1. less than 0.5; essentially nonpolar between 0.5 and 2.0: polar
QUESTION:
Which pair of atoms would form a polar covalent but essentially nonpolar bond? A. N and N, B. H and C, C. Na and Cl, D. Be and I
To answer this question, we need to look up electronegativity (EN) values. Nature of the bond depends on difference in EN values
0, pure covalent
less than 0.5; essentially nonpolar
between 0.5 and 2.0: polar
larger than 2.0, ionic
SCRIPT:
Which pair of atoms would form a polar covalent but essentially nonpolar bond? A. N and N, B. H and C, C. Na and Cl, D. Be and I
PAUSE
CLICK
Polar covalent bonding refers to unequal sharing of electrons.
So we can rule out choice A right away. There’s no reason for electrons to prefer one nitrogen atom over another nitrogen atom.
The bond between two nitrogen atoms is PURE covalent.
CROSS OUT Choice A
To determine which of the other choices is correct, we need to look up electronegativities.
The nature of the bond between two atoms depends on the difference in their electronegativities.
CLICK If the difference is zero, as in choice A, we have a pure covalent bond.
CLICK If the difference is less than 0.5, the bond is, strictly speaking, polar covalent, but is considered to be essentially nonpolar
CLICK If the difference is between 0.5 and 2.0, the bond is considered polar.
CLICK And if the difference is larger than 2.0, the bond is considered ionic. By ionic, we mean that the electron has essentially transferred over to
The more electronegative atom.
CLICK If we look up the electronegativity values, we find that
CLICK the electronegativities are 2.2 for hydrogen and 2.5 for carbon. That’s a difference of Therefore, a carbon-to-hydrogen bond
is nonpolar. The correct answer is B.
CLICK Let’s look at the other choices…
Sodium has an electronegativity of 0.9 while chlorine has an electronegativity of That’s a differenc eof The bond between sodium
And chlorine is ionic. Sodium and chlorine will not be sharing electrons.
CLICK As for beryllium and iodine, the difference is This makes the beryllium-to-iodine bond a polar bond.
You might be find this disconcerting. Beryllium is a metal and iodine is a nonmetal, and
you may have previously learned that a compound between a metal and nonmetal is ionic.
That rule is what we call a rule of thumb. It doesn’t always work.
This is typical in Chemistry. Whenever we classify things, there are always some gray areas.
So how should you deal with these gray areas on a test.
If you’re not given access to electronegativity values on a test, your best bet is
to say that a compound between beryllium and iodine is ionic,
… unless of course your teacher expects you to be very familiar with the gray areas.
PAUSE END RECORDING
H: 2.2, C: 2.5; Difference = 0.3
Na: 0.9, Cl: 3.2; Difference = 2.3
Be: 1.6, I: 2.7; Difference = 1.1

2. Video ID: © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Narrator: Funded by Louisiana Board of Regents Contract No. LA-DL-SELECT-13-07/08