If I have a single molecule of say oxygen; can it be said to be solid, liquid or gas? or do these “states” only apply to how one atom relates to another atom?
Well, the thing that causes the different states of matter is the difference in the amount of energy an atom or molecule has. Therefore, all it takes is one molecule, but it doesn’t really matter until you group it with other ones.
(That should do until someone who knows what they’re talking about chimes in.)
I don’t remember much about physics, but it seems to me that for example the difference between liquids and solids was measured by the difference in “interatomic” attraction (I’m probably dating myself here), so with just one atom, how could you tell?
“State” is merely the behavior a group of atoms exhibit at different energy levels. But if you have a single atom at a particular temperature (say, below its freezing point), it has the same energy level as a group of them does. With a single atom, the concept of states that correlate to temperature just doesn’t apply.
Way outta my league here, but something I’ve always wondered about. In a reaction, some of the matter is converted into energy. So isn’t energy a state of matter? Energy can be converted back into energy, right?
I said this was outta my league. 
Peace,
mangeorge
I only know two things;
I know what I need to know
And
I know what I want to know
Mangeorge, 2000
E=mc^2 essentially means that energy and matter are equivalent.
Stephen
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Urk, that last post wasn’t in the oven long enough, before I accidentally hit “Enter”. I was intending to say something about how it is frequently the energy level, which determines which state a particular matter will be in. Anyway, I made the mistake of looking at my watch, so someone else will have to finish my thought…
Stephen
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The state of matter in the sense we’re talking about (there are other state properties: temperature, pressure, etc.) is related to the entropy–i.e., the state of order or disorder.
Of the three common states (solid, liquid, gas), solids are the most ordered–the arrangement of the molecules that make up the substance are in a highly systematic, predictable arrangement (low entropy). Gases have the most disorder–the odds that you’ll find a molecule in any given space within that substance’s volume are much higher (high entropy).
So, with regards to the OP, I’d agree that you can speak of a single molecule as being in one of the states–if you are able to predict its location, it’s probably a solid; if not, it’s probably a gas. Maybe? Call it a liquid, I guess.
No, the concepts of solid, liquid, and gas apply only to aggregates of molecules. Yes, the “states” only apply to how the molecules relate to each other in a group.
The concepts of solid, liquid, and gas are purely classical, not depending on the idea of the molecular constitution of matter. A solid holds its shape under its own weight; a liquid takes the shape of its container but forms a surface; a gas fills a closed container of arbitrary volume. If you have only one molecule, these concepts are simply meaningless. One grain of sand does not a beach make.
I’ll bet you’re leading up to the bulk properties of the “single molecule” of a BEC, aren’t you?
[QUOTE]
mangeorge:
In a reaction, some of the matter is converted into energy. So isn’t energy a state of matter? Energy can be converted back into energy, right?
Just to set the record straight some of the matter is not converted into energy, the energy comes from the making/breaking of interatomic/intermolecular bonds - breaking bonds requires energy (usually provided by the surroundings or a heat source etc…) and making bonds releases energy into the surroundings. This means that if more bonds are broken than formed then the reaction will be endothermic i.e. it will cool down. Naturally the opposite applies where more bonds are formed than broken thus releasing energy causing an exothermic reaction.
You are right in saying that matter is fundamentally energy as described in superstring theory, but matter is not changed unless you go & start messing around with subatomic particles (this is the way that I understand things anyway, I invite anyone with more knowledge on the subject than me to come & correct/embarras me or to just slag me off :cool
OK that smiley legend confused what I put up - what I meant to put up was 
When you say a single “molecule” of oxygen, do you mean “atom” or do you really mean molecule. There is never such a thing as a single oxygen atom in nature - it’s diatomic so oxygen atoms are always found in pairs.
Wait - it’s even more complicated than that (ions etc.), but I don’t want to get into it.
I think your question was answered by someone else above - states of matter describe the interaction between molecules. If you have a single molecule it therefore has no state by definition.
I would disagree with this. According to my understanding, it doesn’t matter how many atoms or molecules or ions etc… you have, the state that the sample is in depends upon it’s atomic ‘agitation’.
This is where my knowledge gets a bit thin, so be warned…
If, according to quantum theory, everything that effects anything else in the Universe is quantifyable, then atomic/molecular agitation can be represented by the quantum forces acting on that particular atom or molecule. For example, a molecule of oxygen gas is more agitated than a molecule of lead (at, say, room temperature). This means that it either
i) has greater quantum forces acting on it or
ii)is lass able to counter-act the forces
Heating an atom causes it to ‘vibrate’ faster, and when the vibration reaches certain thresholds, it can be classified as either solid, liquid, gas or plasma.
I agree that the states of matter can only be effectively measured when the atoms/molecules are interracting with other ones, but just because a scientist cannot measure something happening does not automatically mean that it isn’t.
Phew, that’s a total mess but if anyone could clean it up for me I’d be very grateful… 
“You can have the afternoon off when you die” - Edmund Blackadder
BIGMatt, you have to be a little careful when you talk about “agitation” or “vibration” in quantum mechanics (or classical mechanics, for that matter). A single atom or molecule, not interacting with other atoms or molecules, doesn’t get agitated and if it is in an excited state (wich can be vibrational for a molecule), it will quickly radiate its vibrational energy away as light. A single particle just propagates merrily along in a straight line doing nothing in particular.
The agitation and vibration of individual particles in bulk matter is due to the fact that they are always banging into each other and bouncing off. If the average energy per particle (AKA the temperature) is larger than the binding energy of two particles trying to stick together, then you have a fluid–a liquid or a gas. If the binding energy is larger then you have a solid. The distinction between a gas and liquid is subtle. For a liquid the “agitation” energy has to be greater then the binding energy when immersed, but less than the binding energy at the surface.
A single particle has no other forces acting on it. You can’t “heat” a single atom; you need to immerse it in a bulk material to even talk about heat.
Can’t you heat a single molecule by bombarding it with photons? or radio waves as they do when creating a plasma?
No, you can’t. When you bombard an atom (or a molecule) with photons (including radio waves) you can give the atom energy in two ways. You can just give it a bump and cause it to move off in a new direction at a new speed, or you can excite some internal degrees of freedom (typically raise an electon into a higher energy level). In the first case, the atom still just cruises along in a straight line. In the second case, the atom very quickly rereadiates the energy as a photon and goes back to its original state.
Heat is the energy of random motions of particles in a collection of many particles. Temperature is the average energy-of-random-motion per particle (actually per degree of freedom) A single particle does not undergo random motion. To make the trajectory of a particle random, it needs other particles to bounce off of. Basically, there’s no such thing as a hot atom, only hot collections of atoms.
CJD wrote “the matter is not converted into energy, the energy comes from the making/breaking of interatomic/intermolecular bonds”.
This is true. However, E=mc2 still applies, and a tiny proportion of the MASS of a material is due to the energy stored in its bonds. If an exothermic reaction occurs, its mass will decrease although I doubt you could find a balance sensitive enough to measure the change.
Rubber bands will also increase in mass when you stretch them, because you are increasing their stored energy.