source : yahoo.com
Which of the following pairs of elements is most likely to form an ionic compound?
Give it up Brooke. And by the way, there are no 100% ionic compounds.
This is a tricky concept because there is not a clear distinction between ionic and covalent bonds and between ionic and covalent compounds. “Ionic” and “covalent” exist on a continuum. Most bonds have characteristics of both ionic and covalent bonds.
In fact there are NO 100 percent ionic compounds. Even the bonds in CsF which have the greatest electronegativity difference (DEN) are 8 percent covalent.
More chemical bonds are covalent than are ionic. All bonds like along a continuum between ionic and covalent and have some characteristics of both. We choose an arbitrary point (where the electronegativity difference is above 2.0) to say that a bond is predominately ionic.
It is possible to compute the percent ionic character in a bond with the following formula:
percent ionic character = 100(1 – e^(-DEN^2/4) )
There are many folks who still don’t understand that bonding cannot be reduced to either “ionic” or “covalent. Keep in mind that there are NO compounds which are completely ionic. Actual chemical bonds lie along a continuum between covalent and ionic. Even the most ionic of bonds (Cs-F) is not completely ionic (it is 8 percent covalent) and share electrons to a certain degree.
Therefore, it really doesn’t make sense to try to peg a bond as either “ionic” or “covalent”. What does make sense is to try to place the bond somewhere along the continuum. The key is to look at the electronegativity differences. The greater the electronegativity difference, the greater the percent ionic character.
For instance, in benzoic acid there are C-O bonds which are quite covalent and O-H bonds which are also very covalent, as well as C-H bonds which are almost 100 percent covalent.
In KCl the bonds are very polar covalent. The electronegativity difference is (3.16-0.82 = 2.34) great enough to say that the bonds behave as if they were ionic (yet the percent ionic character is 75 percent ionic, or 25 percent covalent).
Sodium sulfate also has a high percent ionic character, while sucrose has bonds which are much more covalent in nature (it has the same bonds as does benzoic acid).
Sometimes people think that if they dissolve the compound in water and the solution conducts electricity that the compound will be ionic. This only works part of the time. Lots of molecular compounds with covalent bonds will dissolve in water to produce ions which will make the solution conduct electricity. So this turns out to be an ineffective test for ionic vs covalent compounds. The best example of why this doesn’t work in hydrochloric acid. Hydrochloric acid is produced with molecular HCl gas, with covalent bonds, dissolves in water to produce hydrogen ions and chloride ions. There are many other compounds which will produce ions in solution, but which have predominately covalent bonds.
A better test that involves conductivity is to melt the compound. If the molten substance conducts electricity then there is a greater likelihood that the compound contains ions, and has ionic bonds.
In fact, the oxidation state of an element will determine how ionic or covalent the bonds are. Consider permanganate ion, MnO4-. In permanganate the bonds are much more covalent than the bonds between Mn and Cl in MnCl2 even though oxygen is more electronegative than chlorine.
Sometimes people will try to use melting point as an indicator of ionic or covalent bonds. The melting point is determined by intermolecular forces, not the type of bonding within the molecule. In fact the substance with the highest melting point, diamond, is composed of purely covalent bonds.
The bottom line is that the type of bonding is not as clear cut as some teachers (who aren’t chemists) try to make it out to be. To determine the predominate type of bonding you really need to look at a variety of factors including dipole moment, bond energy, electronegativity difference, and whether the compound forms discrete molecules or exists as a network solid.
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