However, it should be noted that no bond can be 100% ionic--there is always some sharing of electrons between atoms. For instance, NaCl is about 80% ionic and 20% covalent. Conversely, polar covalent bonds can be thought of as paritially ionic (such as the bond in HCl which is often mentioned to be about 10% ionic and 90% covalent).
Ionic and covalent are useful concepts but it must be remembered that they are extremes and that many bonds lie somewhere in between. Only a nonpolar covalent bond is close to 100% pure (and, even then, some valence bond structures of these can be partially ionic).
Problem 7.38: What general trend in electronegativity occur in the periodic table?
What we shall do here is first show another modified periodic table. This time electronegativities are shown for all the elements. We have both a number for each one and a direct indication of trends as a whole.Problem 7.39: Predict the electronegativity of the undiscovered element with Z = 119.
Obviously the trends are now easily stated:
- Electronegativity increases from left to right across a period (excluding group 8A, the snobbish gases).
- Electronegativity decreases down a group. But...
- Be careful here! Note that, with transition elements, it increases going down for some groups.
- What happens with the 8B elements (Fe, Co, and Ni and their relatives below them, Ru-Rh-Pd and Os-Ir-Pt) is particular interesting in that these are all pretty close to each other; these three sets of elements are collectively known as the triads and they have a great many things in common.
- The 1B elements, Cu, Ag, and Au, are also interesting in their behavior. The high electronegativity of Au (gold) is of particular interest in some applications.
- By the time we get to group 2B (Zn, Cd, Hg) things are beginning to "flatten out."
- Things are back to normal and "pHull of pHun" when we go further right with the group 3A elements and beyond.
- Electronegativities are in some respects heuristic (hand-waving) numbers. But for quick estimates of things, they are very very useful!
Again, we show the chart. This has two purposes:Problem 7.42: Which of the following substances are largely ionic and which are covalent?
Here is the chart again: We note that element #119 would be directly below francium.
- It makes things better for those of you who like to use the web pages.
- It makes things more difficult for those who insist on printing everything out on paper. (I really like getting the collective goat of those people!)
All we need do is look at the left-most column. We see the following numbers:
Li 1.0 Na 0.9 K 0.8 Rb 0.8 Cs 0.7
Just extrapolating, and noting that the increase in atomic number, if plotted vs. this would look rather shallow (the atomic numbers go as 3, 11, 19, 37, 55). Fr is 87 and we would probably predict its electronegativity as 0.7 or 0.6. (Since we only go to +0.1 in the estimates, these are probably good numbers.)
The end result here is that we would probably set the E.N. of element #119 to be about 0.6. This is quite low but one would expect it to be the least electronegative of ALL elements in any case.
With electronegativities, there is a fairly simple, albeit crude, rule:Between two atoms:
We apply these rules to the bonds given. In order to facilitate things for the web students and to aggravate those who insist on having hard copy for everything, we again show the E. N. periodic table figure:
- If DEN > 2, the bond is primarily ionic.
- If 2 > DEN > 0, the bond is polar covalent.
- If DEN = 0, the bond is pure covalent (or close to it).
Now, we cruise forth and handle the bonds. Just use the numbers above to get those below.
|(a)||HF||ENF = 4.0; ENH = 2.1: DEN = 1.9||HF is polar covalent.|
|(b)||HI||ENI = 2.5; ENH = 2.1: DEN = 0.4||HI is polar covalent.|
|(c)||PdCl2||ENCl = 3.0; ENPd = 2.2: DEN = 0.8||PdCl2 is polar covalent.|
|(d)||BBr3||ENBr = 2.8; ENB = 2.0: DEN = 0.8||BBr3 is polar covalent.|
So, we look at the
ENO = 3.5; ENH = 2.1: DEN = 1.4
OH- is polar covalent.
|(f)||CH3Li||Here, we know that the C-H bond
covalent (with just a difference of 0.4). So,
what about C-Li? We look at this now.
ENC = 2.5; ENLi = 1.0: DEN = 1.5
CH3Li is polar covalent.
There are lots and lots of bonds which are polar covalent. Just a few are ionic!Problem 7.43: Use the electronegativity data in Figure 7.4 to predict which bond in each of the following pairs is more polar.
Just use the chart for these! In fact, here it is again (to help you and to infuriate the folks who insist on printing out everything):
|(a)||C-H or C-Cl||Respective DEN's: 0.4 & 0.5||C-Cl is more polar|
|(b)||Si-Li or Si-Cl||Respective DEN's: 0.8 & 1.2||Si-Cl is more polar.|
|(c)||N-Cl or N-Mg||Respective DEN's: 0.0 & 1.8||N-Mg is more polar.|
Not really difficult, is it? Note that, in the last line, N and Cl have about the same EN's. Only O and F exceed these--but by quite a bit!