Introduction To Chemical Bonding
Why do atoms form bonds?
To get 8 valence electrons
(also called an octet of electrons.)
Why do atoms need 8 valence electrons?
This makes them stable
(meaning the atom will be in its
lowest potential energy state.)
Do all atoms need 8 valence electrons?
Most do; some can be stable with less than 8
(like H, He, Li, Be, B);
and some can be stable with more than 8
(like S, P, Xe)
these atoms are said to be exceptions to the octet rule
Do any atoms have 8 valence electrons
Yes, the noble gases
What IS chemical bonding?
It is when atoms gain, lose, or share valence electrons to the
mutual benefit of all the atoms involved; the result is the
formation of a chemical compound.
Are ALL the valence electrons involved in
No, typically only the unpaired valence electrons are lost,
gained, or shared
(though there are exceptions…as always!)
How do I know if an atom has unpaired
Draw the atom’s electron dot structure
example of electron-dot structures for neutral atoms
Cl unpaired electron
Are all chemical bonds the same?
No, they can be roughly divided into the following
1. Ionic bonds 2. Covalent bonds
(A.K.A. molecular bonds)
What’s the difference?
Ionic bonds = gain or loss of electrons
Covalent bonds = sharing of electrons
What determines whether a bond will be
ionic or molecular?
the percent ionic character of the bond which is
determined by the electronegativity of the bonded atoms
Well, what is meant by percent ionic
character of a bond?
Percent ionic character = the polarity of the bond
What the heck does THAT mean?
Polarity = the presence of distinct ‘poles’, like on a bar
magnet, at each end of a bond
The more polar the bond the more ionic the character of
Ionic bonds = very distinct poles = greatest % ionic character
Polar covalent = less distinct poles = less % ionic character
Nonpolar covalent = no poles = no ionic character
0% 100 %
type of bond
nonpolar more covalent polar more ionic ionic
covalent than ionic covalent than covalent
What would cause bonds to be polar or
nonpolar in the first place?
Differences in electronegativities (Chp. 5) between the
bonded atoms means unequal sharing of the electrons.
When electrons are shared unequally one atoms becomes
more (–) and the other more (+) and viola! – Poles!
How does unequal distribution of electrons
relate to ionic and covalent bonding?
No sharing = distinct poles = ionic bonding
Unequal sharing = indistinct poles = polar covalent
Equal sharing = no poles = nonpolar covalent
How do you figure out the percent ionic
characteristic of a bond?
Use the following equation:
electro – electro
% ionic character = x 100
What do you do with this information once
you have it?
Use the following table as a general guideline:
% ionic character Type Type
of bond of of
< 5% Nonpolar to barely Nonpolar
polar Covalent covalent
5% ≈ 50 % Polar covalent Polar covalent
> 50% Ionic Ionic
a) Determine the type of bond between Na and Cl
b) Determine if this compound is covalent or ionic
Step 1: Find the electronegativity the two atoms:
Cl = 3.16 Na = 0.93
Step 2: Use the equation above to find % ionic characteristic of
3.16 – 0.93
x 100 = 70.6 % ionic characteristic
Step 3: Consult the table above:
70.6 % ionic character = ionic compound
Why is it important to know whether a
compound is ionic or covalent?
Ionic and covalent compounds have very different physical
and chemical properties…knowing whether a compound is
ionic or covalent allows a scientist to predict both its
physical and chemical behaviors
Bonds/ Compounds Bonds/Compounds
Conductors in molten, dissolved, or Nonconductors
gas phase in any phase
High melting and Low melting and
boiling points boiling points
Brittle bonds Nonbrittle bonds
Gain/ lose electrons Share electrons
Strength of bond measured as Strength of bond measured as
lattice energy bond energy
Forms crystal lattice structures Forms Molecular compounds
whose smallest structure is the whose smallest unit is the
nonindependently existing independently existing molecule
Large differences in Small differences in
electronegativity between the electronegativity between the
bonded atoms bonded atoms
Solid at room temperature (except Solid or vapor at room temperature
Hg) (expect Br)
Usually occurs between a metal and Usually occurs between two
a nonmetal atom nonmetal atoms
Composed of anions and cations the Composed of neutral atoms
least whole-number ratio of which
create the formula unit