Why Doesnt Hydrogen Collapse?

[Fellowes]

Im in gr 9 chemistry and I was wondering if somebody could explain to me why Hydrogen does not collapse because of only one electron and no neutron?

[Crash]

Because the electron cant collapse in, its held in an orbit(1s) according to the schrodinger equation because its angular momentum has to be an exact multiple of h/2pi. this is a rough description of how it behaves because there are more complicated principles of which you'll only learn later.

[Martin]

... because its angular momentum has to be an exact multiple of h/2pi...

 

Crash suppose someone replies "well zero angular momentum is an exact multiple of h/2pi. So what you said so far allows that case. What rules that case out, where the electron does not even go around in an orbit but just sits still on the (proton) nucleus?"

 

what response would you have ready?

[Crash]

well think of it this way, besides you know the awnser..i assume but because you didnt post any sorta explanation kinda makes me wonder whether you do......... but if it didnt work the way i said we wouldnt have hydrogen would we? pffft, anyway because hydrogen on its own becomes an ion (and it dosent collapse) due to the mechanics of how and where forces lie, it wont fall in.

Martin yuo should know the through explanation will go way over the head of a yr 9 student. what i stupid idea trying to suggest i give him the full explanation.....what an idiotic idea, so instead is try and remember what a yr 9 student knows and portay the best possible picture i can (with basic historical notions of why hence bohrs h/2pi because thats the way it is and explain it after). BTW you still havent found me the explanation to the charge question (rather cunningly dodged)

[Martin]

Because the electron cant collapse in, its held in an orbit(1s) according to the schrodinger equation because its angular momentum has to be an exact multiple of h/2pi. this is a rough description of how it behaves because there are more complicated principles of which you'll only learn later.

 

Maybe Fellowes has never heard of the Heisenberg Uncertainty Principle, and maybe he has heard of it.

 

I dont mean to restrict this to Fellowes. If someone asked me that question and said "What rules out the ZERO angular momentum case where the electron just settles down and sits still on the proton?"

 

then i think I might try appealing to the H.U.P.

It cant just sit still there, because then we would know exactly where it is. The more you pin down position the more it makes the uncertainty about the momentum grow without bound. The two are reciprocal.

A surprising number of people have heard of the Heisenberg Uncertainty Principle. So I would try that.

 

 

I thought that might be what you were going to say.

[Crash]

when i started reading on this (principle of interdeterminancy) in year 10 it confused the hell outta me only because the principle undermines a statement i also heard to be true that the exact position and momentum can be known of an electron (of which it certainly is true!) so i decided to let fellowes of the confusion i found through no "thorough" explanation which i would consider to be a class on it

[Martin]

... so i decided to let fellowes off the confusion i found through no "thorough" explanation which i would consider to be a class on it

 

you may well have made the wise choice! one of these days we may get

Swansont or Severian to give a brief discussion of the H.U.P. and some basic applications of it----Severian is a working particle physicist who hangs around here some

[budullewraagh]

simply put, gravitational and electromagnetic forces pull the electron in but centripetal force pushes it out. the difference of the forces pulling the electron in and pushing it out are equal

[Crash]

equal only when the angular momentum is an exact multiple of h/2pi, otherwise it will radiate the energy (moslty in the form of photons) untill it obeys this rule, the 1 quantum state is only a description and a vague one at that, for some interesting reading fellowes and an idea of whats ahead read about the cloud model of the atom, whereas these states are more viewed as electron density clouds are the probability of finding electrons in such positions, the primary quantum numbers represent the 99% chance of finding the electron there.

[swansont]

simply put, gravitational and electromagnetic forces pull the electron in but centripetal force pushes it out. the difference of the forces pulling the electron in and pushing it out are equal

 

There is no outward force, and centipetal = "center-seeking" so a centripetal force will never push anything out.

 

The electrostatic force is a centripetal force for a classical picture (i.e. Bohr model) but don't take that picture too far, since the Bohr model is not accurate in its portrayal of how an atom behaves. So you can equate kQq/r² and mv²/r

 

If the net force were zero, the electron would travel in a straight line, by Newton's second law.

 

Gravity, being so small, can be ignored. An exercise for the interested: calculate the electrostatic and gravitational attraction of a proton to an electron at the Bohr radius (5.29 nm), which is the most-probable-radius for an electron in a Hydrogen atom.

[budullewraagh]

There is no outward force, and centipetal = "center-seeking" so a centripetal force will never push anything out.

sorry i misspoke. the electron is accelerating outwards

[Fellowes]

yeah im not very far yet, but i understand a bit, i know what the bohr model and junk is

[swansont]

sorry i misspoke. the electron is accelerating outwards

 

No, it's not. In a circular orbit (whchc is not how an electron really behaves) the acceleration is inward, toward the center.

[budullewraagh]

damn i really was not thinking earlier. the velocity is going along tangent to the orbit and acceleration is going inwards

[Crash]

owned! but i think your losing fellowes..........

[Fellowes]

hahaha yes sort of, but its interesting

[Martin]

 

Back to the topic in hand

...

 

Yeah, now we know that several of us are very bright and knowledgeable (but could sometimes make mistakes even so) we can get back to topic:

 

the question was why isnt the hydrogen atom radius equal zero?

 

I offered the HUP (heisenb. uncert. princ.) back in post #5

 

which says in one version that there is a tradeoff of unsureness about the position and the momentum

 

and maybe this is helpful to understand the h atom

 

because our uncertainty about the electron's position is the atom's RADIUS

 

and our uncertainty about it's momentum is related to the electron's orbit characteristics like energy and angular momentum by it's MASS

so intuitively shouldnt we be able to estimate the atom's radius

by using HUP and knowing the electron mass?

 

somebody push a button

 

 

(psst. the radius of the atom can't collapse to zero because that means zero uncertainty about the location of the electron and so our uncertainty about its momentum blows up to infinity and it goes whizzing around hysterically out of control like the superball in MEN IN BLACK)

[swansont]

(psst. the radius of the atom can't collapse to zero because that means zero uncertainty about the location of the electron and so our uncertainty about its momentum blows up to infinity and it goes whizzing around hysterically out of control like the superball in MEN IN BLACK)

 

There's also the fact that the ground state energy has a finite value. The electron can be in the nucleus - it just can't stay there. (It doesn't generally combine with the proton because the weak force has a short range, and because it may not be energetically allowed.)

[calbiterol]

Martin yuo should know the through explanation will go way over the head of a yr 9 student. what i stupid idea trying to suggest i give him the full explanation.....what an idiotic idea, so instead is try and remember what a yr 9 student knows and portay the best possible picture i can (with basic historical notions of why hence bohrs h/2pi because thats the way it is and explain it after).

 

I would have to disagree with the thinking that anything and everything that is complex is "way over the head" of all children. Generally, there are some things that children simply cannot comprehend, but that is not because they do not have the ability, it is because they do not have the knowledge base. There are plenty of examples throughout history of very prodigious individuals who were "just" children. They are just few and far between. So next time, please don't assume that just because someone hasn't graduated from MIT with a Doctorate of Philosophy in Chemistry or Quantum Physics or whatever that they are ignorant idiots who do not posess the capability to learn, because if nobody is willing to teach those students, they will never be able to fill the voids of the famous and valued members of society that have since passed.