electrons equally. But sometimes in a covalent bond, one atom tends to hog the
electrons. It still shares them with the other atom, but it tends to keep the
electrons for more than its fair share of the time. This hogging of the electrons is
a result of one atom having a greater electronegativity value than the other.
(Remember that electronegativity increases as we move from left to right across
a period and decreases as we move from top to bottom in a column.)
What Causes a Polar
Covalent Bond?
A polar covalent bond is
caused by a difference in
electronegativity between
atoms.Think about a water molecule. It’s made of 2 hydrogen atoms and 1 oxygen
atom. Each hydrogen atom has 1 valence electron, which it shares with oxygen.
The oxygen atom donates 2 electrons to be shared with the hydrogen atoms.
Then what happens? Basically, each hydrogen atom acquires an electron and has
a configuration like helium’s (which is very stable), and the oxygen atom
acquires 2 electrons and has an electron configuration like neon’s (an octet).
So a water molecule can be represented as follows:
But oxygen’s electronegativity is greater than hydrogen’s. Oxygen “hogs” the
electrons it shares with hydrogen, and the shared electrons spend more time
around the oxygen than they do around the hydrogen. The result? Each hydrogen
atom has a partial positive charge, while the oxygen atom has a partial negative
charge.
When, in a covalent bond, certain atoms have a partial positive charge and others
have a partial negative charge, we say that the covalent bond is polar.