CHEMICAL BONDS
A Dog’s Tale about an
Element’s Search for Happiness
(chemical stability)
CHEMICAL BONDS
The forces that hold atoms together…often
driven by the Octet Rule and stability.
(Mutual electrical attraction between nuclei
and valence electrons of different atoms
that binds them.)
Chemical Bonds
Sometimes it helps to
think of bonds (which you
can't see) in terms of
familiar things you can
see. This is called an
analogy.
Let's use the natural
attraction of dogs to
bones as an analogy to
the attractions that cause
chemical bonds.
Chemical Bonds
The negatively charged electrons
determine how two or more atoms
will interact when they are brought
near each other. In a sense, the
atoms fight over the available
electrons in much the same way two
or more dogs will fight over bones.
The Dog Bone Analogy works quite
well for several types of atomic
bonds.
Dogs are the atoms.
Bones are the electrons.
Octet Rule
Atoms tend to gain, lose, or share
electrons in order to acquire a full set of
valence electrons.
8 (most of the time)
or
Happy dogs!
2 (1
st
energy level)
IONIC BONDS
One big greedy thief dog!
Ionic bonding can be best imagined as one big
greedy dog stealing the other dog's bone.
IONIC BONDS
The bone represents the electron that is up for grabs.
When the big dog gains an electron he becomes negatively charged,
and the little dog who lost the electron becomes positively charged.
These two ions (that's where the name ionic comes from) are
attracted very strongly to each other as a result of the opposite
charges.
IONIC BONDS
Chemical bonding that results from transfer of electrons
and electrical attraction between cations and anions.
Form between metals (on the left side of the Periodic
Table) and non-metals (on the right side of Periodic
Table)
 Metal loses electron(s) and becomes positively
charged…a cation.
 Non-metal gains electron(s) and becomes negatively
charged…an anion.
 The oppositely charged ions are held together because
they are strongly attracted to each other.
 Determine difference in electronegativity

– See page 161
– Difference of greater than 1.7 is often considered ionic
– Ionic character greater than 50%
IONIC BOND?

Cs and F?
Page 161 electronegativity chart
F
en = 4.0
Cs en = 0.7
-------------3.3 is the electronegativity difference


A bond between Cs and F is ionic, greater than 1.7
IONIC COMPOUNDS


Expressed as “formula units” rather than molecules
because they form networks of ions attracted to each
other…not independent neutral units.
Characteristics:
–
–
–
–
–
Crystalline structure
High melting points
Brittle
Dissolve in water
This separates the ions by breaking the ionic bonds…separated
ions move freely making solution of ionic compounds good
conductors of electricity.
– Also conductive when melted.
Formula unit is NaCl, 1 sodium and 1 choride,
the simplest ratio
Ionic compounds form orderly arrangements of the ions , called
crystal lattices.
COVALENT BONDS
The dogs share the bones.
There are two types of sharing:
UNEQUAL
EQUAL
COVALENT BONDS
Bonds formed when electrons are shared
 Covalent bonds form between non-metals
(on the right side of the Periodic Table)
 The electrons are shared by the atoms.
 Electrons may be shared equally or
unequally.
 Molecules are formed because atoms
sharing electrons must be near each other
COVALENT BONDS
 Enormous variety of size, shape, physical, and
chemical properties.




often lower melting point,
not necessarily crystalline
do not conduct electricity,
may or may not dissolve in water
POLAR COVALENT BONDS
Unevenly matched dogs that are willing to
share.
These bonds can be thought of as two or more
dogs that have different desire for bones. They
share unequally.
POLAR COVALENT BONDS
The bigger dog has more strength to possess a larger
portion of the bones. Sharing still takes place but is an
uneven sharing.
POLAR COVALENT BONDS




In a covalent bond, the electrons are shared
between atoms to fulfill the Octet Rule for both.
In a polar covalent bond, the electrons are shared
unequally. They are attracted more to the more
electronegative element.
The electrons spend more time near the more
electronegative element, making it seem more negative
and the other end of the molecule seem more
positive…or “polar”
Determine difference in electronegativity
– See page 161
– Difference of .3-1.7 is often considered polar covalent
– Ionic character 5-50%
POLAR COVALENT BOND?

Unequal sharing of electrons
Partial
positive
charge
Partial
negative
charge
POLAR COVALENT BOND?

H and S?
Page 161
S en = 2.5
H en = 2.1
-------------0.4 is the electronegativity difference


A bond between H and S is polar covalent, .3-1.7
Polar molecules and polar bonds
Two sides to the molecules/bonds
Partial positive
charge towards less
electronegative
element
Partial negative charge towards
element with greater
electronegativity.
S
Polar bonds lead to polar
molecules
Polamolecules
Two sides to Troy Polamalu
Calm, quiet, humble
on the sideline
Fearless, crazed, maniacal on the field
Polar molecules and polar bonds
Two sides to the molecules/bonds
Partial positive
charge towards less
electronegative
element
Partial negative charge towards
element with greater
electronegativity.
S
NON-POLAR COVALENT BONDS
Covalent Bonds: Dogs of equal strength
share equally.
Covalent bonds can be thought of as two or
more dogs with equal attraction to the bones.
They share equally.
NON-POLAR COVALENT BONDS
Since the dogs are identical, then the dogs share
the bones evenly. Since one dog does not have
more of the bone than the other dog, the bone
is equally shared between both dogs. One dog
does not have more than the other.
NON-POLAR COVALENT BONDS
In a covalent bond, the electrons are shared
between atoms to fulfill the Octet Rule for both.

In a non-polar covalent bond, the electrons
are shared equally.

The charge is evenly distributed between
both atoms. The molecule is non-polar meaning
one side does not have more charge than the
other.


Determine difference in electronegativity
– See page 161
– Difference of less than .3 is often considered nonpolar covalent
– Ionic character less than 5 %
NON-POLAR COVALENT BOND?

Cl and Br?
Page 161 for en
Cl en = 3.0
Br en = 2.8
-------------0.2 is the electronegativity difference


A bond between Cl and Br is non=polar covalent, less
than 0.3
NON-POLAR COVALENT BOND?

Equal sharing of electrons
Equal
distribution
of charge
Equal
distribution
of charge
Diatomic Elements
They pair up!
Br2
 I2
 N2
 Cl2
 H2
 O2
 F2

The diabolical DIATOMIC BrINClHOF TWINS!
METALLIC BONDS
Mellow dogs with plenty of bones to go around.
They don’t have to worry about sharing!
METALLIC BONDS
These bonds are best imagined as a room full of
puppies who have plenty of bones to go around
and are not possessive of any one particular
bone. This allows the electrons to move
through the substance with little restriction. The
model is often described as the "kernels of
atoms in a sea of electrons.”
METALLIC BONDs


Electrons are “delocalized”
Electrons move freely around and
between atoms in the network of empty
orbitals. The electrons move through the
substance with little restriction. The
model is often described as the "kernels of
atoms in a sea of electrons.”
METALLIC BONDS

The free movement of electrons explains
the properties of metals…
 Conductivity: free movement of electrons
 Malleability & ductility: bonding is the same in
all directions
 Luster: energy released as electrons move
easily between orbitals
We will look more closely at the
different types of bonds to
understand how millions of
different compounds form from
only about 100 different
elements!
Lewis Dot of Ionic Compounds
(electrons taken, not shared)
Write the dot structures of the neutral
atoms
 Write the ions created by the loss or gain
of electrons, in brackets. The metals have
no dots. The non-metals have a full outer
shell.
 Show the ion charges as superscripts.
 Show the ratio of ions needed to create a
net charge of zero.

Criss-Cross Method to
Determine Ratio of Ions

Write the ion symbols and charges
(superscripts). Get this info from periodic table for
elements, polyatomic ion list for polyatomic ions.

Criss-cross the charge (numbers only) to
subscripts. This shows the ratio of ions required for a
net charge of zero.

Simplify.
Mg2+ N3Mg3N2
Charges of ions from PT
group
1
2
13
14
15
16
17
18
ve
1
2
3
4
5
6
7
2 or 8
charge
1+
2+
3+
4+/-
3-
2-
1-
0
Why does it work?
The goal is a net charge of 0. The charges,
or oxidation numbers, must add up to
zero.
Mg3N2
3 x 2+ = 6 +
2 x 3-= 60 net charge
Lewis Dot Structures of Covalent
Bonds(shared electrons)
Lewis dot structures show atoms near each
other sharing pairs of electrons to create bonds
and satisfy octet rule for both atoms.
 They may share up to 3 pairs of electrons

– 1 SHARED PAIR
– 2 SHARED PAIRS
– 3SHARED PAIRS

:SINGLE BOND
::DOUBLE BOND
:::TRIPLE BOND
It does not matter which atom the electrons
come from because they are shared.
Covalent bonds usually follow the octet rule

Put all of the valence electrons in a pot
and distribute them between the atoms so
each atom has a full octet by sharing up
to 3 pairs of electrons.
:
single bond
::
double bond
::
triple bond