So, deserve to an atom have an ext than 8 valence electrons?
If this is not possible, why can"t an atom have an ext than 8 valence electrons?
Yes. While having actually an octet the valence electrons creates an especially deep energy minimum for most atoms, the is just a minimum, no a an essential requirement. If there room sufficiently solid compensating energy factors, also atoms the strongly like octets can form stable link with an ext (or less) than the 8 valence shell electrons.
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However, the same bonding instrument that allow formation the greater-than-8 valence shells also enable alternative structural interpretions of together shells, depending mostly on whether together bonds are construed as ionic or covalent. Manishearth"s excellent answer explores this issue in much greater detail than I perform here.
Sulfur hexafluoride, $ceSF6$, provides a delightful instance of this ambiguity. As I described diagrammatically in my original answer, the main sulfur atom in $ceSF6$ deserve to be interpreted as either:
(a) A sulfur atom in which all 6 that its valence electrons have actually been completely ionized far by six fluorine atoms, or
(b) A sulfur atom with a stable, very symmetric 12-electron valence shell that is both created and also stabilized by six octahedrally situated fluorine atoms, every of i m sorry covalently shares an electron pair v the main sulfur atom.
While both of these interpretations are plausible indigenous a purely structural perspective, the ionization interpretation has serious problems.
The very first and greatest difficulty is that completely ionizing all 6 that sulfur"s valence electrons would certainly require power levels that space unrealistic ("astronomical” can be a much more apt word).
A 2nd issue is the the stability and also clean octahedral the opposite of $ceSF6$ strongly says that the 12 electrons around the sulfur atom have reached a stable, well-defined power minimum the is various from its usual octet structure.
Both points indicate that the less complicated and much more energetically precise interpretation that the sulfur valence covering in $ceSF6$ is the it has 12 electrons in a stable, non-octet configuration.
Notice additionally that for sulfur this 12- electron stable power minimum is unrelated come the bigger numbers that valence-related electrons viewed in shift element shells, because sulfur merely does no have enough electrons to accessibility those more complex orbitals. The 12 electron valence covering of $ceSF6$ is rather a true bending of the rules because that an atom that in virtually all various other circumstances prefers to have actually an octet of valence electrons.
That is why my all at once answer to this concern is merely "yes".
Question: Why room octets special?
The flip side of whether secure non-octet valence shells exist is this: Why perform octet shells carry out an energy minimum that is for this reason deep and also universal that the whole periodic table is structured into rows that finish (except because that helium) with noble gases through octet valence shells?
In a nutshell, the reason is that for any type of energy level over the special instance of the $n=1$ covering (helium), the "closed shell" orbital collection $s, p_x, p_y, p_z$ is the only combination of orbitals who angular momenta space (a) every mutually orthogonal, and also (b) cover all such orthogonal possibilities for three-dimensional space.
It is this unique orthogonal partitioning the angular momentum alternatives in 3D space that provides the $s, p_x, p_y, p_z$ orbit octet both specifically deep and relevant even in the highest power shells. We see the physical proof of this in the striking security of the noble gases.
The reason orthogonality of angular momentum states is so essential at atomic scale is the Pauli exemption principle, which requires that every electron have actually its own distinct state. Having actually orthogonal angular inert states offers a particularly clean and easy means to provide solid state separation between electron orbitals, and also thus to stop the larger energy penalties applied by Pauli exclusion.
Pauli exemption conversely provides incompletely orthogonal set of orbitals substantially much less attractive energetically. Due to the fact that they force more orbitals come share the exact same spherical room as the fully orthogonal $p_x$, $p_y$, and $p_d$ orbitals of the octet, the $d$, $f$, and greater orbitals are significantly less orthogonal, and thus topic to enhancing Pauli exclusion energy penalties.
A last note
I might later include another addendum to define angular momentum orthogonality in terms of classical, satellite-type circular orbits. If ns do, I"ll also include a bit of explanation regarding why the $p$ orbitals have actually such bizarrely various dumbell shapes.
(A hint: If girlfriend have ever before watched world create a two loops in a single skip rope, the equations behind such dual loops have actually unexpected similarities to the equations behind $p$ orbitals.)
Original 2014-ish answer (Unchanged)
This prize is plan to supplement Manishearth"s earlier answer, fairly than complete with it. My target is to display how octet rules have the right to be valuable even because that molecules that contain more than the usual complement of eight electron in your valence shell.
I contact it donation notation, and it dates back to my high school days as soon as none of the historicsweetsballroom.com of the messages in my small-town library bothered to describe how those oxygen bonds operated in anions such as carbonate, chlorate, sulfate, nitrate, and phosphate.
The idea behind this notation is simple. You begin with the electron dot notation, then include arrows that display whether and also how other atoms space "borrowing" every electron. A dot v an arrow means that the the electron "belongs" greatly to the atom in ~ the base of the arrow, but is being offered by another atom to help complete that atom"s octet. A an easy arrow without any type of dot suggests that the electron has efficiently left the original atom. In that case, the electron is no longer attached come the arrow at all but is instead presented as rise in the number of valence electron in the atoms at the end of the arrow.
Here are instances using table salt (ionic) and oxygen (covalent):
Notice that the ionic link of $ceNaCl$ reflects up merely as an arrow, indication that it has actually "donated" the outermost electron and fallen earlier to its inside octet of electrons to accomplish its own completion priorities. (Such inner octets are never shown.)
Covalent bonds happen when every atom contributes one electron come a bond. Donation notation mirrors both electrons, therefore doubly external inspection oxygen winds up with 4 arrows in between the atoms.
Donation notation is no really essential for straightforward covalent bonds, however. It"s intended much more for showing exactly how bonding functions in anions. Two carefully related examples are calcium sulfate ($ceCaSO4$, far better known as gypsum) and also calcium sulfite ($ceCaSO3$, a typical food preservative):
In these instances the calcium donates via a greatly ionic bond, for this reason its contribution becomes a pair of arrows the donate 2 electrons to the main point of the anion, perfect the octet the the sulfur atom. The oxygen atom then attach to the sulfur and "borrow" entire electrons pairs, without really contributing noþeles in return. This borrowing version is a major factor in why there have the right to be much more than one anion for elements such as sulfur (sulfates and sulfites) and nitrogen (nitrates and also nitrites). Because the oxygen atoms room not needed for the central atom to establish a complete octet, that is feasible for few of the bag in the main octet to stay unattached. This results in less oxidized anions such as sulfites and nitrites.
Finally, a an ext ambiguous instance is sulfur hexafluoride:
The figure shows 2 options. Have to $ceSF6$ it is in modeled together if the sulfur is a steel that gives up every one of its electrons to the hyper-aggressive fluorine atom (option a), or as a case where the octet rule gives method to a weaker yet still workable 12-electron dominion (option b)? there is some conflict even today around how such cases should be handled. The donation notation shows exactly how an octet perspective deserve to still be applied to together cases, though it is never a good idea to count on first-order approximation models for such too much cases.
Finally, if girlfriend are exhausted of dots and arrows and also yearn for something closer to conventional valence link notation, these 2 equivalences come in handy:
The upper straight-line equivalence is trivial due to the fact that the resulting heat is identical in illustration and meaning to the typical covalent link of necessary historicsweetsballroom.com.
The 2nd u-bond notation is the novel one. I developed it out of disappointed in high school ago in the 1970s (yes I"m the old), however never walk anything v it in ~ the time.
The main benefit of u-bond notation is that it allows you prototype and also assess non-standard bonding relationships while using just standard atom valences. As with the straight-line covalent bond, the heat that forms the u-bond to represent a single pair the electrons. However, in a u-bond, that is the atom at the bottom of the U the donates both electrons in the pair. The atom it s okay nothing the end of the deal, for this reason none that its bonding needs are readjusted or satisfied. This absence of bond perfect is represented by the lack of any line ends on the side that the u-bond.
The beggar atom in ~ the top of the U it s okay to use both that the electrons for free, i m sorry in turn method that two the its valence-bond demands are met. Notationally, this is reflected by the truth that both the the line ends of the U are next to that atom.
Taken together a whole, the atom in ~ the bottom that a u-bond is speak "I don"t prefer it, however if you are that desperate for a pair the electrons, and also if you promise to stay really close by, I"ll let you latch ~ above a pair of electron from my already-completed octet."
Carbon monoxide with its baffling "why does carbon suddenly have a valence that two?" structure nicely demonstrates exactly how u-bonds interpret such compounds in terms of more traditional bonding numbers:
Notice that two of carbon"s 4 bonds are resolved by conventional covalent bonds v oxygen, if the staying two carbon binding are addressed by the development of a u-bond that lets the beggar carbon "share" one of the electron bag from oxygen"s already-full octet. Carbon ends up with 4 line ends, representing its 4 bonds, and oxygen ends up with two. Both atom thus have actually their typical bonding numbers satisfied.
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Another an ext subtle insight from this figure is that because a u-bond represents a solitary pair of electrons, the mix of one u-bond and two timeless covalent bonds between the carbon and oxygen atoms requires a complete of 6 electrons, and also so should have actually similarities come the six-electron triple bond between two nitrogen atoms. This small prediction turns out to it is in correct: nitrogen and also carbon monoxide molecules room in truth electron construction homologues, one of the results of which is the they have practically identical physics historicsweetsballroom.com properties.
Below space a few more examples of just how u-bond notation have the right to make anions, noble gas compounds, and odd organic compounds it seems to be ~ a bit much less mysterious: