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Offline Gile na Gile

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What keeps atoms together?
« on: 17/12/2009 14:36:58 »
Hi all I've just got some really basic questions here that hopefully someone will be kind enough to answer.

One is to do with atoms.

(1) What is the force that keeps the energy within an atom? Why doesn't the energy within it simply dissipate outwards? Is this what magnetism is? If so, where does this come from?

(2) Does the distance between atoms change depending on the substance? For example, in a square cm of hydrogen and a square cm of carbon - we will have more atoms tightly packed in the tougher substances, I'm thinking.
When we say that there is a chemical bond between atoms such as in hydrogen and oxygen isn't there still a massive distance between the atoms? I mean, if we were to measure the length of the hydrogen atom's diameter and the length of the oxygen atom's diameter what would be the distance between them in this bond? If both were footballs how far apart would they be - metres apart, kilometres? Also, does this distance change when we progress to the more complex chemical bonds?

(3) On chemical bonds themselves - what is it exactly that is binding these atoms together? Merely saying that that there is a free electron on one shell and a lack of an electron on another doesn't make any sense to me. What are electrons and shells really?

Thanks in advance.
« Last Edit: 17/12/2009 19:55:01 by chris »


 

Offline Soul Surfer

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Re: What keeps atoms together?
« Reply #1 on: 17/12/2009 19:33:42 »
This question will take quite a lot of explaining!

Firstly, the particles that make up the outer areas of the atom, electrons, have fixed negative charges which attracts them towards the positively charged protons in the nucleus.  We can for this question ignore what binds the nucleus together because it plays absolutely no part in the chemistry that you are wishing to understand.

Electrons are very light particles and because of the uncertainty principle are quite "fuzzy" in that it is difficult to pin them down to a precise spot, and while they are attracted to the protons, they are always in effect moving around in what are known a orbitals and cannot collapse on to the nucleus.  These are sometimes shown a being like the sun and its planets but it is not like that.  All you can describe is a vague probability density of finding a particular electron in a particular position with respect to the nucleus.  The moving electrons do create magnetic fields and this affects the way that the electrons arrange themselves around the nucleus but the main processes of chemical bonding are associated with the electrical charges and the orbitals of the electrons.

There is one other important feature about electrons.  They are fermions. This means no two electrons can ever be in the same state so each electron added to a nucleus must have a significantly different quantum state as they are added to the bare nucleus.

so to answer your questions. 

1 The electrical charges are in some ways like gravity an inherent property of the particles themselves and unchanging and cannot be dissipated.  As an aside the gravitational effects between the particles are extremely tiny and play absolutely no part in this process or in the process that binds the nucleus together.

2. The distance between atoms and the size of the atoms depends on the atom and the arrangement of the electrons in orbitals heavier atoms tend to be bigger but the charge on the nucleus and the relative screening of the other electrons also have an effect on the size of the atom. because of the mathematics  of ensuring that all the electrons have significantly different quantum states some states are preferred over others.  The nuclei of the atoms are as you say very much smaller it is the size of the electron that determines the spacing between atoms.  Separations between atoms in substances are of the order of 10-10 of a meter  whereas nuclei are of the order of 10-15 of a meter so the electron clouds are about 100,000 times bigger than the nucleus so 30 cm (football) goes to 3,000,000 cm  or  3,000 KM  around the size of the moon.

3  When an atom has a complete set of electrons it is very difficult to persuade it to form any chemical compounds  The inert gases like helium and neon are members of this set.  To go into the details of which states are preferred and why is beyond a simple answer like this but if you follow the maths it is quite clear.  Now there are two fundamental sorts of chemical bond.  The simplest is when one atom really needs another electron to "complete the set"  and another has one electron over the set so this electron is very loosely bound so the electron is transferred and the atoms stick together by electrostatic charge.  Common salt or sodium chloride is like this and most compounds like this are "salts"  that is, compounds of metals with strong acids.  There is another sort of bond where the atoms in effect share the electrons and the orbitals of the electrons include both nuclei these are called covalent bonds.  Typical examples of this are the bonds in many organic compounds like oils and waxes.



« Last Edit: 17/12/2009 21:42:47 by Soul Surfer »
 

Offline Gile na Gile

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Some basic questions
« Reply #2 on: 17/12/2009 20:13:26 »
Thanks for the reply Soul Surfer.

What puzzles me about electrons is that they are defined as positively or negatively charged. What is a charge anyway? - a unit of energy? - a propulsive force? And this defintion only defines them in terms of their having either a repulsive or attractive relationship towards other components of the atom such as the proton - so our understanding of the properties of either are only being advanced by our talking of how they effect one another. What is it about electrons that make them intrinsically repulsed or drawn towards other particles? How do we visualise such relationships? Are you suggesting that the electron field places a "cap" of sorts on the amount of energy that can be released by the atom?

So electrons are attracted to the protons but what is keeping them both apart - the repulsive power of the neutrons? What is it about a proton and an electron which attracts them to one another, other than saying it is because they are positively and negatively charged? Why are they drawn together? Is it to do with their shapes, their movements, their mass, or what is it?

Sorry for all the questions, btw.
 

Offline Soul Surfer

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What keeps atoms together?
« Reply #3 on: 17/12/2009 22:05:08 »
I think that I have covered most of your supplementary questions in my completed answer.  All electrons are negatively charged and all protons positively charged.  I will now try to explain the rules by which electrons are added to the nuclei.

Each possible orbital of an electron can contain two electons with opposite spins.  Think of the spins as being like little magnets if you have one with the north pole up a mahnet with north pole down will be attracted to it and stick there is one lowest energy orbital so two and only two electrons can orbit it in a structure with a spherical symmetry.  The next electrons must have a slightly higher energy in the form of angular momentum and so there is another slightly higher enery orbital that can contin two electrons in a structure with a spherical symmetry but there are also other structures permitted with an elliptcal symmetry in three seperate directions (like the points on a triangle) each of these orbitals can have two electrons so at the second energy level eight electons can be added and this makes up the main electron structures of the periodic table which includes the full range of chemical properties of the various atoms.

The third level then has even more complex orbitals with an  exra five more extreme structures giving a total of 18 electrons  the next  32  and this level of complexity takes us right up to the limits of the 100 or so elements that exist.

 

Offline Soul Surfer

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What keeps atoms together?
« Reply #4 on: 17/12/2009 22:39:24 »
All these orbitals come out of the precise maths which is very complicated for a simple page like this. There are many excellent physics and chemistry text books at university level that should explain it to you if you really want to know.

However let me try to explain it using the simplest atom, a hydrogen atom as an example.

First let me reiterate two important facts that you must understand and accept.

Firstly the uncertainty principle,  There is a fundamental limit on how accurately you can measure the velocity and the momentum OR the location and the energy of any particle.  This is a totally fundamental property of our universe.

Secondly all particles can be considered as waves.  This wave/particle duality is also a fundamental property of our universe.  The wavelength of any particle is a function of its energy or momentum.  The higher the energy the shorter the wave.

Both of these facts can be fully proved by experiments.

Now consider an electron approaching a proton and trying to form a neutral atom it must loose energy by radiating photons.  These photons are at particular frequencies which define the spectrum of the material that is being formed.  There are fixed sets of energy levels (quantum states) that the electron can occupy the lowest one is the most basic state and the electron cannot collapse on to the nucleus.  Now it is possible to measure the momentum of the electron when it reaches this state and when you do you find that if you consider the electron to be a wave a simple whole number of waves fit into the space so it is just like the electron was resonating in a cavity like the note of a musical instrument.  The higher energy levels also have this property.  This is all precisely calculable and predictable from the measured properties of the proton and the electron in isolation.  Now if you add a neutron to the proton to make deuterium and calculate the spectrum it will be different because the nucleus is heavier and does not move about quite as much.  This is all accurately predictable.  When you get to more complex atoms and multiple electrons they all interact with each other slightly at the classical levels but if you take all this into account the orbitals are exactly predicted.  Now if you add a magnetic field the relationships between the spins of the protons and electrons matter and the energy levels split and this can be predicted and measured and forms a technique for measuring the human body called nuclear magnetic resonance.

I appreciate that this is probably more information than you need but it sometimes annoys me when people (not you I think) come onto these sites thinking that scientists do not know what they are talking about and many physical facts are imprecise. the problem is frequently they know far more than most people will ever understand about these things and many facts are known to degrees of precision that the general public just do not understand.
 

Offline yor_on

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What keeps atoms together?
« Reply #5 on: 17/12/2009 22:42:35 »
First of all, I think Soulsurfer is describing it very nicly.

Never the less I'm guessing that you are stuck on what it 'really means' same as me. What we have and 'know' is a subject for continuous revisions as new theories comes. Einstein, Newton, Maxwell, Planck, Bohr and a lot of others have asked themselves that same question and come up with really interesting contributions as they tried to 'visualize' it. We use mathematics as a primary tool for checking if our ideas make sense, then we use experiments to confirm those mathematics. Sometimes it can be the other way around though. And as Soulsurfer writes, scientists do know what they are speaking of, at least sometimes :)But then again, to really know it all? Why, then you would be 'God', if there now is possible to do so.

As for 'charge'? well that's a loaded question. Take a look at this.
http://amasci.com/elect/charge1.html


1. It's called the atomic 'strong force' http://wiki.answers.com/Q/What_forces_hold_the_various_parts_of_an_atom_together

As for what it 'really means' :)
Wouldn't it be nice to know that one, to be able to answer what it really is is a little of the holy grail. We know that they 'exist' though, and we can count on them.

As for the chemistry? Well, I think Soulsurfer gave you enough to start to look it up?
http://dl.clackamas.cc.or.us/ch104-07/tableof.htm
« Last Edit: 17/12/2009 22:46:33 by yor_on »
 

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What keeps atoms together?
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