Hi all

 

I am going through covalent molecular substances right now and there is a point that I don't quite get.

 

It says 'covalent molecular substances can be elements (e.g. iodine, phosphorus and sulfur) as well as compounds.'

 

Take iodine I[math]_{2}[/math] for example, it is all very well that a covalent bond exists between two atoms of iodine. However, the definition for element goes 'an element is a pure substance which cannot be decomposed into simpler substances'.

 

My question is: Is it not true that iodine can be decomposed into 2 iodine atoms, that is, into simpler substances?

 

Thank you

Generally when speaking of elements, one refers to how a pure sample would exist at standard state. A bunch of lone I atoms does not exist at standard conditions, so it is not considered the "base form", for lack of better terminology.

 

Consider a block of copper. All of the atoms share metallic bonds with each other, so do we only consider copper gas elemental copper?

 

Consider diamond. Do we consider diamond not elemental carbon?

 

The latter phenomenon is known as Allotropy:

 

http://en.wikipedia.org/wiki/Allotropes

That's a rather inaccurate definition of a chemical element. For example, a uranium-238 nucleus can decompose into a thorium nucleus and a helium-4 nucleus (alpha particle) through radioactive decay, yet uranium is a chemical element. Better definitions include "an atom distinguished by the number of protons in its nucleus" and "a chemical substance composed of atoms with the same number of protons in the atomic nucleus", which covers the iodine molecule you mentioned and the block of copper big314mp mentioned.

Thanks for your replies, big314mp and Gilded.

 

Too bad the text book defines element in such a way. I better look up for better definitions next time.

Too bad the text book defines element in such a way. I better look up for better definitions next time.

 

"'an element is a pure substance which cannot be decomposed into simpler substances'. "

 

That definition is fine though. Weather it's an Iodine gas molecule I2 or an iodine ion I-, it's still elemental Iodine. It cannot be broken down to any other element. Basically - if it's on the periodic table then it's an element. It doesn't have to be an individual atom to be called an element. As the other posts said, most of the elements form molecules. H2, Cl2, etc.. they are still elements. The group 8 elements stay as single atoms I think - because they are stable as they are so don't need to form bonds.