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Name:  Jess
Status: educator
Grade: 9-12
Location:TX
Country: N/A
Date: 3/30/2005


Question:
I read the other day in one of my student's textbooks that molecules are only formed via a covalent bond. I have researched all over the Internet and am starting to get information that ionic bonds form compounds but not molecules. This is because the elements that have a tendency to form ionic bonds have already formed molecules between themselves. Can molecules only be formed by covalent bonds or am I misinterpreting the textbook? After spring break I need to explain to my students.


Replies:
AARRRGGGHHH!!!! Once again texts spread confusion trying to "simplify" a concept rather than just "telling it like it is". GENERAL PRINCIPLE: Atoms and/or molecules form bonds by increasing the electron density between the atoms involved. The electrons involved being "attached" to atomic nucleii hold those nucleii together (that is a chemical bond is formed). Increased electron density between atoms ----> chemical bond. The chemical jargon is, "The atoms SHARE their bonding electrons, but that is sometimes misleading because it does not suggest HOW the SHARING is portioned out. In common usage, the word "sharing" suggests an equal division, but that is not the case in chemical bonds. If the two bonding atoms (they may or may not be part of a larger assembly of atoms i.e. a molecule) share their bonding electrons equally (more or less) we call the bond "covalent". On the other hand, if one of the atoms has a much greater affinity for the bonding electrons than the other atom (more or less) we call the bond "ionic". The terms "covalent" and "ionic" are abstractions. No bond is totally one or the other. That is why I say (more or less).

When the nucleii of two atoms have about the same affinity for electrons, the electrons are more or less shared equally. This happens with simple molecules like N2, O2, and a lot of carbon-carbon and carbon-hydrogen bonds in the case of organic compounds.

When the nucleii of two atoms have a very different affinity for electrons, the increased electron density between them tend to be shared unequally. In the limit one atom will "house" the electron almost completely and the other will be left with a deficiency of electrons (that is, the atoms form an ionic bond).

In the real world there are examples of chemical bonds with varying distributions of electron density between atoms from one extreme to the other. That is why some molecules are "polar".

Certain tendencies are characteristic of covalent and ionic bonds. Covalent bonds tend to be directional. Ionic bonds tend to be non-directional. Covalent bonds tend to pair all available bonding electrons (The "bonding electrons" are usually those in the most outer shell.). This leaves none to form other bonds. That is why "covalent" compounds such as organic molecules tend to have low melting and boiling temperatures compared to "ionic" bonds like NaCl.

When two atoms have very different attraction for electrons, the electron density tends to accumulate in the vicinity of one of the atoms rather than an equal sharing. In the conceptual limit we say the atoms form "ionic" bonds. Ionic bonds "tend" to be non-directional.

But things are more subtle than what I have laid out. For example: If asked whether CaCl2 is ionic or covalent, almost everyone would "vote" for ionic, i.e. Ca(+2)[Cl(-1)]2 However, if you heat CaCl2 to a high temperature so that it has a significant number of molecules in the gas phase you would find that CaCl2 is not only the dominant species in the vapor -- but the molecule is also bent!!!

Because ionic bonds "tend" to be non-directional, they tend to form ionic crystals. But I want to make it clear that these are idealizations. Diamond for example is a 3-dimensional crystal, but its bonds are covalent.

Note: I have not raised the question of bonding in metals where the bonding "picture" is different yet. Also I have not raised the question of the bonding in "weak" acids like acetic acid that dissociate slightly in water: HA <----> H(+) + A(-) Here the H to A bond is partly ionic and partly a polar covalent bond.

This forum does not permit a lot of detail. I suggest you visit the site below. It goes into a lot more detail than I can here:

http://www.rsd.k12.wi.us/RAHS/science/webquests/bondwebquest.htm

Vince Calder


In a very basic sense, a molecule is the smallest sample of a compound that still retains the properties of that compound. In general this has come to mean a sample that contains the chemical formula of the compound. Now, while one can argue about what constitutes a sample that exhibits the properties of the macroscopic sample, and whether metallic-, coordination-, ionic-, etc. type bonds constitute molecules, and we can even argue over what constitutes an appropriate smallest sample when talking about polymers, self-organizing molecules, and network compounds --- I think these are small issues that you should avoid and stick to the general conception that a molecule is the collection of atoms that conform to the chemical formula.

Greg (Roberto Gregorius)



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