# Why do atoms need 8 electrons to stabilize? [duplicate]

As the title says. I have surfed all of the net but could never find the answer to this question. Why do atoms need 8 electrons to stabilize? I mean why not 7 or 5 or 10 electrons? Why specifically 8? And what does stabilization of atoms even mean? Are the atoms going to burst without 8? Why do they need 8 electrons?

This is a question which is not often discussed in chemistry and I need an answer which really makes any sense. Not just `it is because the atoms need to fill their octet.

• Unfortunately while this is a very fundamental question, a good answer to this is not easy to write. There are far too many hidden sins in the phrasing that you quote ("8 electrons", "stabilise") and a great deal of elaboration on those points is needed. Perhaps someone will be up to the task. – orthocresol Jul 4 '16 at 16:14
• @orthocresol so this is a very deep and fundamental question that only the professionals can understand? – MartianCactus Jul 4 '16 at 16:37
• Some good pointed questions, like this one, are tricky to answer because it gets just close enough to the philosophy of knowledge to force people to get very precise. You are looking for what we, in philosophy, call a "justification" for an idea. It is clear you have already refuted the justification of "we've done this experiment millions of times, and here's what we've seen," so you are asking for an even better justification than that. Such a justification will invariably stray into the world of quantum physics. – Cort Ammon Jul 4 '16 at 22:20

The valence orbitals of atoms are composed of suborbitals (s and p) there is 1 s suborbital which is spherical and can hold 2 electrons (one with up spin and one with down spin). There are 3 p suborbitals which are dumbbell shaped (look like two balloons tied together at the ends) and align along the x,y&z axies and hold a total of 6 electrons (2 per axis, 3 with up spin and 3 with down spin). Since an atom is energetically satisfied when all of the electrons are paired and 2+6 = 8, an atom must have 8 electrons in its valence shell to pair all of the electrons.

The octet rule does not apply to all atoms though, for transition metals and later elements there is a d suborbital which can hold an addition 10 electrons, meaning that now an atom may have 18 electrons in its valence shell to be satisfied. This allows for materials such as metal complexes and the sulfate poly-atomic-ion to exist despite having more than eight electrons in the valence shell.

• what do you mean by energetically satisfied? How can an atom know if it is satisfied? And what causes this satisfaction? And why is there even a need for satisfaction in atoms? – MartianCactus Jul 4 '16 at 15:06
• @Adi I'm assuming means a low energy state. – DraggyWolf Jul 4 '16 at 15:23
• @DraggyWolf And as I said in your question...what does low energy state has to do with stabilization? – MartianCactus Jul 4 '16 at 15:58
• – Alexander - Reinstate Monica Jul 4 '16 at 17:18
• @Adi High energy states (tend to) have many ways of becoming lower energy states, but very few ways to remain in a high energy state. Consider a tall tower with improper supports. There are many ways to fall, but only a few ways to stay upright. – Alexander - Reinstate Monica Jul 4 '16 at 17:19

I think the most important thing here is to point out some details which I don't believe get emphasized enough in chemistry classes, and yet are extremely important in approaching questions like this.

I mean why not 7 or 5 or 10 electrons? Why specifically 8?

I've been in this situation before. The one where I'm asking a question which I believe to be a question about the fundamental nature of matter, but the more I explore it, it just seems to be a meaningless question. The actual number of electrons required to stabilize a system is not a question we should really care about. The question we care about is whether or not our theories can predict that number. After all, in science, we are asking nature the questions, and nature gives us the answers. So in this case, nature has told us 8 electrons, so the answer is 8 electrons. I think that's one thing which is emphasized a lot more in physics than in chemistry. Nature is the arbiter. We simply try to rationalize the arbiter's decisions.

Why do atoms need 8 electrons to stabilize?

Now that's a question which can be answered by a theory! This can be answered in lots of detail or a little detail, but I get the feeling you are going to ask questions about why we don't just keep filling up the orbitals with more electrons? This is basically what you're asking about A.K.'s answer when saying,

what do you mean by energetically satisfied? How can an atom know if it is satisfied?

This leads us to perhaps the greatest disservice done to chemistry students in their classes: the habit of personifying chemical systems. Talking about the "desires" of the system makes it easier to understand when first learning, but this ought to quickly be abandoned (fluorine just loves electrons!). The answer to your question here is that the system doesn't know what it wants, and that is often a very good question to ask! These sorts of questions can be very productive when studying something like quantum entanglement. But, for the matter at hand, something simpler suffices. Chemical systems are dynamic. They are changing in time by colliding with other molecules, vibrating, rotating, absorbing photons and emitting photons, and doing all these things at once! So then, what it means for a system to be "energetically satisfied" is that all these things have found some kind of balance. It is perfectly possible that at any given moment some atom might lose an electron and another one picks it up, but again we must come back to what nature tells us. Nature says that when we have two atoms flying around, and one of them has seven electrons, that atom is going to take an electron from something else.

This need not be reserved for chemical systems though. In all of physics, we observe that systems tend towards their lowest energy state. This isn't dictated absolutely by some law, but rather it is a general rule. After all, we can find systems that aren't at the global minimum of their potential energy surface, but if the activation energy can be ignored, we will almost always find that system in its lowest energy state.

It's almost as if nature is probing these five, seven, and ten electrons states which you suggest, but simply does not stay there because that is not the way the world works. To be technical, systems tend towards equilibrium (you must convince yourself of this because the mathematics to show it can be involved), so when I find an atom with ten electrons, the forces in the system are out of balance. There might be eight positive charges in the nucleus, but ten negatively charged electrons. Thus, when something with seven electrons comes around, it is likely that system will take one of the extra electrons because this balances out the forces (as well as can be done in this example). Some kind of equilibrium has been established. This is obviously a simplistic example, but you could imagine then that if you had an ensemble of Avogadro's number of particles, that all these things would be exchanging their imbalances and staying around the equilibrium. This equilibrium, again, is observed to be the low energy state.

Concluding Remarks:

If and when you ever study quantum mechanics, you'll find that these things all arise quite naturally out of the theoretical framework laid there. When you find that angular momentum is quantized (and that the electron can have zero angular momentum), you naturally ask what this waving electron thing looks like (say 95% of the time). So, you plot this using the equations that you've found from your theory, and you get out the orbitals that we learn all about in chemistry classes. The next question leads into what you're asking here, both about the number of electrons each atom will naturally have around it, and about the relative stability of each of those cases. If you really want to be satisfied about those questions, study quantum mechanics. Otherwise, what we've said here ought to suffice. All in all, there comes a point when you either to need to look deeper, and be willing to grapple with some complicated ideas in quantum mechanics, or you need to give up on asking epistemological questions because the answer to those kinds of questions in science is almost always, "because mother nature says so."

• thank you for an answer but I believe that everything has a reason behind it..even the most fundamental order of the universe has a reason. Nothing just happens. There is no Because mother nature says so. But for this matter I think that I have to put this question on hold until I learn quantum mechanics as right now I am only a 10th grade student. – MartianCactus Jul 5 '16 at 11:15
• I think that the more you study science you will understand what I mean. Physics and chemistry are simply the act of doing experiments and explaining those observations. An experiment is just asking nature what is true. So, in that sense, there is definitely a "because mother nature says so." And you're right that nothing just happens, but I never said that... – jheindel Jul 5 '16 at 16:48
• I agree with jheindel... We observe what happens in nature and we construct sensible explanations to rationalize what we observe. Technically, a pertinent atom doesn't consciously "desire" 8 electrons, because it can't actually think. But for early education purposes, it's easier to think that they have this "desire". In other words, if we wanted, we could consider the $n$, $l$, $m_l$, and $m_s$ quantum numbers to rationalize our own constructed octet rule, but atoms know nothing of our constructed octet rule. – timaeus222 Jul 6 '16 at 3:08
• @jheindel i understand.But one day I believe I will find the answer. – MartianCactus Jul 6 '16 at 3:29

Atoms generally do not need 8 electrons to stabilize.

Using quantum physics and its models of atoms and bonds we can define four quantum numbers. $n$ being principal quantum number describes the electron shell, $l$ being azimuthal number describes the orbital shape, $m$ being magnetic number describe degeneration of the orbital and $s$ being spin number describes the spin. Pauli's principle says that 2 electrons (fermion) cannot exist in the same (energetic) state and quantum numbers describe such energetic states of electron in the atom.

It was also found, that $n=1,\ldots,~ l=0,\ldots ,n-1, ~m=-l,\ldots,l,~s=\pm1/2$.

Another rule claims that electron is preferably in the state with lowest possible energy and therefore the states with lower $n$ are preferred before the others with similar energy.

Al this together we obtain the sequence of occupying the energetic levels as follows: $$\rm 1s^2, 2s^2, 2p^6, 3s^2, 3p^6, 4s^2, 3d^{10}, 4p^6, 5s^2, 4d^{10}, 5p^6, 6s^2, 4f^{14}, 5d^{10}, 6p^6,\ldots$$ where number is principal quantum number, $\rm s,p,d,f$ stands for $l=0, 1, 2, 3$, respectively, superscript shows the count of available $m$ and $s$ pairs.

The atom is considered stable when all orbitals prior $n\mathrm s$ are fully occupied (noble gasses) and those electrons do not interact in chemical bonds. The electrons with highest $n$ beyond this limit are valence electrons and they can form chemical bonds. Their energies in the atom are highest. For example Lithium electronic configuration can be written as: \begin{align}_3\ce{Li}&:\mathrm{1s^2\ 2s^1}\\ _3\ce{Li}&: [_2\ce{He}]\ \mathrm{ 2s^1}\end{align}

Now for the question, the most common atoms have valence electrons in $\rm s$ and $\rm p$ orbitals and 8 electrons can fill them up, That's why there is the "magic 8 rule" because only 4 atoms do not stabilise in 8-valence configuration:

• Hydrogen stabilises in $_0\ce{H^+}$ or $\ce{_2H^{-}[_2He]}$ configuration;
• Helium is stable as $\ce{_2He}$;
• Lithium stabilises in $\ce{_3Li^{+}[_2He]}$;
• Beryllium stabilises in $\ce{_4Be^{2+}[_2He]}$;
• Boron stabilises in $\ce{_5B^{3+}[_2He]}$;
• Carbon stabilises in $\ce{_6C^{4+}[_2He]}$ or $\ce{_6C^{4-}[_{10}Ar]}$

According the question in DraggyWolf's comment, why is not 1 electron per atom considered as stable?

Let's consider that energy electron relative to the core is zero when it is not bound. If proton ($\ce{H^+}$) accepts electron they form hydrogen and release $13.6~\rm eV$ of energy (photon). Energy of electron in hydrogen atom is $-13.6~\rm eV.$ In case of Helium the energies are $-54.4$ and $24.6~\rm eV.$ For Lithium we obtain energy $-30~\rm eV$ for $\rm 1s^2$ state and $\rm -5 eV$ for $\rm 2s^1$.

It is clearly seen how strong the core-electron bond near the core is and that it neglect the electron-electron repulsion - The core with all positive charge is "located" in space 23,000 - 145,000 times smaller that whole electron cloud.

• Dude, use mchem. – user5764 Jul 5 '16 at 6:35
• @mafia A little more friendly would be appreciated. – Martin - マーチン Jul 5 '16 at 8:05
• Hmm @Mart, I never thought it would sound mean and that's why I suggested an edit so that OP might learn checking the edits. Nevertheless, noted for the future. – user5764 Jul 5 '16 at 9:44
• Dude, it doesn't soud mean to me. I haven't heard of mchem yet, so if you think it is worth editing, edit :-) – Crowley Jul 5 '16 at 16:39
• @Crowley: check How can I format math/chemistry expressions here? – user5764 Jul 6 '16 at 3:31

My 2 cents: as noted by others, the stability is really only there for n = 1, 2 and 3, beyond that the situation becomes complicated (and He has only 2 electrons, so really we are only talking about neon and argon).

That means that the orbitals in question are: 1s, 2s, 2p, 3s and 3p. For each possible occupation of these orbitals by up to 18 electrons, energy levels can be obtained for the system as a whole. It's important to remember that only discrete energy levels exist.

Turns out that energy levels within the two periods are minimal, respectively, for electron configurations $1s^2 2s^2 2p^6$ and $1s^2 2s^2 2p^6 3s^2 3p^6$.

Incidently, that's 8 electrons in the outer shell.

As for the other questions:

And what does stabilization of atoms even mean? - In this context, it means that certain atoms are more likely to give up or take additional electrons, nothing else. It does not describe a switchlike situation - "is it stabilized or not", it only says that some are more stable that others.

Why do atoms need 8 electrons to stabilize? I mean why not 7 or 5 or 10 electrons? Why specifically 8? - This is already answered - different electron configurations have different stability.

Are the atoms going to burst without 8? - No.

Why do they need 8 electrons? - They try to obtain it because in that configuration, they have the maximum possible stability (in other words, minimum energy).

The octet rule is overcomplicated.

Real answer is - for many organic chemistry-relevant atoms, a shell with 8 electrons is by far the one with low energy state if one performs the quantum mechanical calculations.

However, the octet rule doesn't apply for other types of elements.

• Why aren't atoms on the lowest energy with only 1 electron?And what does being on lowest energy have to do with the stabilization of atoms? – MartianCactus Jul 4 '16 at 15:18
• @Adi Sorry, I cant explain this better. – DraggyWolf Jul 4 '16 at 16:32
• this does not answer the question in any meaningful way. You effectively say , its 8 because its 8. – porphyrin Jul 5 '16 at 5:57
• and so down vote. What are ' organic-chemistry relevant atoms'. Not just C, what about H (1 electron) , S, P and many metals. Are not chlorophyll (with a Mg atom) and vitamin B12 (with Cobalt) not organic molecules? – porphyrin Jul 5 '16 at 6:06

Please excuse my rather layman reply, my specialism is not in chemistry or physics, but I felt this question is asking more why than how.

Why do atoms need 8 electrons to stabilize? I mean why not 7 or 5 or 10 electrons? Why specifically 8?

I suspect this more has to do with the fact that force is equalised. Even numbers. An odd number would suggest an unpaired electron which would suggest an unstable state. Think of it like having two cars pushing against each other evenly, or two equally attracted magnets. I don't think anyone knows why it's specifically 8, as opposed to 6 or 4 or 2.

And what does stabilization of atoms even mean?

It means they are in a low energy state. A high energy state is where an atom holds a potential to react. Take for example sodium - when introduced to water it reacts violently and explodes. That is because, in simplistic terms, it transfers electrons between sodium and water. When an atom achieves 8 electrons, there's no more electrons for it to gain or lose (IE if it was less than 8 it would have to gain additional electrons from some other material, and more than 8, it could lose some - essentially a reaction would occur).

So when we say stabilisation has occurred, we are essentially saying that conventionally the atom won't react. It has become stable.

Are the atoms going to burst without 8?

The atom itself won't burst, per se, but like in the example, the sodium will.

Edit: Not sure why the downvotes. No feedback, no correction. Electron transfer produces reactions: https://en.wikipedia.org/wiki/Electron_transfer https://en.wikipedia.org/wiki/Redox

• The downvotes (at least mine) is because of the very unscientific answer that simplifies in the wrong direction, states wrong facts etc. – Jan Jul 5 '16 at 1:00
• While I agree that this is not helpful, I do not agree, that it formally does not qualify as an answer. Such answers should be down votes, but not deleted. – Martin - マーチン Jul 5 '16 at 8:04
• @Jan I upvoted your honesty, but what facts are wrong or unscientific? Do not chemical reactions result due to electron transfer, which is often due to a difference in the number of electrons in given shells, and thus the energy differential transfers, that is, the reaction occurs as a result of the energy attempting to equalise? And if so, would not a stable set of 8 electrons be a result of an atom reaching it's energy resting point? – c1646091 Jul 7 '16 at 5:51
• Your centering around eight electrons being stable. It is not as simple as that. In some cases, eight electrons are stable, in others they simply are not. Stable in itself is also a highly problematic term implying thermodynamic stability, which is only really true for the most stable state of a given element. On the other hand there are metastable states, which don’t react for quite some time, no matter whether they have eight, seven or a different number of electrons per atom. Also, quantum mechanics tell us very clearly why eight is stable and not six, four or ten. – Jan Jul 27 '16 at 19:44