Two questions. For some reason,it is never explained much in chemistry why the nucleus holds together or why electrons don’t hit protons, even though everyone is told particles of opposite charges attract and like charges repel. Eventually I was enlightened a little and learned and learned the strong nuclear force holds the nucleus together (woohoo gluons!). For some reason I always assumed the weak force kept electrons away, but the other day I heard it is only apparent in certain radioactive occurences. So, whats the deal? And why don’t they explain these things in chemistry?.. even just little, so those of us who actually think about this stuff don’t go around confused like we’re missing something.
What keeps electrons from slamming into the nucleus of the atom?
First of all, in classical mechanics we have many fairly stable systems of orbitting bodies (such as the solar system). One of the first models of the atom suiggested that the electrons simply “orbit” the nucleus. Problem is, a moving charge necessarily radiates energy which causes it to slow down and spiral into the nucleus. This would imply if it was simply classical “electric” orbits holding the electrons away from the nucleus, they’d decay extremely rapidly. Since we see atoms around, we know that’s not the case. Enter quantum mechanics. Basically, electrons have allowed “states” when they get close to a nucleus. They don’t act like little orbiting planets at all. Think of them as sort of a cloud that can surround the nucleus. If you try to measure the actual position of the electron, you will find it maps out a probability distribution rather than simply occupying an “orbit” as the planets do. The weak nuclear force doesn’t have anything to do with it.
The weak nuclear force is the force that causes certain nuclei to decay. It is a force of repulsion that is generally WEAKER than the strong nuclear force which is a force of attraction. (Good thing, or heavy nuclei wouldn’t be possible). Actually, the model has unified the Weak Nuclear force and the Electromagnetic force into the Electroweak Force. This nuclear force seems to come from different treatments of the same phenomenological origin of electricity and magnetism.
The weak nuclear force can do everything that the electromagnetic force can, but much more weakly (especially at long distances). It can also do some things that the electromagnetic force can’t do, but only at very short distances. Most notably, the weak nuclear force is the only force[sup]*[/sup] which can change one sort of quark into another. Quarks are the things which make up protons and neutrons, and if you change one of the quarks, you can change a neutron into a proton, or vice versa (with some other particles thrown in, too).
*Actually, gravity can (in extreme circumstances) do this, too, but it’s so insignificant, even compared to the weak force, that particle physicists almost always ignore it. You basically have to be right smack next to a black hole for it to matter.
JS has it exactly right. One thing I would point out is that, in fact, some properties (if I remember correctly, this shows up in things like NMR) actually require having electrons slamming into the nucleus (although, as was pointed out, it’s not really correct to think of electrons at being at any one point).
As far as ordinary chemistry is concerned, the weak force can be completely ignored, and I actually can’t think of any mainstream computational chemists (i.e. people who make a living doing the quantum mechanics of molecules) who ever worry about it (or, for that matter, about gravity or the strong force either).
Thanks for the replies. I suppose the word chemistry in the title is misleading, I just meant that it gives chemistry a disatisfying feel when something like that is bothering me.
I always thought the answer would be much simpler than that. I still don’t quite understand. I’m not sayign you’re wrong JS, but, basically it seems to me that you just explained the quantum view of the electron. What exactly do you mean by allowed states? I’m just re-taking chemistry for the first time in 3 years, so unfortunately, although my physics is decent, my knowledge of the states of elctrons is paltry. I do remember there was a set of numbers describing the general state of elctrons in a given orbit. It just seems to me that such a fundamental concept as “electrons are around the nuclues”, one taught to kids in middle school, should be understood a bit more clearly. Even if it is simply that as electrons get closer to the nucleus the possible states (or probability of it’s position) approach 0, shouldn’t there be some underlying reason or phenomonon for this?
Well, gravity can of course be important on the right scales… Just think of all the talk about growing crystals on the Space Station. And I’m pretty sure that weak force effects are measureable in some chemical reactions, even if they’re so insignificant that you hardly ever care about them.
Radioactivity is studied in certain parts of chemistry. To that end chemistry can be said to be “affected” by the Weak force.
Ryan, you should investigate some of the threads on the board on Quantum Mechanics further. You seem to be bothered by “probability” and the fundamental concepts embodied in the theory. You are in good company. Many of the “Founding Fathers” of quantum theory were similarly disturbed. After 100 years we now have a bit better handle on the peculiarities of the quantum world, but people still think it’s weird.
You ask for the “reasons” behind quantum mechanics. The “reason” simply is the mathematical formalism known as the “wavefunction”. Every particle in theory has a wavefunction that is effected by the laws of quantum mechanics (operator algebra, for the most part). We look at the wavefunctions as being intrinsic properties of a “waveparticle”, and the “allowability” is simply an artifact of discrete mathematical modes in a system. Discrete modes may seem weird at first glance, but they are all around you. To bring it back to a more visualizable idea, consider vibrating strings. There are only certain “allowable” waves on the string. Why should that be? Well, it all has to do with what the intitial conditions (that there are nodes on the string).
Similarly, when you are looking at the “allowable” states of electrons bound to an atom, you have to consider the initial conditions (electron mass, electromagnetic attraction, normalization, etc.) and what comes out is this crazy world of quantum mechanical states for the electrons when you work through the implications. It’s weird and strange, but it’s the way nature works.
Here are some semi-intuitive ways to think about it, but be careful none of them are really correct. (Please don’t correct me)
In order for an electron to spiral into the nucleus it would have to continuously radiate energy, but energy can only be radiated in quanta and there is no allowed quantum of energy that would permit the electron to jump to the nucleus.
A subatomic particle has wave properties and the lowest non-destructively interfering orbit is the ground state where a complete wavelength of a 3 dimensional standing wave can be accommodated.
If an electron were to be in a smaller atomic shell than its ground state, it’s position would be more localized and the Heisenberg Uncertainty Principle would insist that its momentum be more uncertain. This would cause it to have higher energy, and it would therefore emit energy and expand to the ground state.
None of this is of really correct but it may serve to give some idea of what is going on. (Again please don’t correct me)
Yeah, it’s certainly true that the weak force does have chemical implications, but certainly for ordinary, everyday discussions of chemistry (the kind you’d have in a general chemistry class, or an organic chemistry class, or even a physical chemistry class, so long as we ignore radioactivity), all that you ever need is the electromagnetic force.
And heck, you don’t even need a full description of that; for most phenomena you can get away with solving the Schrodinger equation, ignoring relativistic effects, gravity, weak and strong interactions, and so forth, and treating the electron-electron interaction approximately. Which is a darned good thing, or else we’d never be able to make predictions about anything much more complicated than, oh, gas phase Neon.