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Name: dcheng
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: 1999 


Question:
What force (or theory) keeps the electrons in the 1s orbital from hitting the nucleus or eventually running out of energy? Also how does that relate or does it ever relate to the decay of an atom?



Replies:
This is a fantastic question -- try the Physics section. Seriously, I think the folks in physics would be more capable of addressing this particular subject. I will point out that the electron has very low probability of ever being in the nucleus. As you know there is no node at the nucleus of a 1s orbital and its amplitude is greatest there but the probability of it being there is very low because the nucleus occupies such a small volume. The probability of finding the electron at the center of the nucleus is zero (for an infinitesimally small space) and the distance from the origin at which you are most likely to find a 1s orbital is _FAR_ outside of the nucleus.

Be sure and post your question in the Physics section. I'd like to see more details on this too.

gregory r bradburn


*Ahem*....I'd like to mention to gregory that there are two full-time theoretical physical chemists answering questions on NEWTON (frank brown and myself)...plus, I have a degree in physics. So atomic questions can be posted here to good effect.

Greg's points are all correct, but I'd like to add something. Fundamentally, the student's question is not answerable in the sense that atoms do not behave like macroscopic objects. If they did, the electron would collapse onto the nucleus due to their mutual electrostatic attraction....

One of the predictions of quantum mechanics is that any particle (or system of particles) which experience some sort of attractive interaction will, instead of settling down to a motionless "collapsed" state, settle into a state which has a distribution of momentum and position values. If this were not the case it would be inconsistent with the fact that particles have wavelike properties (electrons are diffracted by crystals, for example)...and any system that behaves like a wave must satisfy an uncertainty principle for the measurement of its position and momentum (or group velocity if you prefer). So, in an idealized sense, if an electron were to spiral down onto the nucleus and stick to it, there would be virtually no uncertainty in its position or in its momentum...which violates the uncertainty principle.

rtopper


Electrons with net velocities along paths circling the nucleus (those not in "s orbitals") don't hit the nucleus for the same reason that a rock you threw from a moving car wouldn't hit a billboard if you aimed directly at it: The sideways part of the velocity would cause it to miss. (A closer analogy to the electron is trying to bean the calliope of a carousel while riding one of the horses.)

What about electrons in s orbitals? Well, they can and do hit the nucleus. Not often. Although the probability per unit volume of an s electron being at the nucleus is very high, the volume of the nucleus is only about a trillionth the volume occupied by the typical electron. But occasionally one strays in, and then (if certain factors relating to the physics of the nucleus are right) it gets absorbed by a proton, which turns into a neutron and emits a neutrino. This, as you've guessed, is a form of radioactive decay of elements, called "K-shell capture" because it normally only happens for electrons in the lowest (K) shell. The atom loses 1 in atomic number (e.g. a chlorine atom turn into a sulfur atom) but stays neutral. The neutrino is hard to detect, but the hole the captured electron leaves is filled by an electron "falling down" from a higher energy level in the atom, which results in an X-ray being emitted that you can detect.

Incidentally the s electrons that come very close to the nucleus and don't get captured are the source of "spin-spin coupling" in nuclear magnetic resonance (NMR) spectra. This effect basically allows you to count the number of hydrogens bonded to each carbon atom in a molecule. Pretty helpful if you're trying to figure out what the molecule looks like.

christopher grayce



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