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For example, what makes Hydrogen flammable, what makes Oxygen support burning and what makes water behave unlike either components?

The crux of the question is, what gives an element its chemical properties and what makes the compounds behave differently from the component elements? Further, on what basis could behavior of new molecules be predicted?

While the answer you give here are welcome, it would be really helpful if you could point me to sources that explain the underpinnings that eventually would help me get my answers.

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migrated from May 25 '13 at 18:48

This question came from our site for active researchers, academics and students of physics.

For elements the answer has to do with the electron configuration of the outer electrons. For compounds the answer involves both the outer electron configurations as well as the types of bonds between atoms. You'll get much better answers at Chemistry.SE. – Brandon Enright May 25 '13 at 18:35
Not only does the electron configuration matter, but so does the amount of protons in the nucleus (and to a much smaller extent, the amount of neutrons). For example, $\ce{Cl^-}$ and $\ce{Ar}$ both have $1s^22s^22p^63s^23p^6$ configurations, but they most certainly do not behave the same way! – Nicolau Saker Neto May 25 '13 at 19:07
I'm sorry, this question is just too broad to be answered. It would take the subject matter of several chemistry courses. – Eric Brown May 25 '13 at 19:27
@Eric I understand that but isn't it possible to give the major steps that leads to the answer? I am sure it should be. – Dirt May 25 '13 at 19:34
@Dirt are you asking why does hydrogen gas burn (e.g. hindenberg disaster) and why does oxygen support combustion, but water, which contains both O and H, does not burn? – Eric Brown May 25 '13 at 19:46
up vote 2 down vote accepted

To get answers on these question on the same footing, one has to address quantum chemistry which explains reactivity pretty well from very general starting points. In other words, to understand all these reactions in details one has to dig into nanoworld, so I suggest you textbooks on physical chemistry and then quantum chemistry:

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I regret to say: this person should start with a basic chemistry text before diving into quantum chemistry. But I do appreciate the fundamental approach: indeed, the answer lies here. – Eric Brown May 26 '13 at 0:34
Ah, "quantum chemistry"! I just ordered Donald McQuairre book. That is exactly the kind of source I was looking. Your answer helped. Thanks. – Dirt May 26 '13 at 9:07

The reason hydrogen gas burns (e.g. hindenberg disaster) is because of the tremendous exothermicity (gives off heat) of the reaction:

$\ce{2 H2 + O2 -> 2 H2O}$

Oxygen support combustion in reactions such as:

$\ce{CH4 + 2 O2 -> CO2 + 2 H2O}$

Both $\ce{H2O}$ and $\ce{CO2}$ are very stable molecules, and this has something to do with the exothermicity of these reactions, rather than exclusively any intrinsic instability of the reactants.

So why doesn't water burn/explode/whatnot? It's an O/H mixture already at a very stable energy structure, and has very little energetic incentive to do anything else.

This is not to say that water does not violently react with other compounds. This one is quite vigorous:

$\ce{2 CaH + 2 H2O -> 2 Ca(OH)2 + H2}$

and this is for as much the favorable Ca-O bonds as it is providing more $\ce{H2}$ to perform reactions like the one at the top of this answer.

A lot (all?) of chemistry can be described by the recombination of atoms to form lower-energy structures. Since there are so many possibilities, what with over a hundred known elements and their various combinations, it is impossible to summarize the rules and observations succinctly.

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