Why Aren't Electrons Pulled Into the Nucleus?

[PhilosophicalBluster]

This is probably a question with an obvious answer, but I just started hon. chem & phys this year, so I don't really know. Why aren't the electrons in the orbitals pulled into the positive nucleus by magnetic force? This is very puzzling to me.

 

[pgp_protector]

Simple answer the speed.
Complicated answer has more to do with Quantum Physics If I Recall correctly.

 

[Syrokal]

As it is, if the Electron goes to move past the Nucleus, the force of gravitation pulls it towards it, and its path curves. If its speed were too high then it would go past the Nucleus on a curved course, but would not be held in orbit. If its speed were too low then it would spiral in and eventually crash into it. But if its speed is just right, it is held in a stable orbit, which is circular or elliptical.

Its momentum makes it want to move away from the Nucleus in a straight line, but the attractive force pulls it towards it, and it can't get away. The result is that it stays in orbit around the Nucleus.

So basically it's just a combination of it's natural speed working against the orbit it's on to keep it on a steady course.

To answer the question about electrons, the same thing happens, except that there is a nucleus instead of the Earth, one or more electrons instead of the moon, and the attractive force between them is electrostatic instead of gravitational.

 

[pgp_protector]

As it is, if the Electron goes to move past the Nucleus, the force of gravitation pulls it towards it, and its path curves. If its speed were too high then it would go past the Nucleus on a curved course, but would not be held in orbit. If its speed were too low then it would spiral in and eventually crash into it. But if its speed is just right, it is held in a stable orbit, which is circular or elliptical.

Its momentum makes it want to move away from the Nucleus in a straight line, but the attractive force pulls it towards it, and it can't get away. The result is that it stays in orbit around the Nucleus.

So basically it's just a combination of it's natural speed working against the orbit it's on to keep it on a steady course.

To answer the question about electrons, the same thing happens, except that there is a nucleus instead of the Earth, one or more electrons instead of the moon, and the attractive force between them is electrostatic instead of gravitational.

Actually the orbits aren't circular or elliptical, that's just a simple way of describing the potential location of them.

It's more of a cloud area of probable location. (IIRC)

 

[keith99]

This is probably a question with an obvious answer, but I just started hon. chem & phys this year, so I don't really know. Why aren't the electrons in the orbitals pulled into the positive nucleus by magnetic force? This is very puzzling to me.

Excellent question. I honestly never thought about it.

Short non-answer is that the atomic level is far different than the lavel of 'large' objects.

 

[sfs]

I don't think there is a good classical analogy. For an electron in the potential well of the nucleus, there is a minimum allowed energy state, the ground state. There just is no lower energy state available to it, so it cannot be closer to the nucleus. (Note, by the way, that the ground state wave function actually does overlap the nucleus, and so in some sense the electron does spend part of its time in the nucleus.)

 

[PhilosophicalBluster]

I don't think there is a good classical analogy. For an electron in the potential well of the nucleus, there is a minimum allowed energy state, the ground state. There just is no lower energy state available to it, so it cannot be closer to the nucleus. (Note, by the way, that the ground state wave function actually does overlap the nucleus, and so in some sense the electron does spend part of its time in the nucleus.)

But then why wouldn't the nucleus absorb it like a beta particle?

 

[sfs]

But then why wouldn't the nucleus absorb it like a beta particle?

The nucleus does absorb orbital electrons; the process is called electron capture. It only happens, though, when the nucleus is unstable because it has too many protons, i.e. the isotope with one fewer proton and one more neutron is energetically favored over the original isotope.

 

[Cabal]

This is probably a question with an obvious answer, but I just started hon. chem & phys this year, so I don't really know. Why aren't the electrons in the orbitals pulled into the positive nucleus by magnetic force? This is very puzzling to me.

(very rough answer)

Uncertainty principle.

If the electron moves towards the nucleus, it can't keep doing that forever, eventually it'll attach to the nucleus and effectively come to rest. However that would give it a precise position and a precise momentum (i.e. both zero). This applies to all quantum systems and this is why all quantum systems have a zero point energy - or nonzero ground state, to put it another way - as a result of the inherent uncertainty.

So an electron can't go any further than the ground state due to this zero point effect. They can hop around the excited levels all they want, as long as they don't try and double occupy the same level.

 

[Cabal]

As it is, if the Electron goes to move past the Nucleus, the force of gravitation pulls it towards it, and its path curves. If its speed were too high then it would go past the Nucleus on a curved course, but would not be held in orbit. If its speed were too low then it would spiral in and eventually crash into it. But if its speed is just right, it is held in a stable orbit, which is circular or elliptical.

Its momentum makes it want to move away from the Nucleus in a straight line, but the attractive force pulls it towards it, and it can't get away. The result is that it stays in orbit around the Nucleus.

So basically it's just a combination of it's natural speed working against the orbit it's on to keep it on a steady course.

To answer the question about electrons, the same thing happens, except that there is a nucleus instead of the Earth, one or more electrons instead of the moon, and the attractive force between them is electrostatic instead of gravitational.

I'm sorry, but this isn't quite correct.

The fatal flaw in the solar-system model was that even orbital motion counts as acceleration, and according to Maxwell's EM laws, the electrons would then be giving out a continuous stream of radiation (think aurorae or Cerenkov detectors) and gradually bleeding out energy until they collided with the nucleus - which as my previous post pointed out, would violate the uncertainty principle.

It was this big flaw that prompted Bohr to make his quantum mechanical model, but even he didn't figure out the uncertainty element giving the atomic ground state a nonzero value - Heisenberg didn't come with the uncertainty principle until a good few years later. Bohr basically just said "the electrons just stay in their levels BECAUSE" and left it there.

Additionally, while some helpful analogues can be drawn by comparing quantum mechanical systems to classical systems like gravitational orbits (I've just come out of a seminar on Rydberg atoms, which are a hot topic at the moment and all of the maths was right out of the Bohr model, which was quite refreshing after paaaages of nasty wavefunctions) - ultimately quantum mechanical systems simply have no classical analogues. The best understanding of electrons is that they occupy a probability cloud or "orbital" of differing shapes depending on their energy, rather than fixed orbits of fixed radius.

 

[catzrfluffy]

I would say energy balance.
In the nucleus, perhaps the neutrons weigh out the protons just enough so that the electrons aren't overly attracted by magnetism, as an electron orbits the nucleus the magnetism is just enough to pull the electron back till the electron's own energy swings it back away. So the electrons just orbit the atom under their own energy, as long as there is enough energy for them to do so. When the proton-neutron balance is upset (like in an unstable atom), the extra positive energy in the nucleus creates too strong a magnetism, which causes the electrons to be irresitably attracted to the protons, making them collide, until the absorbation of negative energy causes the proton to become a neutron again, stabilising the atom.
I would then assume that any lost electrons were picked up again from free ones in the surrounding environment.
(An unstable element is one with too many protons, but you probably already knew that.)
I don't think an atom can become unstable by having too many electrons, because they would just zoom past the nucleus because the nucleus' magnetism wouldn't be strong enough to attract them into orbit.
Can a nucleus have too many neutrons? Or doesn't this matter?

How do the neutrons weigh out the protons? Perhaps energy sharing?

 

[sfs]

I would say energy balance.
In the nucleus, perhaps the neutrons weigh out the protons just enough so that the electrons aren't overly attracted by magnetism, as an electron orbits the nucleus the magnetism is just enough to pull the electron back till the electron's own energy swings it back away.

No, that's wrong. The electrons are attracted by the positive electric charge, not by magnetism. The neutrons, which are neutral, have no effect on the attraction between the protons and the electrons.

(An unstable element is one with too many protons, but you probably already knew that.)
I don't think an atom can become unstable by having too many electrons, because they would just zoom past the nucleus because the nucleus' magnetism wouldn't be strong enough to attract them into orbit.
Can a nucleus have too many neutrons? Or doesn't this matter?

Unstable nuclei can have either too many protons or too many neutrons; for heavier nuclei, any combination of protons and neutrons is unstable. The instability has very little to do with the electrons in the atom, however.

 

[catzrfluffy]

No, that's wrong. The electrons are attracted by the positive electric charge, not by magnetism.

I thought that's what magnetism was - negative charge being attracted by positive charge, and vice versa.

The neutrons, which are neutral, have no effect on the attraction between the protons and the electrons.

But then why is an atom with too many protons unstable?

Unstable nuclei can have either too many protons or too many neutrons; for heavier nuclei, any combination of protons and neutrons is unstable.

You're talking uranium, etc.?
Does having too many neutrons have the same effect on the electrons?

 

[Wiccan_Child]

I thought that's what magnetism was - negative charge being attracted by positive charge, and vice versa.

It's both: that's why we call it electromagnetism, because it's all basically the same thing.

But then why is an atom with too many protons unstable?

Because protons have a positive charge, so they repel each other. But, protons and neutrons are also attracted to each other because of the strong nuclear force.

So it's a balance: too few protons and the repulse EM force can't overcome the attractive strong nuclear force, so they stick together. Too many, and the EM overcomes the nuclear, making them fly apart.

You're talking uranium, etc.?
Does having too many neutrons have the same effect on the electrons?

Neutrons have no affect on electron number, but there are still limits to how many (and how few) neutrons they can have.
The more neutrons you have, the weaker each one is held together. So, eventually, some just fly away because there's not enough to hold them. This graph plots stable nuclei according to their proton and neutron numbers (it's big, so it's split into three sections).

 

[Wiccan_Child]

This is probably a question with an obvious answer, but I just started hon. chem & phys this year, so I don't really know. Why aren't the electrons in the orbitals pulled into the positive nucleus by magnetic force? This is very puzzling to me.

Because if they got any closer they'd lose energy. But there's a minimum amounts of energy they have to have (it's called the zero point energy).

 

[canphys]

Coming back to the original post,
This is probably a question with an obvious answer, but I just started hon. chem & phys this year, so I don't really know. Why aren't the electrons in the orbitals pulled into the positive nucleus by magnetic force? This is very puzzling to me.

It is indeed very puzzling! The really short answer is basically the one above - the electron isn't allowed to drop beneath the ground state energy - but it's maybe a bit difficult to see why this should be if you haven't studied much quantum mechanics. A slightly more detailed answer might help clear things up.

To start we need to tweak your question just a bit. The force we are discussing here is actually the electric force, not the magnetic force. The electric force is the force that arises between two charged objects, such as an electron and a proton. It's also what makes clothes stick to each other after coming out of the dryer. The electric force tends to pull oppositely charged particles, such as those in atoms, together. The magnetic force is a little different and very weak compared to the electric force. It is not really what's at issue when discussing atomic stability.

So, what you are wondering is why a force (in this case the electric force) that pulls two objects together can lead to stable orbits, instead of always causing them to crash into one another. If we ignore the problem of radiation, the answer to this question is actually fairly simple.

You can think of the electric force as a sort of "tether" that ties the electron to the nucleus. As an analogy, imagine attaching two rocks together with a tense rubber band. If you just let the two rocks go - if the electron and nucleus are both at rest - the rubber band will pull them together and they will indeed collide. However, if you pull a David and swing one rock around your head by the other, the force of the rubber band doesn't pull the rocks into one another, but rather forces them to move about in a circle.

What's going on here is that the rubber band, or the electric force, is acting to change the direction of the rock's/electron's motion. If no force were there, the electron would simply continue in a straight line instead of staying attached to the atom. But there is a force, and this holds the electron in a stable orbit.

Quantum mechanics comes in when you consider radiation. In classical electromagnetics, an accelerating charge (such as an electron moving in a circular path) ought to emite electromagnetic waves. These would carry away energy, causing the electron to move closer and closer to the nucleus, and quickly crash. The resolution of this problem has to do with the concept of "wave-particle duality", which is the consideration that the electron cannot be adequately described as either a particle or a wave alone. When we consider the electron as a wave, we see that it can only exist in certain orbits about the nucleus, where that wave interferes constructively with itself. In other orbits, the electron tends to cancel itself out, and so we don't see it. What we find is that there is a single minimum "constructively interfering" orbit the electron can have. We call this orbit the ground state, and believe that no bound state of lower energy (such as the bound state of having collided with the nucleus) is possible. This is a bit of a simplification, and I can explain in a bit more detail if you're interested.

 

[Agonaces of Susa]

This is probably a question with an obvious answer, but I just started hon. chem & phys this year, so I don't really know. Why aren't the electrons in the orbitals pulled into the positive nucleus by magnetic force? This is very puzzling to me.

Great question PhilosophicalBuster!

No one knows. It's a mystery.

And it makes no sense.

If protons have a positive charge and electrons have a negative charge why aren't they attracted to one another?

The same question can be asked of gravitation.

If the Sun and Earth are attracted to one another, why don't they fall on eachother?

According to the hypothesis of gravitation: God did it.

"...lest the systems of the fixed stars should, by their gravity, fall on each other, he [God] hath placed those systems at immense distances from one another." -- Isaac Newton, mathematician, 1687

 

[ArnautDaniel]

This is probably a question with an obvious answer, but I just started hon. chem & phys this year, so I don't really know. Why aren't the electrons in the orbitals pulled into the positive nucleus by magnetic force? This is very puzzling to me.

Quantum Mechanics.

Classical Mechanics ceases to apply at this level.

Basically the physics is different. The only allowed trajectories don't involve crashing into the nucleus.

That said, QM does give an electron (in the right orbital mind you) a non-vanishing chance of existing inside the nucleus now and again.

...

Or...if you want more detail...to crash into the nucleus the electron has to lose energy, to lose energy it has to radiate light. So long as it can't radiate the light it can't crash into the nucleus.

(Classically) a charged object radiates light when it accelerates. Of course you expect an electron in a circular orbit to be accelerating and thus radiating following classical physics.

QM doesn't allow the electron in a particular orbital to radiate light. An electron can radiate light when it transitions to a lower energy orbital. However, there is a lowest energy orbital, so when the electron gets into it, it has nowhere with lower energy to go to.

Not much of an answer...but like I said, it really boils down to the physics not being classical.

 

[kangitanka]

If the Sun and Earth are attracted to one another, why don't they fall on eachother?

~sigh~
Because the Earth is falling *around* the sun. If not for gravity, the Earth would go in a straight line
But the Sun's gravitational pull changes all that.
Before you *REJECT* gravity, you might want to read up on it, and actually try to understand it.
And quit trying to lead others into your deception.

<staff edit>

 

[Agonaces of Susa]

For those of us who have never read Isaac Newton or the Principia and who don't know that the hypothesis of gravitation says God did it: Isaac Newton, General Scholium

lest the systems of the fixed stars should, by their gravity, fall on each other, he hath placed those systems at immense distances from one another.

This Being governs all things, not as the soul of the world, but as Lord over all; and on account of his dominion he is wont to be called Lord God ..., Or Universal Ruler; for God is a relative word, and has a respect to servants; and Deity is the dominion of God not over his own body, as those imagine who fancy God to be the soul of the world, but over servants. The Supreme God is a Being eternal, infinite, absolutely perfect; but a being, however perfect, without dominion, cannot be said to be Lord God; for we say, my God, your God, the God of Israel, the God of Gods, and Lord of Lords; but we do not say, my Eternal, your Eternal, the Eternal of Israel, the Eternal of Gods; we do not say, my Infinite, or my Perfect: these are titles which have no respect to servants. The word God* usually signifies Lord; but every lord is not a God. It is the dominion of a spiritual being which constitutes a God: a true, supreme, or imaginary dominion makes a true, supreme, or imaginary God. And from his true dominion it follows that the true God is a living, intelligent, and powerful Being; and from his other perfections, that he is supreme, or most perfect. He is eternal and infinite, omnipotent and omniscient; that is, his duration reaches from eternity to eternity; his presence from infinity to infinity; he governs all things, and knows all things that are or can be done. He is not eternity and infinity, but eternal and infinite; he is not duration or space, but he endures and is present. He endures forever, and is everywhere present; and, by existing always and everywhere, he constitutes duration and space. Since every particle of space is always, and every indivisible moment of duration is everywhere, certainly the Maker and Lord of all things cannot be never and nowhere. Every soul that has perception is, though in different times and in different organs of sense and motion, still the same indivisible person. There are given successive parts in duration, coexistent parts in space, but neither the one nor the other in the person of a man, or his thinking principle; and much less can they be found in the thinking substance of God. Every man, so far as he is a thing that has perception, is one and the same man during his whole life, in all and each of his organs of sense. God is the same God, always and everywhere. He is omnipresent not virtually only, but also substantially; for virtue cannot subsist without substance. In him** are all things contained and moved; yet neither affects the other: God suffers nothing from the motion of bodies; bodies find no resistance from the omnipresence of God. It is allowed by all that the Supreme God exists necessarily; and by the same necessity he exists always and everywhere. Whence also he is all similar, all eye, all ear, all brain, all arm, all power to perceive, to understand, and to act; but in a manner not at all human, in a manner not at all corporeal, in a manner utterly unknown to us. As a blind man has no idea of colors, so have we no idea of the manner by which the all-wise God perceives and understands all things. He is utterly void of all body and bodily figure, and can therefore neither be seen, nor heard, nor touched; nor ought he to be worshiped under the representation of any corporeal thing. We have ideas of his attributes, but what the real substance of anything is we know not. In bodies, we see only their figures and colors, we hear only the sounds, we touch only their outward surfaces, we smell only the smells, and taste the savors; but their inward substances are not to be known either by our senses, or by any reflex act of our minds: much less, then, have we any idea of the substance of God. We know him only by his most wise and excellent contrivances of things, and final causes; we admire him for his perfections; but we reverence and adore him on account of his dominion: for we adore him as his servants; and a god without dominion, providence, and final causes, is nothing else but Fate and Nature. Blind metaphysical necessity, which is certainly the same always and everywhere, could produce no variety of things. All that diversity of natural things which we find suited to different times and places could arise from nothing but the ideas and will of a Being necessarily existing. But, by way of allegory, God is said to see, to speak, to laugh, to love, to hate, to desire, to give, to receive, to rejoice, to be angry, to fight, to frame, to work, to build; for all our notions of God are taken from the ways of mankind by a certain similitude, which, though not perfect, has some likeness, however. And thus much concerning God; to discourse of whom from the appearances of things, does certainly belong to Natural Philosophy.