BONDING
Let’s get together…
Barbara A. Gage
PGCC
CHM 1010
Why Do Atoms Bond?
• Chemical bonds form because they lower
the potential energy between the charged
particles that compose atoms
• A chemical bond forms when the potential
energy of the bonded atoms is less than
the potential energy of the separate
atoms
Barbara A. Gage
PGCC
CHM 1010
Types of Bonds
• We can classify bonds based on the kinds
of atoms that are bonded together
Types of
Atoms
metals to
nonmetals
nonmetals to
nonmetals
metals to
metals
Type of Bond
Ionic
Covalent
Metallic
Barbara A. Gage
PGCC
CHM 1010
Bond
Characteristic
electrons
transferred
electrons
shared
electrons
pooled
Types of Bonding
Barbara A. Gage
PGCC
CHM 1010
Ionic Bonds
• When a metal atom loses electrons it becomes a
cation
– metals have low ionization energy, making it
relatively easy to remove electrons from
them
• When a nonmetal atom gains electrons it
becomes an anion
– nonmetals have high electron affinities,
making it advantageous to add electrons to
these atoms
• The oppositely charged ions are then attracted
to each other, resulting in an ionic bond
Barbara A. Gage
PGCC
CHM 1010
Covalent Bonds
• Nonmetal atoms have relatively high ionization
energies, so it is difficult to remove electrons
from them
• When nonmetals bond together, it is better in
terms of potential energy for the atoms to
share valence electrons
– potential energy lowest when the electrons are
between the nuclei
• Shared electrons hold the atoms together by
attracting nuclei of both atoms
Barbara A. Gage
PGCC
CHM 1010
Metallic Bonds
• The relatively low ionization energy of metals
allows them to lose electrons easily
• The simplest theory of metallic bonding
involves the metal atoms releasing their
valence electrons to be shared as a pool by all
the atoms/ions in the metal
– an organization of metal cation islands in a
sea of electrons
– electrons delocalized throughout the metal
structure
• Bonding results from attraction of cation for
the delocalized electrons
Barbara A. Gage
PGCC
CHM 1010
Lewis Electron-Dot Symbols
For main group elements The A group number gives the number of valence electrons.
Place one dot per valence electron on each of the four
sides of the element symbol.
Pair the dots (electrons) until all of the valence electrons are
used.
Example:
:
Nitrogen, N, is in Group 5A and therefore has 5 valence
electrons.
.
.
.
.
.
.
.
.
:N
N
N:
N.
.
.
.
:
Barbara A. Gage
PGCC
CHM 1010
Determining the Number of
Valence Electrons in an Atom
• The column number on the Periodic Table will
tell you how many valence electrons a main
group atom has
– Transition Elements all have two valence electrons.
Why?
Barbara A. Gage
9
PGCC
CHM 1010
Lewis Structures of Ions
• Cations have Lewis symbols without
valence electrons
– lost in the cation formation
• Anions have Lewis symbols with eight
valence electrons
– electrons gained in the formation of the
anion
Barbara A. Gage
10
PGCC
CHM 1010
Ionic Bonding & the Crystal
Lattice
• Ionically bonded substances form a structure in
which every cation is surrounded by anions, and
vice versa
• This structure is called a crystal lattice
• The crystal lattice is held together by the
electrostatic attraction of the cations for all
the surrounding anions
• The crystal lattice maximizes the attractions
between cations and anions, leading to the most
stable arrangement
Barbara A. Gage
PGCC
CHM 1010
Crystal Lattice
• Electrostatic attraction is nondirectional!!
– no direct anion–cation pair
• Therefore, there is no ionic molecule
– the chemical formula is an empirical formula,
simply giving the ratio of ions based on
charge balance
– One unit of the
empirical formula is
called a formula unit
Barbara A. Gage
PGCC
CHM 1010
Electrical conductance and ion mobility.
Solid ionic
compound
Molten ionic
compound
Barbara A. Gage
PGCC
CHM 1010
Ionic compound
dissolved in
water
Lewis Theory of
Covalent Bonding
• Lewis theory implies that another way
atoms can achieve an octet of valence
electrons is to share their valence
electrons with other atoms
• The shared electrons would then count
toward each atom’s octet
• The sharing of valence electrons is called
covalent bonding
Barbara A. Gage
PGCC
CHM 1010
Covalent Bonding
• Atoms with incomplete octets can share
rather than transfer electrons.
• Each pair of shared electrons = 1 bond
• Shared electrons move around the
nuclei of both atoms in the bond so both
atoms have possession of the shared
electrons.
Barbara A. Gage
PGCC
CHM 1010
Lewis Dot Structures for
Covalent Compounds
• Sum the valence electrons of all atoms.
• Determine the central atom.
• Position the central atom and place the
additional atoms equally around it.
• Place the required number of electrons
around the outside atoms first and then
around the central atom to be each one
meets the octet rule (or the number
needed if it is an exception).
Barbara A. Gage
PGCC
CHM 1010
Lewis Dot Structures for
Covalent Compounds
CCl4 Total electrons = 1(4) + 4(7) = 32
Cl
Cl C Cl
Cl
Cl
Cl
C
Cl
Cl
Barbara A. Gage
PGCC
CHM 1010
Lewis Dot Structures for
Covalent Compounds
SO3 Total electrons = 1(6) + 3(6) = 24
O
O
S O
O
O
S O
Barbara A. Gage
O
PGCC
CHM 1010
O
S
O
Covalent Bonding:
Bonding and Lone Pair Electrons
• Electrons that are shared by atoms are called
•
bonding pairs
Electrons that are not shared by atoms but
belong to a particular atom are called lone
pairs
aka nonbonding pairs
Bonding pairs
..
..
..
.. O .... S .. O ..
..
Barbara A. Gage
PGCC
CHM 1010
Lone pairs
Single Covalent Bonds
• When two atoms share one pair of electrons it is
called a single covalent bond
 2 electrons
• One atom may use more than one single bond
to fulfill its octet
 to different atoms
 H only duet
••
••
H• • O
•H
••
••
H O H
••
••
••
••
F
Barbara A. Gage
PGCC
CHM 1010
••
F
••
••
••
••
• F
••
••
•• ••
F F
•• ••
•
F •
Double Covalent Bond
• When two atoms share two pairs of
electrons the result is called a double
covalent bond
•
•O
••
••
•O
••
•
– four electrons ••
O ••
•• O
Barbara A. Gage
PGCC
CHM 1010
Octet Rule Exceptions
• Some elements are stable with fewer or
more than 8 e-.
H 2eBe 4eP, Cl, Br (and more) 10eS, Se, Xe (and more) 12e-
Barbara A. Gage
PGCC
CHM 1010
B 6e-
Resonance: Delocalized Electron-Pair Bonding
O3 can be drawn in 2 ways
-
O
O
O
O
O
O
Neither structure is actually correct but can be drawn to represent a
structure which is a hybrid of the two - a resonance structure.
B
B
O
O
O
A
O
O
O
O
O
C
A
O
C
Resonance structures have the same relative atom placement but
a difference in the locations of bonding and nonbonding electron
pairs.
is used to indicate that resonance occurs.
Barbara A. Gage
PGCC
CHM 1010
Writing Resonance Structures
PROBLEM:
Write resonance structures for the nitrate ion, NO3-.
SOLUTION: Nitrate has 1(5) + 3(6) + 1 = 24 valence e-
O
O
O
O
N
N
N
O
O
O
O
O
N does not have an
octet; a pair of ewill move in to
form a double
bond.
O
O
O
O
N
N
N
O
O
Barbara A. Gage
O
PGCC
CHM 1010
O
O
Bond Lengths
• The distance between the
nuclei of bonded atoms is
called the bond length
• Because the actual bond
length depends on the other
atoms around the bond we
often use the average bond
length
– averaged for similar bonds from
many compounds
Barbara A. Gage
PGCC
CHM 1010
Bond Energies
• Chemical reactions involve breaking bonds in
reactant molecules and making new bonds to
create the products
• The DH°reaction can be estimated by comparing
the cost of breaking old bonds to the income
from making new bonds
• The amount of energy it takes to break one
mole of a bond in a compound is called the bond
energy
– in the gas state
– homolytically – each atom gets ½ bonding electrons
Barbara A. Gage
PGCC
CHM 1010
Barbara A. Gage
PGCC
CHM 1010
Silberberg, Principles of Chemistry
What is the relationship between bond order and bond length for bonds
between the same two elements?
What is the relationship between bond length and bond energy for bonds
between the same two elements?
Barbara A. Gage
PGCC
CHM 1010
Comparing Bond Length and Bond Strength
PROBLEM:
Using the periodic table, rank the bonds in each set in order
of decreasing bond length and bond strength:
(a) S - F, S - Br, S - Cl
(b) C = O, C - O, C
O
(a) The bond order is one for all and sulfur is bonded to
halogens; bond length should increase and bond strength
should decrease with increasing atomic radius. (b) The same
two atoms are bonded but the bond order changes; bond length
decreases as bond order increases while bond strength
increases as bond order increases.
SOLUTION:
(a) Atomic size increases going down a group. (b) Using bond orders we get
Bond length: S - Br > S - Cl > S - F
Bond strength: S - F > S - Cl > S - Br
Barbara A. Gage
Bond length: C - O > C = O > C
Bond strength: C
PGCC
CHM 1010
O
O>C=O>C-O
Break
1 mol C─H +414 kJ
1 mol Cl─Cl +243 kJ
Barbara A. Gage
1 mol C─Cl
1 mol H─Cl
PGCC
CHM 1010
Make
−339 kJ
−431 kJ
Electron Distribution in a
Covalent Bond
• Are electrons shared equally in a
covalent bond?
• If not, why not?
• Distance of electrons from nucleus and
number of protons in the nucleus
• Electronegativity – attraction of one
atom in a bond for the electrons in that
bond
Barbara A. Gage
PGCC
CHM 1010
Polar Covalent Bonding
• Covalent bonding between unlike atoms results in
unequal sharing of the electrons
– one atom pulls the electrons in the bond closer to its
side
– one end of the bond has larger electron density than
the other
• The result is a polar covalent bond
– bond polarity
– the end with the larger electron density gets a
partial negative charge
– the end that is electron deficient gets a partial
positive charge
Barbara A. Gage
PGCC
CHM 1010
The Pauling electronegativity (EN)
scale.
Barbara A. Gage
PGCC
CHM 1010
Polarity
• When atoms in a bond have different
electronegativities, the electron sharing
is unequal.
• As the ΔEN increases, the electron
distribution becomes more uneven and
the molecule becomes polar.
Barbara A. Gage
PGCC
CHM 1010
Polarity
• HCl
• ENH = 2.1
ENCl = 3.0
ΔEN = 0.9
• The end with the higher EN will be
slightly negative and the other will be
slightly positive
δ+H
– Clδ-
H – Cl
Barbara A. Gage
PGCC
CHM 1010
Electronegativity Difference and Bond Type
• If difference in electronegativity between bonded atoms
is 0, the bond is pure covalent
– equal sharing
• If difference in electronegativity between bonded atoms
is 0.1 to 0.4, the bond is nonpolar covalent
• If difference in electronegativity between bonded atoms
is 0.5 to 1.9, the bond is polar covalent
• If difference in electronegativity between bonded atoms
is larger than or equal to 2.0, the bond is ionic
4%
0
0.4
Percent Ionic Character
51%
2.0
Electronegativity Difference
Barbara A. Gage
PGCC
CHM 1010
“100%”
4.0
Bond Dipole Moments
• Dipole moment, m, is a measure of bond
polarity
– a dipole is a material with a + and − end
– it is directly proportional to the size of the
partial charges and directly proportional to the
distance between them
• m = (q)(r)
• not Coulomb’s Law
• measured in Debyes, D
• Generally, the more electrons two atoms
share and the larger the atoms are, the
larger the dipole moment
Barbara A. Gage
PGCC
CHM 1010
Determining Bond Polarity from EN Values
PROBLEM:
(a) Use a polar arrow to indicate the polarity of each
bond: N-H, F-N, I-Cl.
(b) Rank the following bonds in order of increasing
polarity: H-N, H-O, H-C.
(a) Find EN values; the arrow should point toward the
negative end.
(b) Polarity increases across a period.
SOLUTION:
(a) The EN of N = 3.0, H = 2.1; F = 4.0; I = 2.5, Cl = 3.0
N-H
F-N
I - Cl
(b) The order of increasing EN is C < N < O; all have an
EN larger than that of H.
H-C < H-N < H-O
Barbara A. Gage
PGCC
CHM 1010
Percent Ionic Character
• The percent ionic character is the
percentage of a bond’s measured dipole
moment compared to what it would be if
the electrons were completely
transferred
• The percent ionic character indicates the
degree to which the electron is
transferred
Barbara A. Gage
PGCC
CHM 1010