Thread: Exceptions to the octect rule?

What causes the exceptions to the octet rule? for instance Boron only requiring 6 valence electrons and it's 'happy'?
thanks!

Well, Boron just doesn't have enough e- to achieve an octet. That having been said, it will be stable as something like BF₃, but it will form BF₄ if it can. Then, you have elements which overviolate because they have d and f orbitals involved in bonding.

Almost all rules have their exceptions.

Also, if you ever see the Bohr model of the atom, throw something heavy at your teacher.

unfortunately it was on my course which book should I throw

thanks ill have a read atkins is 1000+ pages light reading I guess ps the bohr model of the atom appears in the atkins book though it does specify that features of it are untenable according to quantum,mechanics but then so did my old physics and chem books it was a history of atomic theory question

Originally Posted by cycloneash

What causes the exceptions to the octet rule? for instance Boron only requiring 6 valence electrons and it's 'happy'?
thanks!

Just to build on the replies already given, the octet rule really only works for elements for which the outermost s and p orbitals do all the bonding. This more or less applies to the whole of the S block of the Periodic Table and quite a bit of the upper part of the P block. But once you get to the lower part of the P block, or the D block, the energy of d, or even f, orbitals can become low enough (i.e. once they have been pulled in enough by the higher nuclear charge) for them to take part in bonding - and then the octet rule ceases to work. At the other end of the scale Boron is another exception, being too electron deficient to easily complete its octet, but at the same time too non-metallic to release easily the 3 valence shell electrons, as Aluminium does.

In my view the octet "rule" should be treated as no more than an empirical guide to the preferred bonding of most light elements, notably in organic chemistry.

Originally Posted by PhDemon

Just to clarify something you said above, the dependence of the energy of atomic orbitals on nuclear charge is a lot more complicated than being "pulled in by higher nuclear charge", for hydrogen-like systems (one electron) the s, p, d and f orbitals for a given prinipal quantum number are the same. The splitting of these orbitals to different energies only occurs when the lower energy orbitals are filled due to the difference in "effective" nuclear charge experienced by the electrons (look up penetration and shielding in atomic orbitals).

Indeed, just as you say. I was merely looking for a simple phrase to give an - admittedly hand-waving - idea of why d and f orbitals tend to take part in bonding on the right hand side of the Periodic Table.

By the way I see you were once a pupil of Peter Atkins. I had the enormous pleasure of attending his lectures on quantum chemistry as an undergraduate in the 1970s. He was an inspirational lecturer, even though the maths was hairy. I'm terribly rusty on all that these days, after a career in industry, but the subject still fascinates me.

Originally Posted by PhDemon

I was a liitle later - I was one of his students 1994-1997. The lectures were hard work, the one-on-one tutorials were even harder, nowhere to hide when you made the inevitable cock up in you maths or showed your ignorance

Ha! Yes, I recall he had the reputation of not suffering fools gladly, as it were - not that you would be by any means a fool for getting occasionally tangled in QM maths. My tutor was Richard Wayne, who later was prof at the PCL, now retired. I remember him saying to me once, "It's very interesting you say that, because it's ballocks." He was always good-humoured about it though. The main challenge with his tutorials was to get out of the doorway at the end, without bouncing off the sides, after consuming whatever random form of alcohol he had found in his cupboard.

Originally Posted by PhDemon

Originally Posted by exchemist

Ha! Yes, I recall he had the reputation of not suffering fools gladly...

And people wonder why I'm sometime harsh with the fools around here - I had good training

I did my Part II project (and got my first publication) with Richard - it's what got me started as an atmospheric chemist and remember well (although hazily) his fondness of a tipple...

Respect!

yeah....made you the rounded individual [twitch, shuffle] that we see before us today, eh?