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Name: Scott B.
Status: educator
Age: 50s
Location: N/A
Country: N/A
Date: 2001 - 2002


Question:
While observing a chemistry class on chemical bonding the teacher ended the lesson with the statement "Ionic bonds are generally stronger than covalent bonds that is why ionic materials melt at higher temperatures".

The statement has two parts,

First "Ionic bonds are stronger generally than covalent bonds" Second "that is why ionic materials melt at higher temperatures"

What is the truth here?

Can any such generalizations be made about Ionic and Covalent bonds and their melting points?

I can think of many contradictory examples. I have several texts on mineralogy that specifically say that covalent bonds are stronger than ionic because they are directional and most of the bond energy in concentrated in the area of the shared electrons where as ionic bonds are a general attraction of + and - charged atoms due to the almost complete transfer of the electron. Is this true for all materials like say CO2 or O2 or just covalently bonded solids?


Replies:
Several points:

1. The idea of a purely ionic bond and a purely covalent bond is not correct. The "covalency" whatever that means is a continuum. It is our abstraction of a "chemical bond". The bonding is what it is, and all bonds have some features of both concepts.

2. It depends upon what reaction you use as a bond energy: MXgas ------> (M+)gas + (X-)gas, or the neutral atoms.

3. "Ionic" solids and metals are held together by interactions between many atoms, i.e. the bonding is delocalized, so it requires more energy, usually, to vaporize a single atom and/or ion from the solid.

4. "Covalent" compounds e.g. organic compounds, are held together into discrete molecules by very strong bonds, but the crystal lattice is held together by comparatively weak bonds. As a result they have, in general, lower boiling and melting points, but strong bonds between atoms within a given molecule.

If you look up typical bond energies in a text or reference book such as CRC Handbook of Chemistry and Physics, you will find a lot of overlap in the bond energies. The big effect in ionic solids and metals is the energy required to vaporize those substances, compared to "covalent" compounds. If you take that effect away. It's pretty much a wash, depending upon the examples you choose. Extreme example: RbI has a bond energy of 80 Kcal/mol vs. N2 = 226 Kcal/mol. but the former clearly has the higher melting and boiling points.

Vince Calder



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