http://www.ck12.org Chapter 9. Covalent Bonding
FIGURE 9.12
Electronegativities of the elements.The general trend in electronegativity is that the value increases from left to right across a row in the periodic table
and decreases down a column. The most electronegative element is fluorine, which has a value of 4.0. As discussed
in the lesson "Trends in the Periodic Table," electronegativity increases across a row as the number of protons in the
nucleus increases and therefore has a stronger pull. Electronegativity decreases down a column due to an increased
effect of electron shielding in larger atoms.
Polar Bonds
If there is a difference between the electronegativity values of the two atoms involved in a covalent bond, the bond
is said to be polar. In this situation, the more electronegative atom has a stronger tendency to attract the shared
electrons toward itself. For example, in the water molecule, oxygen has an electronegativity value of 3.5, while
hydrogen has a value of 2.1. Since O has a higher electronegativity value, the electrons in each covalent bond will be
pulled closer to the oxygen atom. Because the negatively charged electrons are spending more time near the oxygen
atom and less time near the hydrogen atoms, each atom appears to have a partial electric charge when averaged over
time. The Greek letterδ(delta) is used to designate a partial charge. As we see in the illustration, oxygen has a
partial negative charge, and the hydrogens each have a partial positive change. These charges are fractional charges,
less than the full +1 or -1 charge that we might see for a sodium or chloride ion.
FIGURE 9.13
Partial charges in the water molecule.A similar situation occurs in the hydrogen fluoride molecule. The difference in electronegativity between fluorine
(4.0) and hydrogen (2.1) is quite high, so the shared electrons spend much more time in the vicinity of the fluorine
atom. As a result, fluorine carries a partial negative charge in this molecule, whereas hydrogen carries a partial
positive charge.