Atoms Seeking Valence Electrons: Why 8?

[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?

 

[Dual Op Amp]

YES! I finally got it! The electrons cannot have the same four quantum numbers. So, the reason is that the second shell has an s orbital and a p orbital. A p orbital is a dumbell, so it can have 6 electrons, because the orbitals can be X, Y, and Z. In the second shell, there are two subshells, one having an s orbital and the other having a p orbital, which adds to 8.
Only a certain part of the periodic table has two shells, so the second one has to fill up to eight electrons to stableize it!
THANK YOU EVERYONE!
MUCH APPRECIATED, AMP!

 

[what_are_electrons]

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.

Elsewhere in this Forum I was told by a Mentor that I should not ask "Why" questions. Is there a uniform policy on this type of question?

 

[quantumdude]

Elsewhere in this Forum I was told by a Mentor that I should not ask "Why" questions. Is there a uniform policy on this type of question?

Not that I'm aware of.

 

[Dual Op Amp]

Here's a great site I found.
http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch6/quantum.html

 

[Dual Op Amp]

Electrons are attracted to protons, but repell electrons. So, instead of all the electrons being bunched up right next to the nucleas, they orbit around the nucleas in shells. These shells can sometimes contain sub-shells. For example, the first shell contains only one sub-shell. As an electron gets further away from it's atom, it must have more "quantum energy." Electrons want to get as close to the nucleas as possible, but according to quantum physics, no to electrons can have the same "quantum energy." So, they orbit in shells. The electrons orbit in orbitals. The sub-shells have orbitals. For example, the 1 shell has an S orbital. Because it's an s orbital and it's the first shell it's labelled 1S. For 1-First shell-, S-S orbital. An S orbital has the shape of a sphere. An orbital wants to fill it's self. Alright, so why would the atom want to have 8 electrons in it's outer most shell, good question. The second shell has two sub-shells. One sub-shell has an S orbital, and the second has three P orbitals. The reason it has three is because they can arrange themselves according to X,Y,Z. Each orbital has only two electrons, because no two electrons can have the same "quantum energy." So, for the valence shell of an atom with two shells, one S orbital and three P orbitals. Two electrons an orbital adds to...8. Hydogen, on the other hand, only has one shell. So, to fill it's valence shell, it only needs two electrons. It already has one - Hydogen = one proton, one electron - so, it only needs to bond with one atom to fill itself. Carbon, on the other hand, has two shells, so it needs 8 to fill it's valence shell. So...

H
H C H Methane! CH4.
H

If you were to count it up everyone's filled. The carbon atom has 6 electrons. 2 in it's first shell, and 4 in it's valence shell. It needs 8 in it's valence shell. So, it shares one with hydrogen, and the hydrogen shares one of the carbons. This gives the carbon an extra electron, and the hydrogen it's desired two. The carbon, then, bonds with three more to add to 8.

HOH Water! H20. Oxygen has six valence electrons, meaning it needs 2 to gain, which it does with 2 hydrogen molecules.

O=O Oxygen! O2.

You're probably wondering, why is there an equals sign between the Oxygen molecules?
This indicated a double bond. Oxygen has six valence electrons, when it bonds with another oxygen, it gets 7. That's not the desired 8. So, it makes a double bond, and they share two electrons each. Which adds to 8.

O
O O Ozone! O3. Each one of these atoms share with each other, making 8.

That's covelant bonding!
This "quantum energy I told you about is somewhat true. What's really true is that there are four "quantum numbers" that cannot match.
The first is N.
N is the energy of an electron. For example, an electron in the first shell would have an N of 1. An electron in the second shell would have an N of 2. An electron in the third shell would have an N of 3.
N=1, means it's in the first shell.
The second is L. It's actually a greek cursive L kind of like this. l. Okay. This sign is the orbital. L = N - 1. That's the equasion. So, if N = 1, then, L = 0. 0 is an S orbital.
If N = 2, L can equal either 0 or 1. If it is 1, that's a P orbital. If N = 3, then that can be either 0,1 or 2. An S,P or...a D orbital.
Now, the third quantum number is M. It is the orientation of the orbitals, you know XYZ.
M can equal anything between -L and +L. For example if L is 1, then M can equal -1,0,1.
This is 3 different ways of arranging the P orbital.
Now the final one is Ms. For Spin. The spin of the electron can equal - 1/2 or 1/2.

Okay, so let's look at the possible arrangements of some electrons.

N L M Ms
1 0 0 -1/2
1 0 0 1/2 First shell, only can have two electrons.

2 0 0 -1/2
2 0 0 1/2
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 Second shell, eight electrons, but none of them, nor the one's in the first shell have the same 4 quantum numbers.

HOPE YOU UNDERSTAND. IT TOOK ME A WHILE TO WRITE, I'D HATE TO LOSE IT AT THE LAST MOMENT, LIKE THE POWER SHUT DOWN OR SOMETHING. IF YOU UNDERSTAND THIS, YOU WILL UNDERSTAND THE REST.
HERE'S SOME SITES.

http://chemed.chem.purdue.edu/gench...h6/quantum.html

http://lectureonline.cl.msu.edu/~mm...od/electron.htm

 

[yobeht]

As an atom approaches a full valence shell, the magnetic charge in the first electron orbit cloud is too negative, causing strong repulsion of electrons away from the cloud. Redirecting the electrons to a wider orbit, to not interfere with the first set of valence electrons.

~~~~peace

 

[pallidin]

As an atom approaches a full valence shell, the magnetic charge in the first electron orbit cloud is too negative, causing strong repulsion of electrons away from the cloud. Redirecting the electrons to a wider orbit, to not interfere with the first set of valence electrons.

~~~~peace

"...first set of valence electrons."

You mean there's more?
I thought there is only one valence shell. See here: http://en.wikipedia.org/wiki/Valence_electron