Electronegativity
How Badly Does an Element Want Electrons?
Electronegativity (EN) is the relative ability of a bonded atom of an
element to attract the electrons from the another element
participating in the bond.
As with most aspects of chemistry, the EN of an element
is a direct result of its Atomic Structure. We shall
discuss this relationship between electronegativity and atomic structure
at length in this section.
We have previously discussed the two most common forms of bonds
between elements Ionic Bonds and Covalent Bonds. In most actual
chemical compounds, however, the type of bonding that occurs between
elements falls somewhere between these two extremes of Ionic and Covalent
Bonds.
There are some very real and common examples of true Ionic Bonds, and a
number of true nonpolar covalent compounds. However, the great majority
of compounds have bonds that are more accurately described as polar
covalent bonds. These polar covalent bonds are partially ionic and
partially covalent as shown in the picture below.
Ionic - Covalent - Polar Covalent Bonds
In an Ionic Bond the electrons are completely transferred from one
element to the other. In a covalent bond, in this case a nonpolar
covalent bond, the electrons are fully and equally shared between the
two elements forming the bond.
However, a Polar Covalent Bond takes on certain aspects of both and an
Ionic Bond and a Covalent Bond in that the electrons are shared but one
element attracts them more strongly than the other. In the resulting
polar covalent bond, a partial positive and partial negative charge is
created at the poles of bond thus giving the Covalent Bond some Ionic or
Polar properties.
One of the most important concepts in chemical bonding is
Electronegativity which is abbreviated EN. More than 50 years ago
the great American chemist Linus Pauling developed the most common scale
of relative EN values for the elements.
The Pauling values for EN are shown in the following two pictures.
Pauling EN Values
As stated before the electronegativity of an element is a function of
its Atomic Structure. Because the nucleus of a smaller
atom is closer to the shared pair of electrons in a bond than that of a
larger atom, the nucleus of the smaller atom attracts the bonding
electrons more strongly than that of the larger atom.
So EN is inversely related to the size of an atom.
That is as the size of an atom increases, the EN
decreases. As the size of an atom decreases, the EN
increases.
So, in general, since the size of an atom will go up as we proceed down
a column (Group) of elements in the Periodic Table, the
EN will go down as we go down a Periodic Table Group
(Column).
Also, as we proceed to go up in atomic number across a Row (Period) in
the Periodic Table, the size of atoms generally decreases due to larger
numbers of electrons in the outer shell being attracted to the nucleus.
Thus the atomic size will generally decrease and the EN
will increase as we go left to right across a Periodic Table Row (Period).
We have illustrated this for you in the picture below.
EN Periodic Table
One important use of electronegativity is in determining an atom's
oxidation number (O.N.) The oxidation number is sometimes called
the valence number of an atom. In Ionic Compounds we have defined the
valence number as the number of electrons that an atom of an element
must give up or accept to achieve an electron-full outer shell.
For covalent compounds, which share electrons, we need a slightly
different definition of the oxidation number. We use the
electronegativity and some other atomic structure information to
calculate the oxidation number as follows:
EN and Oxidation Number (O.N.)
For example in NaCl (Sodium Chloride or table salt), Cl is more
electronegative (EN = 3.0) than Na (EN = 0.9). Therefore, according to
the rules above, we assign both (2) shared electrons to Cl and 0 shared
electrons to Na.
Cl has 7 valence electrons and 6 unshared electrons in its outer shell. Na has 1 valence
electron and 0 unshared electrons in its outer shell.
Using the equation above for Cl: O.N. = 7-(2+6) = 7-8 = -1.
Using the equation above for Na: O.N. = 1-(0-0) = 1-0 = +1.
Delta EN and Bond Type
When you look at a bond between any two elements X and Y and ask "Is the
X-Y bond ionic or covalent?", the answer in almost every case is "Partly
Ionic and Partly Covalent".
The partial-ionic/partial-covalent character of any given bond X-Y is
directly related to the Electronegativity Difference (Delta EN), which
is the arithmetic difference between the EN values of X and Y.
The relationship between Delta EN and Bond Type is shown in the picture
below.
Delta EN vs Bond Type
A greater Delta EN gives a larger partial charge across the bond and a
higher partial ionic character. A Delta EN of zero yields a Nonpolar
covalent bond.
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