IV. INTRODUCTION TO CHEMICAL BONDING
There are two principal types of chemical bonding: Ionic and Covalent. We will approach each separately,
but later we will see that the type of bonding is not always black and white, but some shade of gray.
A. Ionic Bonding
1. Lewis symbols: For each valence electron the atom has place a dot (or mark) next
to the symbol, keeping the electrons unpaired if possible.
Li
Be
B
C
N
O
F
Notice that when done correctly, the Lewis symbols reflect what we learned about the
number of unpaired electrons in atomic orbital theory.
2. Formation of ionic bonds
a) Ionic bond: the mutual attraction between the oppositely charged ions.




The ions form when an electron(s) is transferred form an element with a
low IE to an element with a high EN.
The elements with low IE’s are metals, and those with high EN’s are
nonmetals
Therefore, a metal and a nonmetal make an ionic compound.
All ionic compounds are solid due to the strong attraction between the
oppositely charged ions.
Example(1) Na with Cl
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INTRO TO CHEMICAL BONDING / Ionic Bonding
Example(2) Mg with S
Example(3) K with S
Example(4) Al with F
b) “Octet” “rule”: After chemically combining atoms end up with eight electrons
in their outer most occupied principal energy level.
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INTRO TO CHEMICAL BONDING / Ionic Bonding
3. Writing formulas from charges
After you understand why and how ionic compounds form from elements, you can use this shortcut
method to determine formulas for ionic compounds.
This method is based on two facts:
a) All compounds are electrically neutral.
b) The charges on many ions are predictable.
Group I Metals: Form 1+ charged ions upon making an ionic compound.
Remember elements do NOT have charges! The charges develop
as a result of forming a compound.
Group II:
Group III Metals:
Group VII:
Group VI:
Group V Nonmetals:
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INTRO TO CHEMICAL BONDING / Ionic Bonding
Example(5): What is the formula of the compound that forms between Ca and Br?
Example(6): What is the formula of the compound that forms between Na and N?
Example(7): What is the formula of the compound that forms between Al and S
43
INTRO TO CHEMICAL BONDING / Covalent Bonding
B. Covalent Bonding
1. Formation of covalent bonds
a) When nonmetals bond to each other they share electrons, since one nonmetal
is incapable of pulling an electron away from another nonmetal.
Example(1): F with F
F1+
+
F1-
b) Single covalent bond: the sharing of one pair of electrons between two atoms.
Example(2): F with F
Example(3): H with H
Example(4): H with F
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INTRO TO CHEMICAL BONDING / Covalent Bonding
2. Atomic orbital overlap
To understand how nonmetals form bonds, we must look at their atomic orbitals.
a) A covalent bond is formed between two atoms by overlapping their atomic
orbitals. The attraction of the 2 nuclei for the region of increased electron cloud
density holds the atoms together.
Example(5): H with H
b) The energy of the atoms depends on the distance between them.
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INTRO TO CHEMICAL BONDING / Covalent Bonding
c) If an orbital is completely filled it cannot overlap with another orbital that also
contains electrons. This follows from the basic principal that no more than 2
electrons can occupy any single orbital.
Example(6): He with He
d) If an orbital contains only one electron (i.e. an unpaired electron) it is available
for bonding.
Example(7): F with F
Example(8): H with F
Example(9): H2S
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INTRO TO CHEMICAL BONDING / Covalent Bonding
3. Dot formulas
Example(10): HF
Example(11): H2S
Example(12): SiCl4 (For almost all simple molecules, such as this, the single atom is in the center
and the multiple atoms are bonded to it.)
4. Using dot formulas to determine the formula of simple covalent compounds
Example(13): What is the formula of the compound that forms between S and Cl.
Example(14): What is the formula of the compound that forms between P and F.
47
INTRO TO CHEMICAL BONDING / Covalent Bonding
5. Multiple bonding between two atoms
a) Double covalent bond: the sharing of two pair of electrons between two
atoms
Example(15): O2
Example(16): CO2
b) Triple Covalent Bond: the sharing of three pair of electrons between two
atoms
Example(17): N2
6. More dot formulas
a) Hydrocarbons (compounds containing only carbon and hydrogen)
Example(18): CH4
Example(19): C2H6
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INTRO TO CHEMICAL BONDING / Covalent Bonding
Example(20): C3H8
Example(21): C2H4
Example(22): C2H2
b) Oxygen attached to a central atom
When doing the dot formula of a compound or polyatomic ion containing oxygen
attached to a central atom, put the electrons around the central atom in pairs.
Example(23): CO2
Example(24): SiO2
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INTRO TO CHEMICAL BONDING / Covalent Bonding
7. Polyatomic ions
a) Recognizing polyatomic ions
b) Dot formulas of anions
Example(25): SO42-
Example(26): PO43-
Example(27): ClO31-
c) Dot formulas of cations
Example(28): NH41+
Example(29): BrCl21+
50
INTRO TO CHEMICAL BONDING / Covalent Bond Properties
C. Covalent Bond Properties
1. Bond Length (BL):The distance between two atoms, measured from nucleus to
nucleus. BL is determined by two factors:
a) Atom size: The larger the atoms bonded together, the longer the bond.
Example(1): Group VII diatomic elements
o
F2
1.43
A (143 pm)
Cl2
1.99
A (199 pm)
Br2
2.28
A
I2
2.66
A (266 pm)
o
o
(228 pm)
o
o
1 A (angstrom) = 1 x 10-8 cm = 1 x 10-10 m
Example(2):
HF
HCl
HBr
HI
b) Number of bonds: The more bonds between two atoms the shorter the BL.
Example(3):
o
C2H6
C —C
1.54 A
C2H4
C=C
1.34 A
C2H2
C C
1.20 A
(154 pm)
o
(134 pm)
o
(120 pm)
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INTRO TO CHEMICAL BONDING / Covalent Bond Properties
2. Bond Energy (BE): The amount of energy required to completely dissociate two
bonded atoms.
AB  A + B
H = BE
a) Breaking a bond requires energy - endothermic.
Making a bond releases energy - exothermic.
(Recall when we first bonded two H atoms together. We said that the energy was lower when bonded then
when separate.)
The same two factors that determine BL also determine BE.
b) Atom size: the larger the atoms the weaker the bond (lower BE).
Example(4):
Cl2
60.7 kcal/mol (254 kJ/mol)
Br2
52.3 kcal/mol (218 kJ/mol)
I2
42.5 kcal/mol (179 kJ/mol)
(F2 is an exception, it can be explained, but we won’t worry about it.)
c) Number of bonds: the more bonds between two atoms the stronger the bond
(higher BE).
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INTRO TO CHEMICAL BONDING / Covalent Bond Properties
Example(5):
C —C
83 kcal/mol (348 kJ/mol)
C=C
145 kcal/mol (615 kJ/mol)
C C
194 kcal/mol (812 kJ/mol)
d) Effect of bond strength on chemical reactivity
Example(6): Nitrogen and chlorine both have an EN of 3.0, but at room temperature nitrogen is essentially inert and
chlorine is very reactive.
Example(7): Oxygen is only slightly reactive at room temperature, but at elevated temperatures it is very reactive.
3. Vibrational frequency
The bond between atoms is not rigid, like masses attached by a spring, the atoms vibrate. When the
bonded is stretched, the atoms are pulled together; when the bond is compressed the atoms are pushed
apart.
a) The stronger the bond, the greater the vibration frequency.
Example(8): Which C to O bond has the higher vibrational frequency, H2C=O or H3COH ?
b) The more massive the atoms, the lower the vibrational frequency.
Example(9): Which Cl2 molecule has the higher vibrational frequency,
35
Cl35Cl or ,
37
Cl37Cl ?
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INTRO TO CHEMICAL BONDING / Covalent Bond Properties
4. Resonance
a) A molecule exhibits resonance if it has more than one valid dot formula.
Example(8): SO2
b) The actual structure is the average structure of all of the dot formulas
c) Bond order (BO): The number of bonds between two atoms.
Example(9): What is the nitrogen to nitrogen bond order in N2?
Example(10): What is the carbon to oxygen bond order in CO2?
Example(11): What is the sulfur to oxygen bond order in SO2?
For a molecule with resonance: BO =
# of .bonds
# of . attached . atoms
Example(12): What is the sulfur to oxygen bond order in SO3?
Example(13): What is the carbon to oxygen bond order in CO32-?
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INTRO TO CHEMICAL BONDING / Polar Covalent Bonding
D. Polar Covalent Bonding
1. Polar bonding
a) Role of electronegativity difference in determining bond type
Example(1): Na with F
Example(2): F with F
Example(3): H with F
b) Polar covalent Bonding: the unequal sharing of electrons between atoms. It
occurs anytime nonmetals with a difference in EN
are bonded.
Example(4): Cl with F
Example(5): Cl with S
Example(6): H with S (Assume that H has the lowest EN of the nonmetals.)
c) Degrees of polarity
Example(7):
HF
HCl
HBr
HI
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INTRO TO CHEMICAL BONDING / Polar Covalent Bonding
Example(8):
NaCl
MgS
AlP
2. Polar Molecule: has a partial positive end and a partial negative end (a dipole).
Formula
F2
HF
CO2
H2O
BF3
NH3
Structure
Polar
Bonding?
Polar
Molecule?