons
and nuclei are weakened by larger distances. Bond
energy decreases.
Comparison of Bond Strengths as the
bond becomes more polar
Type of
Bond
C-C
C-N
C-H
C-Cl
C-O
C-F
EN
0
0.4
0.5
0.6
0.9
1.4
Bond Length
(pm)
154
147
110
176
143
141
Avg. Bond
Energy (kJ/mol)
346
305
413
339
358
485
C, N, O, F all in period 2; H in period 1; Cl in period 3
Bond Strength vs. Bond Polarity
• Given that bond lengths increase with
increasing atomic size, there does seem to be
some correlation between the polarity of a
bond and the bond strength when the
elements are in the same row.
• As polarity & % ionic character increase, bond
strength increases.
Other Bond Dissociation Energies
• Ionic & metallic > covalent
• Ionic compounds are all solids at room temp.
• All metals but Hg are solids at room temp. but
some have low melting points.
Coordinate Covalent Bonds
• One atom contributes both electrons to the
bond.
• NH3 + H+  NH4+
• H2O + H+  H3O+
• The H+ has no electrons at all!
Properties
• Know the properties of the molecular covalent
substances (the vast majority)
• Know the properties of the network covalent
substances (C as diamond or graphite), SiO2,
SiC.