Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: Fuzzy Logic on 09/08/2006 21:49:07
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Never fully understoof why somethink like caesium would cause a massive explosion when added to water, but stick a lump of gold in there and it will just sit at the bottom...doing nothing much. How does the number of electrons protons neutrons have an effect on the reactivity of an element?
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It's not a simple question!
You should have a complete knowledge of atom's quantum mechanics.
For the moment, I can only say this:
Imagine to add one proton and two neutrons to hydrogen nucleus and an electron around: you have helium atom. Why is it so different, chemically, from hydrogen? Electrons, at least inside an atom, are not little balls, instead they are sort of "waving clouds". These "clouds", among other things, shield the positive charge of the nucleus. With two electrons, the electronic clouds of helium are localized in such a way to shield very well the electric charge of the nucleus, so the electrons of another atom are not attracted from this positive charge.
For this reason helium atoms are almost non-reactive chemically.
The opposite between two hydrogen atoms: the charges of the nucleus are badly shielded and when two hydrogen atoms, for example (or an hydrogen atom with a chlorine atom or many other kinds of atoms) approach each-other, the electron of one atom is attracted from the nucleus of the other, generating a Very strong (covalent, in this case) bond between the atoms.
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Sounds good, but would do little to explain why lithium, with 3 electrons, is more reactive than Helium with 2, or why Caesium is more reactive than Sodium, although it clearly has far more electrons with which to shield its nucleus.
Hydrogen is unusual in having a fairly exposed nucleus, but chemistry is not very much about the nucleus, it is about the way the electrons themselves interact. As Lightarrow suggests, it does help if you know the quantum behaviour of electrons in an atom (which I do not claim to know), but it basically boils down to electrons preferring some configurations over others.
At the simplest, the comparison between hydrogen and helium – it is not really to do with the nucleus, it is more to do with electrons liking to be in pairs. Electrons have (like most common particles) two possible spin states, and they are more stable when an electron in one spin state is paired with an electron in the opposite spin state. When two hydrogen atoms meet, the electrons each one of them hold can be shared between them, forming a more stable pair of electrons, and thus binding the two atoms together.
All of the group 1 atoms (hydrogen, lithium, sodium, potassium, caesium; all share the characteristic that they have an odd number of electrons, and that one of those electrons is relatively unstable. The reason that the heavier atoms are more reactive is quite contrary to the argument that Lightarrow put forward – it is not because of a stronger electrical reaction with the nucleus, but because of the larger number of electrons in the bigger atoms, they are actually more weakly attached to their own nucleus, and so more readily interact with the electrons of other atoms.
Another, even more stable configuration for the electrons around an atom requires 8 electrons. This gives the noble gases (apart from Helium) their stability, but it also gives atoms like chlorine and fluorine their reactivity. Atoms like those of chlorine and fluorine are only one electron short of having a group 8 electrons available to them, and so will readily snatch an electron from another atom (particularly if it is an atom that has a single loose electron, such as sodium or caesium) in order to make up that group of 8 electrons.
The above explanation is very crude, and really does need a proper understanding of the quantum states of electrons to give a better quantitative answer (it is probably the kind of answer that might have been acceptable in the 1920s or 1930s – the Bohr orbital model of the atom, but has now been superseded by better explanations of what goes on amongst the electrons of an atom).
George
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Have a read at this
http://www.visionlearning.com/library/module_viewer.php?mid=54
Steven
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To another someone:
Yes, very good.
I didn't want to explain all of this, I forgot to write that mine was only an example, a little part of the story (it's not trivial for me to translate into english). I wrote about quantum mechanics of an atom, just to explain that simple reasonings (as mine) are only intuitive ideas, not the reality. To understand why two atoms react you have to solve the Schrodinger equation associated with them and find the wave function of the electrons. In the Schrodinger equation, of course, you have to write H (Hamiltonian), which also depends on the potential, which also depends on nucleus, their positions and their charges. I think the only intuitive way to explain (to an high school student) the covalent bond, for example between two hydrogem atoms, is the one I wrote.
Of course, when we talk about atom's reactivity, we should also specify which kind of reactivity. I would explain alcali metal's reactivity, for example, beginning, as you say, with stating that the last electron is weakly bound to the atom, so quite available to react with other atoms. But why it is weakly bound? Same consideration as up: Quantum Mechanics. However, if we want to answer to an high school student, for example, and considering that it could be very difficult to solve Schrodinger equations for heavy atoms, we can say, intuitively, that the last, outer electron is weakly bound because is far from the nucleus and because the others, inner electrons, shield quite well the charge of the nucleus to the outer electron.
In alcali metals, the last electron is forced to occupy an outer zone because the inner ones are already all occupied (see Pauli's principle, at least).
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Just to keep it simple (too simple maybe, but... )
All elements strive to an electron configuration like that of a "noble gas". The less effort it takes, the less "trigger energy" is needed or the more active our element will be. Now caesium needs only to get rid of one electron out of 55 to have a configuration like that of xenon; and this sole electron is situated on a level far away from the nucleus. Which means that is not actracted very much to that specific nucleus anyway.
Okay, I'm oversymplifying, not every reaction leads to a noble gas electron configuration, but you have to get this far before we can start discussing quantum behaviour. Like George stated, it has everything to do with electrons preferring one configuration over an other, and they prefer to be coupled rather than single. Which should no seem so odd after all.
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Thanks for all your responses, I have in the past asked several people this (obviously the wrong people) and have never got a decent explaination until now...thanks