Naked Science Forum

Non Life Sciences => Chemistry => Topic started by: HealerMelissa on 29/01/2020 07:24:42

Title: How to know if an element can form more than 8 bonds
Post by: HealerMelissa on 29/01/2020 07:24:42

Hi all,
Okay, there is this thing called H₂SO₄, and the lewis structure has me baffled.
In this over on youtube (search for H2SO4 Lewis Structure: How to Draw the Lewis Structure for H2SO4 ), it says S can take more than eight electrons in the valence shell.
Any way of telling that from the Table?
Thanks
Melissa

Title: Re: How to know if an element can form more than 8 bonds
Post by: chiralSPO on 29/01/2020 12:21:06

In general, any elements in the p block below the first row of the p block: Al, Si, P, S, Cl, (kinda Ar?) and the lower rows of the p block). Depending how you draw it, even N can go "hypervalent" in N oxides. And actually, even hydrogen can form multiple bonds (not talking about hydrogen bonds, I'm talking about bridging hydrides, and H₃⁺. And the carbon (carbide?) in the active site of nitrogenase has six bonds (https://en.wikipedia.org/wiki/FeMoco)

https://en.wikipedia.org/wiki/Hypervalent_molecule

There are different ways to explain this, depending on which model is used to think about them in the first place.

Some invoke d orbitals for their hybridization and bonding (this is not a realistic explanation, but it is simple and predictively useful). Or you can think about assigning formal charges instead of making bonds, and thinking of the additional attraction as an ionic bond (more realistic, but less elegant).

3-centered two electron bonds are a more complex way to think about the electronic structures of these, that effectively considers delocalization of bonds. https://en.wikipedia.org/wiki/Three-center_two-electron_bond

MO theory gives a more complete picture, but is more abstract, dealing with "bond order" rather than how many bonds are there and where are they? https://en.wikipedia.org/wiki/Molecular_orbital_theory

Title: Re: How to know if an element can form more than 8 bonds
Post by: HealerMelissa on 29/01/2020 19:13:51

Hi,
Thanks for this info. Now I have learned something.
I will dive into the info you have left in your answer.
Basically: orbitals get hybridised and therefore are capable of more electrons, if I understand correctly...

Title: Re: How to know if an element can form more than 8 bonds
Post by: chiralSPO on 30/01/2020 00:19:04

Well... each orbital can only hold 2 electrons. So if we need to fit more than 8 electrons (4 orbitals-worth) around an atom, we will need to invoke more than 4 orbitals. We can choose to invoke only atomic orbitals (hybrid or pure), in which case, the 5th orbital (and beyond) would need to be a d orbital (after the s and 3 p orbitals are the first 4). Or we could invoke molecular orbitals made only of s and p orbitals from all of the constituent atoms.