Kossel Lewis Approach To Chemical Bonding

[andyrk]

We know that a shell can accommodate a maximum of 2n² electrons because a shell can have a maximum of n² orbitals depending upon the value of n, i.e n=1 for K shell, n=2 for L shell, n=3 for M shell, n=4 for N shell and so on. According to Lewis, he pictured the atom in terms of a positively charged Kernel (the nucleus plus the inner electrons) and the outer shell that could accommodate a maximum of eight electrons. This means that 2n²=8 => n=2? Means an outer shell can be nothing more than an L shell? I got stuck up here and I am still confused..

 

[andyrk]

Anybody there!?

 

[DrDu]

This electron counting rule is exact only for hydrogen. In all other atoms, sub-shells are formed, namely s, p, d, f subshells according to the angular momentum of the electrons. The d and f subshells are energetically higher and more diffuse. So, at least in the main group elements, the valence shell is formed by the s and p orbitals which can take 2 and 6 electrons, respectively.
However, you have to take in mind that Lewis theory is more than 100 years old and has been superseeded in many respects by more modern descriptions.

 

[andyrk]

This electron counting rule is exact only for hydrogen. In all other atoms, sub-shells are formed, namely s, p, d, f subshells according to the angular momentum of the electrons. The d and f subshells are energetically higher and more diffuse. So, at least in the main group elements, the valence shell is formed by the s and p orbitals which can take 2 and 6 electrons, respectively.
However, you have to take in mind that Lewis theory is more than 100 years old and has been superseeded in many respects by more modern descriptions.

Hydrogen? But hydrogen can't have 8 electrons! And yes, firstly the subshells are formed namely 1s,2s, 3p, 3d and so on.. these get get combined under the name of s,p,d and f and then these s,p,d,f are under the K,L,M,N shells. Can you explain it once again? Basically I didn't understand the 8 electron thing that the theory says. I get confused up there and end up getting 2 as an answer.

 

[Borek]

For iodine the electron configuration is:

s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p⁶4d¹⁰5s²5p⁵

of those only last 7 electrons (5s²5p⁵) are considered the valence electrons (or outer shell, to use your nomenclature). The kernel is s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p⁶4d¹⁰. Valence electrons are not from the L shell, L shell is buried deep below other electrons. Valence electrons have n=5, so they are from the O shell (but no, it is not a complete O shell, just a part of it).

(Note: there are some shortcuts in what I wrote, I am fully aware of them, but nitpicking at this stage will only confuse the OP even more.)

 

[andyrk]

For iodine the electron configuration is:

s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p⁶4d¹⁰5s²5p⁵

of those only last 7 electrons (5s²5p⁵) are considered the valence electrons (or outer shell, to use your nomenclature). The kernel is s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p⁶4d¹⁰. Valence electrons are not from the L shell, L shell is buried deep below other electrons. Valence electrons have n=5, so they are from the O shell (but no, it is not a complete O shell, just a part of it).

(Note: there are some shortcuts in what I wrote, I am fully aware of them, but nitpicking at this stage will only confuse the OP even more.)

My question is: How can the outer shell accommodate a maximum of ONLY 8 electrons?

 

[Borek]

It doesn't. It can accommodate much more. However, in iodine atom O shell is filled only partially. This is not its maximum yet.

 

[andyrk]

So why does the Lewis Theory say that the outermost shell can accommodate a maximum of only 8 electrons?

 

[Borek]

Because - as DrDu already said - it is oversimplified and in general it works reasonably well only for the second period elements.

 

[andyrk]

Because - as DrDu already said - it is oversimplified and in general it works reasonably well only for the second period elements.

Can you explain what you said above a bit more? Sorry but didn't get the hang of what you said...

 

[Borek]

What I am saying is that Lewis theory is just a rule of thumb, which works for a limited number of cases.

 

[DrDu]

What I wanted to say is that the ns, np, nd ... orbitals are only degenerate for the same n in hydrogen like atoms.
In higher main group elements, like in the example of iodine of Borek, the 5d electrons are energetically much higher than the 5s and 5p electrons, so effectively the valence shell of iodine can carry at most 8 electrons (2 in 5s, 6 in 5p).