*A Chemical Bond is a mutual electrical
attraction between the nuclei of one
atom and the valence electrons of
another atom.
*This attraction binds the atoms
together.
a.
Metallic bonding – “metal to metal”
• Results from the attraction between metal
atoms and the surrounding sea of electrons
b.
Ionic bonding – “nonmetal to metal”
• Results from the electrical attraction between
large number of cations and anions. It involves
the transfer of electrons from one atom to
another
c.
Covalent bonding – “nonmetal to nonmetal”
• Results from the sharing of electron pairs
between two atoms
*
* Many
atoms transfer electrons and other
atoms accept them. This creates cations
(positive ions) and anions (negative ions).
sodium atom
Na
sodium ion - Na+1
Na
chlorine atom
Cl
chlorine ion – Cl-1
Cl
Ionic bonding is the electrostatic
attraction between positive cations and
negative anions. This is the strongest
bond type.
*In many cases electrons do not completely
transfer from one atom to another. The
electrons between atoms are shared -like a
“tug of war”
*Consider two hydrogen atoms:
Hydrogen
Hydrogen
H
H
The two hydrogen atoms SHARE the 2 electrons equally,
giving each atom 2 electrons
H:H
the electron orbitals overlap or “share” electrons
When electrons are shared equally the bond is called
a nonpolar or pure covalent bond
*
*A polar covalent bond is when the
electrons between two atoms are NOT
shared equally. It is an unequal tug-ofwar.
*Each Atom donates one electron to be
shared to give each a stable, filled outer
energy level
*
But sometimes, the two electrons are shared
UNEQUALLY…one atom “tugs” harder on the
shared electron pair (in this case the Cl is
stronger). This creates an uneven distribution
of charge. The H will have a partial positive
charge (
) and the Cl will have a partial
negative charge ( ).
*
Since this bond has two “poles” – a positive
pole and a negative pole – it is called a
Polar Covalent Bond
H
Cl
*We use electronegativities to determine
whether we have an ionic, polar or non-polar
bond
*An electronegativity value is a number
ranging from 0.0 to 4.0 that shows the
tendency of an atom to attract electrons
towards itself.
The electronegativity values for the elements are found
on an “electronegativity chart”, which looks like the
periodic table. The numbers on the chart are the
electronegativity values for that element. The HIGHER the
number, the STRONGER the attraction for the electrons.
*
*How do we determine what type of bond
will occur between two atoms?
*The DIFFERENCE between their
electronegativity values (EN) will tell us:
electronegativity
bond
difference (
type
EN)
1.7 and higher
ionic
0.3 – 1.7
polar
0.0 – 0.3
non-polar
*What type of bond will occur between
Iodine (EN= 2.7) and the following
elements: cesium (EN=0.8), iron
(EN=1.8) and sulfur (EN=2.5)
Bond between
Iodine
and
Electronegativity
difference
cesium
2.7 – 0.8 = 1.9
iron
2.7 – 1.8 = 0.9
sulfur
2.7 – 2.5 = 0.2
Bond Type
More negative
atom
*
Bonding
between:
EN difference
Bond Type
Na and Cl
S and O
Ca and Br
P and H
Si and Cl
S and Br
1.7 and greater
0.3 – 1.7
0.0 – 0.3
IONIC
Polar
Non-Polar
More negative
atom
*What if the electronegativity difference
is 0.3 or 1.7? What type of bond is it?
*The cut-off numbers are guidelines, not
definite delineation points. It is more
of a gradual change from one bond type
to another.
Sodium and chlorine
ionic bond
Sulfur and oxygen
polar covalent
S
O
*Calcium and Bromine
*Phosphorus and hydrogen
*Silicon and chlorine
*Sulfur and bromine