Chemical Bonding- Polarity Notes
Polarity
Both covalent and ionic bonds are affected by a property called polarity. When something
demonstrates polarity, that means it has two regions of opposite character. For example,
North/South poles or opposite poles on a battery.
If, in a chemical bond, the electrons are shared unequally, the bond is “polar.”
When 2 H’s combine to form H2, each atom will attempt to take the other’s electron to
become more stable (filling the 1st energy level). Instead of them taking away electrons
from each other, they share equally.
No poles form because the two atoms are equal and they share electrons equally.
What about diatomic fluorine? (Diatomic means two atoms of the same element bond to
each other). In this case, two fluorines combine to form F2.
Each F atom has identically strong electron affinity; they each need one electron for an
octet. No pole is formed because the atoms share electrons equally.
Hydrogen Fluoride
What about when an H and an F combine to form HF? The H has one electron to offer
while F really wants that electron, to form an octet. It has a HIGH electron affinity
(MUCH higher than H). Because F pulls on the electron much more strongly, the electron
is shared unequally.
The electron spends more time around the F atom. The shared electron will spend more
time around the F nucleus, so that side of the molecule will have a slightly negative
charge. The shared electron will spend less time around the H nucleus, making that side
of the bond slightly positive.
The HF bond is therefore POLAR!
H2 O
What about when 2 H’s and 1 O combine to form H2O?
The O has much greater electron affinity that the H’s.
These three atoms (all non-metals) combine in a covalent bond, but the oxygen atom
pulls more strongly on the shared electrons, therefore the electrons are shared unequally.
Water is a polar molecule!
Bonds can be anywhere from completely non-polar to extremely polar. The polarity of a
bond depends on the difference between the atoms’ electronegativity. Remember,
electronegativity is the ability of an atom to attract electrons to itself (in a chemical
bond).