CHEM 121
Introduction to Fundamental Chemistry
Summer Quarter 2008 SCCC
Lecture 7
http://seattlecentral.edu/faculty/lcwest/CHE121
Forces Between Particles
ƒ Noble Gas Configurations9
ƒ Ionic Bonding9
ƒ Ionic Compounds9
ƒ Naming Binary Ionic Compounds 9
ƒ The Smallest Unit of Ionic Compounds 9
ƒ Covalent Bonding 9
ƒ Polyatomic Ions 9
ƒ Shapes of Molecules and Polyatomic Ions 9
ƒ The Polarity of Covalent Molecules
Electronegativity can be defined as:
“The tendency of an atom to pull electrons towards itself in a
covalent bond.”
Electronegativity increases from left to right across a period
and decreases going down a group.
What element do you expect to be the most electronegative?
Note that electronegativity is not defined for the noble gases.
Atoms of the same type have identical electronegativities.
When atoms of the same type form covalent bond electrons will
be shared equally.
• This is called a nonpolar covalent bond.
Atoms of different types have different electronegativities. When
atoms of form covalent bonds the electrons will be shared
unequally.
• This is called a polar covalent bond.
Table 4.4 in our text book gives numerical values for the
electronegativities of selected elements.
An important rule is if the numerical value for the difference in
electronegativites of two atoms is > 2.1 then electrons are
completely transferred and the bond is ionic.
When atoms with different electronegativities share electrons in a
covalent bond the shared electrons will be closer to the more
electronegative element resulting in a polar covalent bond.
C
δ+
O
δ-
What happens in a molecule with multiple covalent bonds?
We can define two types of covalent molecules containing polar
covalent bonds:
1. If there is a symmetrical charge distribution about
the central atom then the molecule is classified as
nonpolar.
δ−
F
F
δ−
B
δ3+
δ−
F
δ−
δ2+
δ−
O C O
2. If there is a nonsymmetric charge distribution about
the central atom then the molecule is classified as
polar.
δ2-
H
δ+
O
H
δ+
When we name binary ionic compounds formed by simple ions
we give the name of the cation followed by the name of the
anion.
e.g. NaCl is called “sodium chloride”, MgCl2 is called “magnesium
chloride”
The name does not specify the number of ions of each type, this
is meant to be understood.
How do we name covalent compounds?
For covalent compounds there are three naming rules:
1. Give the name of the less electronegative element
first.
2. Give the stem name of the more electronegative
element and add the suffix –ide.
3. Indicate the number of each atom using Greek
prefixes.
You will need to learn the Greek numerical prefixes (Table 4.6):
Number
1
2
3
4
5
6
7
8
9
10
Prefix
MonoDiTriTetraPentaHexaHeptaOctaNonaDeca-
Lets do some examples:
Name the following:
a) CO2
a) Carbon dioxide
b) CO
b) Carbon monoxide
c) N2O5
c) Dinitrogen pentoxide
d) NO2
d) Nitrogen dioxide
e) CS2
e) Carbon disulfide
As you can see there are some instances where the rules are
broken.
To name compounds containing polyatomic ions we write the
name of the cation followed by the name of the anion.
The names of some polyatomic ions are given in Table 4.7.
Memorize the following:
Formula
NH4+
CO32CNOHNO3PO43SO42-
Name
Ammonium
Carbonate
Cyanide
Hydroxide
Nitrate
Phosphate
Sulfate
Some rules to keep in mind when writing the formulas for
compounds containing polyatomic ions:
1. The overall charge of the compound must be zero.
e.g. NH4Cl 9
NH4Cl2 8
2. If there is more than one of the ion parenthesis are put
around the ion and the number indicated with a
subscript.
e.g. (NH4)2Cr2O7
Mg(CN)2
Na2SO4
Ba(OH)2
Forces Between Particles
Ionic Bonding (polyatomic ions or metal/nonmetal)
Covalent Bonding (nonmetals)
• Molecules
• Network Solids
Metallic Bonding
• attraction between “sea of valence electrons” and +ve
“core of atoms”
Interactions of polar molecules
• dipole-dipole interactions (H-bonding)
• dipole-ion interactions
We have previously talked about ionic crystalline compounds
where we had lattice sites occupied by ions which formed ionic
bonds with adjacent ions to form an infinitely repeating crystal.
We have talked about covalent
bonds in molecules.
Crystalline compounds form
when we have covalent bonds
between atoms at lattice sites.
We have talked about covalent bonds in molecules.
Crystalline compounds form when we have covalent bonds
between atoms at lattice sites.
Another compound of this type
is silica (quartz). Quartz has
the formula SiO2.
There are two types of lattice sites in silica. In the first is a
silicon atom that is covalently bonded to four oxygen
atoms in a tetrahedral arrangement. In the second is an
oxygen atom covalently bonded to two silicon atoms in a
bent arrangement.
Quartz and diamond are an example of a class of substances
called network solids.
Network solids have extremely high melting points and
hardness as the atoms they are composed of are held by strong
covalent bonds.
In metals lattice sites are
occupied by metal atoms. The
valence electrons of metal
atoms are loosely held and
they move freely throughout the
lattice.
The attraction between the positive core of the metal atoms and
this mobile “sea of electrons” is called a metallic bond.
When a molecule has an asymmetric distribution of charge we
say it is polar or that it has a dipole.
A molecule that has a dipole will have regions that have a net
positive charge and regions that have net negative charge.
Electrostatic attraction between the positively and negatively
charged regions of polar molecules is called a dipole-dipole
interaction.
Dipole-dipole interactions are weaker than ionic or covalent
bonds. However, they are significant and effect boiling point,
melting point and other properties.
It is also possible to have an electrostatic interaction between a
polar molecule and an ion. These kinds of interactions are
called dipole-ion interactions.
We can also have dipole-dipole interactions between
molecules of different types.
A special class of dipole-dipole interactions occurs between
molecules that have a hydrogen atom covalently bound to a
very electronegative element (O, N, S).
This type of interaction is called a hydrogen bond.
Hydrogen bonding is very important in living systems. For
example between base pairs in DNA.
Link these together and we
get the DNA double helix.
Hydrogen bonding also determines how proteins fold to form
characteristic structures such as β-sheets and α-helixes.