Atoms Seeking Valence Electrons: Why 8?

[Dual Op Amp]

Well, first thing I still don't understand this, that's why I'm asking. I'm talking about the covelent bonding rule. For example the molecule of oxygen shares a double bond with another oxygen atom (O2). The oxygen atom has six valence electrons, and seeks to have eight. The atoms share two electrons, and for a brief moment one of them own all eight valence electrons. This is because one oxygen atom donates an electron, and so does the other. The two electrons orbit around the two atoms. Okay, so the oxygen has six, donates one, and now has five.
When the two electrons orbit around it, it has seven valence electrons. That's not suitable. There's where the double bond come's in. Actually, each oxygen atom donates two valence electrons. So, it now has four valence electrons. The four valence electrons orbit the oxygen atoms. For a very brief moment one atom has it's eight valence electrons. This is proven in every almost every molecule. Water, methane, ozone, oxygen. If you calculate the valence electrons, it always adds to eight.

 

[ZapperZ]

Well, first thing I still don't understand this, that's why I'm asking. I'm talking about the covelent bonding rule. For example the molecule of oxygen shares a double bond with another oxygen atom (O2). The oxygen atom has six valence electrons, and seeks to have eight. The atoms share two electrons, and for a brief moment one of them own all eight valence electrons. This is because one oxygen atom donates an electron, and so does the other. The two electrons orbit around the two atoms. Okay, so the oxygen has six, donates one, and now has five.
When the two electrons orbit around it, it has seven valence electrons. That's not suitable. There's where the double bond come's in. Actually, each oxygen atom donates two valence electrons. So, it now has four valence electrons. The four valence electrons orbit the oxygen atoms. For a very brief moment one atom has it's eight valence electrons. This is proven in every almost every molecule. Water, methane, ozone, oxygen. If you calculate the valence electrons, it always adds to eight.

Two things: (i) you're confusing what you see in CERTAIN, LIMITED instance as being the general rule; (ii) you already are contradicting yourself with your own example.

Look at water molecule, for instance. While you are busy occupying your attention with what is going on with the "oxygen" part of the molecule, why don't you take a look at what is going on with the "H" part. Each of the hydrogen atom within the molecule only share a total of TWO electrons with the O atom - one electron that it originally had, and one electron from the O atom. This is a total of <gasp> TWO valence electrons for each H atom in the water molecule. So where is your "8 valence electron" rule here? And I hate to repeat my earlier example of H2 molecule, etc., since obviously that example was completely ignored.

I still want to know where is this -8e ions that are so stable.

Zz.

 

[Dual Op Amp]

Okay, first thing there is a difference between arguing and fighting. Fighting is being angry at one other, and rudely saying or doing something in order of getting rid of anger. Arguing is discussing a topic, in order to find an answer.
I'd hate to be more mature than you, since I'm <gasp> only sixteen. I'm sorry, I don't mean to be rude to my elders, but you, a fourty-two year old, instigated this. Now, I am sorry that I didn't adress hydrogen for you. Hydrogen is an exception to this rule, because it can only donate one electron, but if you notice, every atom that has more than two electrons fills up the lowest shell with, always, two electrons. Methane, four hydrogen atoms surrounding a carbon atom, has a total of eight electrons. Each hydrogen atom has only two, because it is an exception to the rule. Imagine water, it adds up to eight. Oxygen, with six valence electrons, has two hydrogen atoms bonded to it. This adds up to eight. Type in chemical bond in your search engine, an you'll find out this rule is every where.

 

[ZapperZ]

Oxygen, with six valence electrons, has two hydrogen atoms bonded to it. This adds up to eight. Type in chemical bond in your search engine, an you'll find out this rule is every where.

EVERYWHERE??!

Here are some COMMON IONS that do not form nor want the Noble Gas "8-valence electron" structure:

Fe3+ [Ar]3d5
Cu2+ [Ar]3d9
Zn2+ [Ar]3d10
Ag+ [Kr]4d10
Pb2+ [Xe]4f14 5d10 6s2

A compound form with these do not have 8 valence electrons. Try it! Look at copper-oxide, for example! A lot of the transition metals (when you have the d-orbitals as part of the valence band) certainly do not form molecules and compounds with 8 valence electrons.

For some odd reason, the "examples" you seem to be focusing on, or maybe these are the only ones you were exposed to, seems to be only the ones having 2s 2p/3s 3p-type as the "valence" band (count this - this is EIGHT electrons total). If you look closely, only a limited range of element in the periodic table would want to either gain electrons or loose electrons to gain the equivalent noble gas structure. Once you get into elements with the 3d orbitals (the transition metals), your 8-electron rule is no longer valid! You could have gotten that from my earlier question regarding why the 4s orbital gets filled up first ahead of the 3d.

.. and please don't tell me I'm "fighting" after I spent all this time trying to explain why, and show you examples where, your 8-electron rule isn't as "everywhere" as you thought!

Zz.

Edit: additional info. Why I even bother with this, I don't know. But just to prove that I'm not making this up as I go along, here's a reference:

http://wine1.sb.fsu.edu/chm1045/notes/Bonding/Ionic/Bond02.htm

Look at the bottom of the page where there are examples using exactly the transition metals, where the octet rule fails!

 

[balkan]

Oy vey!

If you agree with Balkan, then the valence shell is 6 electrons, which is the filling of the p orbital. This clearly contradicts your illusion of all atoms wanting 8 valence electrons. Your assertion that the atom will try to fill the "final orbit" (??) with EIGHT (count 'em) electrons, "even if it makes an ion" is ridiculous. What "orbit" is this, since the NEXT orbital is the d-shell?Show me where you can easily find an ion with -8e.

Zz.

i think you must excuse our friend here and stop tripping so much
i agree wholehearted with you personally, but i think he's talking about the bonding processes that leads to a lower energy level... like when two hydrogen atoms combine, or two oxygen atoms combine, obtaining a lower energy level by getting a partially shared amount of electrons which (almost) equivalents to the noble states...

am i right, Dual Op Amp?
cause in that case, it has to do with quantum mechanics, and that'll come later if you keep studying physics... it involves a whole lot of theory and will also reveal that it's really not even the case, it's just a convenient way to predict bonding... cause in a molecule, electrons will find them heavily subjected to the pauli principle and hunds rule, and everything goes sha'bang... just accept is as a convenient "rule" and learn the theory later...

 

[Dual Op Amp]

Okay, I was going to say that some atoms don't do this, but I didn't. Some atoms are an exception to this rule. I'm sorry for assuming you were fighting. Now, if you'll give me another chance, could you explain to me why, only in these certain atoms, do they become stable, by filling their valence electrons?

 

[shrumeo]

I don't understand why ZZ is arguing so fervently about something any freshman General Chemistry course can teach you.

So, the main group "follows" the 8-electron rule AKA the octet rule, which is often broken. The transition group "follows" the 18-electron rule, which is less often violated.

But, they both add up to filled orbitals. When an orbital is full, it is lower in energy than a non-filled one. Noble gases already have filled orbitals and are very stable by themselves, no need to go sharing. Maybe your question is WHY are filled orbitals so stable? That's when you'd have to take the QM class.

 

[Dual Op Amp]

Why is a filled orbital lower in energy?

 

[balkan]

Why is a filled orbital lower in energy?

an basic explanation for this would fill a dozen pages and have unpleasent (= lovely for some of us that really like that ****, hehe) calculations that, if you're sixteen, you're probably not ready for...

 

[Dual Op Amp]

Thank you, anyway.

 

[evac-q8r]

Hey, I'm not sixteen and I would really like the know the answer to that. There are others viewing this thread you know. Please share because I never knew that was possible.

-EVAC

 

[balkan]

Hey, I'm not sixteen and I would really like the know the answer to that. There are others viewing this thread you know. Please share because I never knew that was possible.

-EVAC

but if you don't have quantum mechanics, it's going to be totally confusing... it is very confusing for many of my fellow students, and they have quantum mechanics...
if you're interested, i suggest taking a class in quantum mechanics or picking up a book about the basic concepts, cause it really takes some reading on the concepts before you can begin diving into the math...

 

[evac-q8r]

but if you don't have quantum mechanics, it's going to be totally confusing... it is very confusing for many of my fellow students, and they have quantum mechanics...
if you're interested, i suggest taking a class in quantum mechanics or picking up a book about the basic concepts, cause it really takes some reading on the concepts before you can begin diving into the math...

Balkan, why are you assuming that I haven't taken QM! Very bad. I have had more courses in advanced QM and QFT, String theory than you probably even knew existed and honestly if I wanted to know I'd just figure it out for myself. Would you like for me to explain to why or why not this is possible? The point of my post was that there are many individuals reading this thread, including myself, who may not know why this is true because there is so much to learn about QM you can never retain it all, but would like to see it explained.

-EVAC

 

[balkan]

Balkan, why are you assuming that I haven't taken QM! Very bad. I have had more courses in advanced QM and QFT, String theory than you probably even knew existed and honestly if I wanted to know I'd just figure it out for myself. Would you like for me to explain to why or why not this is possible? The point of my post was that there are many individuals reading this thread, including myself, who may not know why this is true because there is so much to learn about QM you can never retain it all, but would like to see it explained.

-EVAC

i'm assuming because this is pretty basic quantum mechanics... already in second semester we did calculations on several atoms to discover how their valence electrons acted...
if you've taken so many courses, you should know how it works, and you should furthermore know, that it's not something you can just jot down in an online forum...

 

[evac-q8r]

First of all it is not of your concern to tell me what I should and should not know. That's a big problem within the real world of physics which you have not reached yet. Everyone, or maybe just the hot shots, wants to prove that they know so much and that the next person doesn't know anything. Basically, I would encourage you to be more careful with the assumptions you make about people who you know nothing about.

So, again, if it soooooo simple (pretty basic QM), then give a simple explanation for that phenomenon. I'll spare you the dozens of pages that you say is required to adequately explain the concept. We are waiting.

EVAC

 

[Gokul43201]

EVAC,

I think you are being unreasonable here.

If you really want to know why (say) an 8 electron shell (for n=2) is lower in energy than if it had 7 (or 6 or 5, etc) electrons, please write down the Hamiltonians and plug into the time indep. Schrodinger Equation, and solve them. You will find that the energy is lowest when the shell is filled. Clearly, adding an extra electron makes it go into the next shell (n=k+l=3) which increases the energy, since, roughly speaking (E(n) = -E/n^2).

The problem with this is that you really just can't write this down on a pice of paper and solve it like it were a quadratic equation. You need a powerful numerical algorithm to get close.

 

[evac-q8r]

Thanks for that explanation. If I was being unreasonable, it is only because I don't need to be told that I should pick up a book or take a course in something that I have already taken. This was mainly for the people who wanted to see (including myself) how these eigenstates and eigenvalues might be calculated instead of being told that it is too difficult. I don't care how old the individual is, no one likes being told nor needs to be told that they are not ready to achieve something just because of their age.

an basic explanation for this would fill a dozen pages and have unpleasent (= lovely for some of us that really like that ****, hehe) calculations that, if you're sixteen, you're probably not ready for...

-EVAC

 

[balkan]

I don't care how old the individual is, no one likes being told nor needs to be told that they are not ready to achieve something just because of their age.
-EVAC

well perhaps nobody likes it (personally i don't care), but that doesn't make it any less true... and if they are making projects at university their supervisors will be saying the same thing, all the time... and in that case, maybe it's time to learn how to handle such a simple, and in no sense offensive, statement...

 

[so-crates]

The answer to why a certain configuration fo electrons are more stable than others is pretty complicated, and if you are looking for a one or two sentence answer, others have provided some good ones. I will try to give more detailed understanding.

First of all, a five minute introduction to quantum mechanics. Classically, energy can be absorbed or emitted in any amount. For example, I can hit a pool ball at 1 m/s, 1.5 m/s, 2 m/s or any value in between. However, when physicists tried to apply classical mechanics to small pecies such as atoms, there arose three problems: (1) Black Body Radiation(also called The Ultraviolet Catastrophe), (2) The Photoelectric Effect and what I call (3) The Electron Orbital Catastrophe. (1) and (2) are quite involved, but let me see if I can explain (3) briefly . By that time (circa 1900) the results of Rutherford's experiments had shown that the atom consisted of a small, positively charged nucleus surrounded by negatively charged electrons. They then began to theorize how the electrons moved about the nucleus. In the Bohr model, you can imagine the atom as forming a minuature planetary system, where the electrons rotate around the nucleus in more or less circular orbits. As you will learn later in physics moving charges should radiate electromagnetic energy, so the electron as it revolves, or moves somehow about the nucleus should lose energy and crash into the nucleus. Clearly that does not happen.

The failure of the classical theory to these phenomenon led physicists, among them Max Plank, to develop a model whereby energy could only be omitted in certain bundles, or quanta. These quanta are represented by photons (light, radio, and other electromagnetic radiation) The energy carried by a photon is equal to E = hv, where h is the universal constant called Plank's constant. and v is the greek leter "nu" standing for frequency. It turns out that small particles like electrons, protons, atoms, and molecules are not like pool balls. They can only absorb quantized energy. In other words, they can only absorb or emit photons, and in addtion only photons or a certain wavelength or frequency. Since the particles can only absorb energy in certain bundles, we can speak of energy 'states' at which the particle has absorbed n = 1, 2, 3 ... etc. photons. The state in which the particle can emit no more photons is said to be its 'ground' state, n = 1. As the particle absorbs the photon, it gains energy. As it loses energy, it will re-emit the photon. This effect can easily be seen in light bulbs: a filiament is heated, which excites an electron. The electron will return to its ground state and re-emit the photon, which produces light in the visible spectrum. Knowing the charge and mass of the proton and electron allows us to calculate these energy levels, using the principles of electromagnetic theory.

But first, we have another problem, and it has to do with something called Wave-Particle Duality. For a very long time, physicists were debating over what light consisted of. Newton suspected it consisted of particles, which he called corpsucles. Others such as Robert Hooke, one of Newton's contemporaries, believe it to be carried by a wave. By the middle of the 19th century it appeared the question was settled when Henry performed his infamous double slit experiment. When you pass a wave through a small aperture that is on the order on the wavelength of the wave, you get a phenomenon known as diffraction. This means that the wave spreads out like it originated from a point source. You can then detect the intensity of the wave(related to its amplitude) at some distance from the apeture. Waves also exhibit another property called interference. This means that waves can pass through one another, and as they do they will add or subtract together. You can see this phenomenon if you get a long rope or slinky, hold it fixed at one end, and then send one wave down it and another a short time afterwards. You will observe the two waves interfere with each other as they meet, then pass through one another. If you are still not convinced(they could merely be rebounding off of each other), send a small wave then a big one. If you do the calculations as you pass a wave through two slits, you can calculate what the intensity will be at a plane that is some distance from the slits and parallel to both slits. You should get a 'band' pattern, where you have alternating fringes of constructive and destructive interference. Well when Henry performed his experiment, he detected these bands of light, and it was accepted that light traveled as a wave.

You may notice that in order for something to travel as a wave, it requires a medium. For example, water waves travel in water, sound waves in air, etc. Without the medium, the wave cannot propogate. Well if light travels as a wave, what is the medium? Physicists in the mid 19th century called this medium the 'ether' and set out to try to understand it or detect it somehow. If light can travel from the moon, the sun and even faraway stars and galaxies, then it must permeate all space. Finally around 1885 two physicists, Michelson and Morley performed an experiment that showed that there was no ether. They constructed an apparatus, called an interferometer, which is a bit complex but basically measured interference of light that was rebounded off of mirrors. They performed the experiment and found interference as expected. Then they waited six months and performed the experiment again, and found the same interference pattern. Well if the ether is everywhere, then the Earth must be moving through it, so you would expect to get a different interference pattern depending on the motion of the Earth through the ether.

So that experiment showed that there was no ether. In addition Einstein's explanation of the photoelectric effect which I mentioned before, required a particle theory of light. So as it turned out, light was not a wave. But it was not a particle either, as Henry's double slit experiment showed So what it is it? You can consider it to be both a particle and wave, or something that our language is inadequate to describe.

(to be continued...)

 

[elas]

As a visitor from the Theory Development asylum I find this forum particularly interesting, it makes some of the points made by us nutcases. But on this forum I must stick to reality.
Have you not noticed that the formation of electrons is similar to the formation of quarks (try pentaquark foe example)? Also that good bonding requires additional shared particles or that mass fluctuates as the shared particles enters or departs. Or that radius shrinks on addition to a shell but expands with the addition of the first particle to a new shell (valence particle?). Or that the number of particles needed to complete the expansion cycle increases from one to three as atoms become more massive.

Not all behaviour patterns are repeated at both atomic and particle level but there are no unrepeated actions. Some actions are repeated on the cosmic scale, for example-
The fractional charges found in fractionally charged electrons are the same as the fractional differences in distances found on the cosmic scale but, whereas no one would include the waves radiating from the sun, in its (the sun's) radius; particle physicist always imply that particle waves should be included in particle size, I wonder why.

My point being that you can probably get a better grasp of what is happening if you accept that nature repeats its actions on all scales, therefore logically the true explanation is likely to be the one that can be applied to all scales. That explanation of course, is not known at present.

 

[Dual Op Amp]

To be continued...Don't stop there, continue please.
For Zapperz:
This part knew, not all of it, but this part.
The answer to why a certain configuration fo electrons are more stable than others is pretty complicated, and if you are looking for a one or two sentence answer, others have provided some good ones. I will try to give more detailed understanding.

First of all, a five minute introduction to quantum mechanics. Classically, energy can be absorbed or emitted in any amount. For example, I can hit a pool ball at 1 m/s, 1.5 m/s, 2 m/s or any value in between. However, when physicists tried to apply classical mechanics to small pecies such as atoms, there arose three problems: (1) Black Body Radiation(also called The Ultraviolet Catastrophe), (2) The Photoelectric Effect and what I call (3) The Electron Orbital Catastrophe. (1) and (2) are quite involved, but let me see if I can explain (3) briefly . By that time (circa 1900) the results of Rutherford's experiments had shown that the atom consisted of a small, positively charged nucleus surrounded by negatively charged electrons. They then began to theorize how the electrons moved about the nucleus. In the Bohr model, you can imagine the atom as forming a minuature planetary system, where the electrons rotate around the nucleus in more or less circular orbits. As you will learn later in physics moving charges should radiate electromagnetic energy, so the electron as it revolves, or moves somehow about the nucleus should lose energy and crash into the nucleus. Clearly that does not happen.

The failure of the classical theory to these phenomenon led physicists, among them Max Plank, to develop a model whereby energy could only be omitted in certain bundles, or quanta. These quanta are represented by photons (light, radio, and other electromagnetic radiation) The energy carried by a photon is equal to E = hv, where h is the universal constant called Plank's constant. and v is the greek leter "nu" standing for frequency. It turns out that small particles like electrons, protons, atoms, and molecules are not like pool balls. They can only absorb quantized energy. In other words, they can only absorb or emit photons, and in addtion only photons or a certain wavelength or frequency. Since the particles can only absorb energy in certain bundles, we can speak of energy 'states' at which the particle has absorbed n = 1, 2, 3 ... etc. photons. The state in which the particle can emit no more photons is said to be its 'ground' state, n = 1. As the particle absorbs the photon, it gains energy. As it loses energy, it will re-emit the photon. This effect can easily be seen in light bulbs: a filiament is heated, which excites an electron. The electron will return to its ground state and re-emit the photon, which produces light in the visible spectrum. Knowing the charge and mass of the proton and electron allows us to calculate these energy levels, using the principles of electromagnetic theory.

But first, we have another problem, and it has to do with something called Wave-Particle Duality. For a very long time, physicists were debating over what light consisted of. Newton suspected it consisted of particles, which he called corpsucles. Others such as Robert Hooke, one of Newton's contemporaries, believe it to be carried by a wave. By the middle of the 19th century it appeared the question was settled when Henry performed his infamous double slit experiment. When you pass a wave through a small aperture that is on the order on the wavelength of the wave, you get a phenomenon known as diffraction. This means that the wave spreads out like it originated from a point source. You can then detect the intensity of the wave(related to its amplitude) at some distance from the apeture. Waves also exhibit another property called interference. This means that waves can pass through one another, and as they do they will add or subtract together. You can see this phenomenon if you get a long rope or slinky, hold it fixed at one end, and then send one wave down it and another a short time afterwards. You will observe the two waves interfere with each other as they meet, then pass through one another. If you are still not convinced(they could merely be rebounding off of each other), send a small wave then a big one. If you do the calculations as you pass a wave through two slits, you can calculate what the intensity will be at a plane that is some distance from the slits and parallel to both slits. You should get a 'band' pattern, where you have alternating fringes of constructive and destructive interference. Well when Henry performed his experiment, he detected these bands of light, and it was accepted that light traveled as a wave.

 

[what_are_electrons]

Unfortunately, short of being a psychic, one doesn't know that. It is compounded by the fact that the original question is rather terse, and no references or sources were cited to where this "eight electrons" rule was found.

In fact, the ONLY atom that would want "eight electrons" in its valence shell is an atom with eight protons! This is only one particular atom. [Remember, the question asked about ATOMS, not molecules, not atoms in a solid state configuration, etc... or am I being "picky" again?] So there isn't even any notion of generalities here.

From where I stand, the whole original question is based on a false idea, or at best, extremely vague. So that's why I was puzzled that there was really an active followup with people attempting to answer it without anyone actually pointing out that the whole thing is based on a false premise in the first place.

Zz.

Maybe you should read a book on General Chemistry before complaining.

 

[what_are_electrons]

Oh puhleeze! I'm older than dirt, so that's no excuse! :)

Zz.

Just how old is the dirt where you live? My seems to be several billion years old. So, logically you should be several billions years old too. Did I do the math right ZZ?

 

[what_are_electrons]

EVERYWHERE??!

Here are some COMMON IONS that do not form nor want the Noble Gas "8-valence electron" structure:

Fe3+ [Ar]3d5
Cu2+ [Ar]3d9
Zn2+ [Ar]3d10
Ag+ [Kr]4d10
Pb2+ [Xe]4f14 5d10 6s2

A compound form with these do not have 8 valence electrons. Try it! Look at copper-oxide, for example! A lot of the transition metals (when you have the d-orbitals as part of the valence band) certainly do not form molecules and compounds with 8 valence electrons.

For some odd reason, the "examples" you seem to be focusing on, or maybe these are the only ones you were exposed to, seems to be only the ones having 2s 2p/3s 3p-type as the "valence" band (count this - this is EIGHT electrons total). If you look closely, only a limited range of element in the periodic table would want to either gain electrons or loose electrons to gain the equivalent noble gas structure. Once you get into elements with the 3d orbitals (the transition metals), your 8-electron rule is no longer valid! You could have gotten that from my earlier question regarding why the 4s orbital gets filled up first ahead of the 3d.

.. and please don't tell me I'm "fighting" after I spent all this time trying to explain why, and show you examples where, your 8-electron rule isn't as "everywhere" as you thought!

Zz.

Edit: additional info. Why I even bother with this, I don't know. But just to prove that I'm not making this up as I go along, here's a reference:

http://wine1.sb.fsu.edu/chm1045/notes/Bonding/Ionic/Bond02.htm

Look at the bottom of the page where there are examples using exactly the transition metals, where the octet rule fails!

Perhaps you should study the many energy vs structure trends that exist in the Periodic Table. There's a nice short text (100 pp) called " The Periodic Table of the Elements" 2nd ed by RJ Puddephatt and PK Monaghan (1986) that will provide most of those trend relationships.

 

[quantumdude]

Maybe you should read a book on General Chemistry before complaining.

Perhaps you should study the many energy vs structure trends that exist in the Periodic Table. There's a nice short text (100 pp) called " The Periodic Table of the Elements" 2nd ed by RJ Puddephatt and PK Monaghan (1986) that will provide most of those trend relationships.

The problem with your responses is that none of those references has the "why?" explanation that is being sought after here. That is to be found in books on quantum mechanics.

 

[Dual Op Amp]

Doesn't anyone have the answer?

 

[quantumdude]

You've asked a question that has the "simple answer" and the "not-so-simple answer". The simple answer is the one that you would find in your general chemistry book: when atoms form molecules, they like to have full shells. For instance consider carbon, which has 6 electrons, has an electron configuration:

1s²2s²2p².

So carbon's outer shell (n=2) has 4 electrons. The 2s subshell is full, but the 2p subshell can have 6 electrons, so when C bonds it tends to pick up the 4 electrons it needs to fill the shell, giving it a total of 8 valence electrons (mind you, there will not always be 8 valence electrons for every atom, as has been noted).

Now the physicist would not accept that as an explanation because it merely changes the question of, "Why do most atoms tend towards 8 valence electrons when bonding?" to "Why do atoms tend towards full shells when bonding?"

Enter quantum mechanics.

The physicist would like to explain this mathematically, by solving the Schrodinger equation. Unfortunately, for an atom with even 2 electrons, there is a noncentral term in the equation, which makes it impossible to solve exactly. And the more electrons an atom has, the more noncentral terms accrue (one for each unique pair). So, physicists and theoretical chemists have developed methods to solve these equations approximately, and it is to these solutions that a physicist would appeal for an explanation to your question.

Now one might object that all this does is change the question from, "Why do most atoms tend towards 8 valence electrons when bonding?" to "Why are atoms described by the Schrodinger equation?", but the physicist would overrule the objection, saying that that reduces the question as far as we can reduce it, and so constitutes an "explanation".

 

[Dual Op Amp]

1s22s22p2.

So carbon's outer shell (n=2) has 4 electrons. The 2s subshell is full, but the 2p subshell can have 6 electrons, so when C bonds it tends to pick up the 4 electrons it needs to fill the shell, giving it a total of 8 valence electrons (mind you, there will not always be 8 valence electrons for every atom, as has been noted).

What does that mean?
p2?
1s2?
I don't understand this.
Feeling stupid!

 

[quantumdude]

Atoms have quantities called quantum numbers associated with them.

1. Principal Quantum Number: n
In the nonrelativistic treatment of hydrogen, the quantum number n parameterizes the energy of the lone atomic electron. In multi-electron atoms, it is still associated with the energy of the electronic state, but we cannot write down neat little expressions for electronic energies as functions of n, as we can for hydrogen (and hydrogen-like atoms).

n can take on as its value any counting number, and each n represents a shell.

2. Orbital Quantum Number: (l)
l characterizes the orbital angular momentum of an electron about the nucleus. It can take on any nonnegative integer value up to n. That is, once n is specified, the domain of l is fixed.

For example, if n=2, then the allowed values of l are 0 and 1.

That is, there are 2 orbital angular momentum states accommodated by the n=2 shell. Each unique value of l within a shell is called a subshell. They are denoted in spectroscopic symbology as follows:

l=0: s
l=1: p
l=2: d
l=3: f
l=4: g
.
.
.
(continues in alphebetical order after f).

3. Magnetic Quantum Number: m_l
m_l paramterizes the z-component of the orbital angular momentum of each electron. It can take on any integer value between -l and +l. So back to our example of carbon, if we take a look at each subshell we find:

l=0: m_l=0
l=1: m_l=-1,0,1

This would continue in the same pattern if l were larger.

4. Spin Quantum Number: m_s
m_s parameterizes the z-component of the spin angular momentum of each electron. Since the electron is a spin-1/2 particle, the only values m_s can take on are 1/2 or -1/2.

Quantum states of electrons are completely described by (n,l,m_l,m_s).[/color]

Now let's see how shells and subshells are filled.

n=1
For n=1, we can have l=0, m_l=0, and m_s=(+/-) 1/2. Listing out the quantum states, we have:

(1,0,0,1/2)
(1,0,0,-1/2)

Note that there are two quantum states. So, the n=1 shell can accommodate 2 electrons. We denote this in an electron configuration by 1s². The "1" stands for "n=1", the "s" stands for "l=0" (in spectroscopic notation, as outlined above), and the superscript "2" stands for the number of electrons occupying the shell.

Next shell...

n=2
For n=2, the allowed values of the other quantum numbers are l=0,1, m_l=0 (for l=0), m_l=-1,0,1 (for l=1), and m_s=(+/-)1/2. Let's list out the quantum states.

s-subshell: 2 quantum states
(2,0,0,-1/2)
(2,0,0,1/2)

These are denoted spectroscopically by 2s².

p-subshell: 6 quantum states
(2,1,-1,-1/2)
(2,1,-1,1/2)
(2,1,0,-1/2)
(2,1,0,1/2)
(2,1,1,-1/2)
(2,1,1,1/2)

These are denoted spectroscopicaly by 2p⁶.

Now my fingers are about to fall off, so I am going to post a link that will take you into more detail.

Applet: Electron Configurations
http://wine1.sb.fsu.edu/chm1045/notes/Struct/EPeriod/Struct09.htm
Chemical Elements Dot Com: Electron Configuration It's got a clickable periodic table.
Electron Configuration from Wikipedia

 

[Dual Op Amp]

Okay, I am on the brink of understanding it, I just need to know how can there be more than one orbital per subshell?