The Octet Rule
All atoms “want” a full valence shell of eThis makes them more stable, like the Noble
Gases, which have 8e-, a full valence shell.
For H and He a full valence shell is 2 eFor all bigger atoms, a full valence shell is
8 e- (hence “octet”)
The Octet Rule
All atoms “want” their valence shell to mimic
a Noble Gas e- configuration:
Most chemical reactions are the result of
atoms gaining or losing or sharing e- with
other atoms in order to have 8 valence e-
Covalent Bonding
Definition: two or more atoms sharing
valence e- so that each atom has a full
valence shell (2 e- for H and He, 8 e- for
all bigger atoms)
H
Cl
Covalent Bonding
Definition: two or more atoms sharing
valence e- so that each atom has a full
valence shell (2 e- for H and He, 8 e- for
all bigger atoms)
H Cl
When does covalent bonding
occur?
1) Between elements that are non metals
2) Between two elements with an
electronegativity difference of less than
1.7
How do I find the electronegativity
difference between 2 atoms?
Using Table S, look up the electronegativity values
for each atom. Then find the difference by
subtracting the smaller value from the bigger
value:
H = 2.1 Cl = 3.2
Electronagativity difference = 3.2 -2.1
= 1.1
Since 1.1 is less than 1.7, this bond is covalent
Forming a Covalent Bond
Non- polar covalent bonds
NON POLAR covalent bonds occur when
the electronegativity difference of the two
bonded atoms is zero.
When does this happen?
When the two bonded atoms are identical
Non- polar covalent bonds
All Diatomic atoms have non-polar covalent
bonds:
H2 N2 O2 F2 Cl2 Br2 I2
Non- polar covalent bonds
In a non polar covalent bond, the shared
electrons are shared equally between
both atoms
This means they spend equal amounts of
time orbiting both nuclei
Double covalent bonds
Some atoms can share TWO electrons with
another atom. This results in a double
bond.
O
O
Each oxygen atom has 6 valence eBy sharing 2 e- pairs, they will each fill their
Valence shell with 8 e-
Double covalent bonds
O
O
Double bond
Each oxygen atom has 6 valence eBy sharing 2 e- pairs, they will each fill their
Valence shell with 8 e-
Lewis dots can get tedious to draw for
double bonds, so often the “stick” model is
used instead:
One
represents a shared pair of e-
O O
Triple covalent bonds
Some atoms, like nitrogen, can form a triple
covalent bond with other atoms
Each atom shares 3 of its valence e-
N
N
POLAR covalent bonds
Except for diatomic molecules, most
covalent bonds are POLAR
This means one atom is more
electronegative than the other, and the eare NOT SHARED EQUALLY
The e- spend more time near the nucleus of
the more electronegative atom
Since Cl is more electronegative than H,
The H-CL bond is polar
Since the shared e- spend more time near the
Cl nucleus, the Cl side of the molecule has a
partially negative charge
+ H Cl
-
The H side has a partially positive charge
The whole molecule is neutral
Water Molecule
The water molecule H20 has polar covalent
bonds between the O and H.
The shape of the molecule is also important:
It looks like Mickey Mouse
Water Molecule
The Mickey Mouse shape also makes the
molecule polar: The O end is partially
negative, and the H end partially positive
+
The non-bonding
e- are called
“lone pairs”
XX
X
_
The bonding eare called the
“shared pairs”
Polar Molecules
Just because a molecule contains polar
bonds does not mean the molecule itself is
POLAR, like water.
C=O=C
carbon dioxide has polar bonds between C
and O but the symmetrical shape
eliminates the polarity of the whole
molecule
Molecules
Atoms bonded together by covalent bonds
form MOLECULES
Molecules are the smallest discrete particle
of an element or compound formed by
covalently bonded atoms
Molecules
Each atom in a molecule usually has 8
valence e- (a noble gas e- configuration)
Covalent molecules make up Molecular
Substances
Water is a molecular substance
Molecular Substances
Properties of molecular substances:
1)
2)
3)
4)
Soft (like butter)
Poor conductors of electricity and heat
Low melting points (melt easily)
Low boiling points (boil easily)
Metallic Bonding
Metallic Bonds
Are a special type of bond between metal atoms of
the same element
These bonds are characterized as:
Positive ions (the kernel) immersed in sea of
mobile (valence) electrons
The kernel is the nucleus and the non-valence e-
Metallic Bond:
Positive ions immersed in a sea of mobile
electrons
-
-
-
+
-
+
-
-
+
-
-
-
-
-
+
-
-
+
-
+
-
Metallic Bonds
The valence e- in a metal bond are free to
move between the different atoms.
These MOBILE ELECTRONS give metals
the ability to conduct electricity and heat
Also the metallic properties of malleabilty
and ductility