Central atoms in Lewis structures: basic question

[nomadreid]

TL;DR Summary

Two rules that seem to contradict each other: given the same number of subscripts, (a) the central one will be the one with the lowest electronegativity, (b) the following elements will be preferred in this order: C, Si, N, P, S and O. Which rule takes precedence?

The two rules are Methods 1 and 3 in https://sciencing.com/how-to-name-covalent-compounds-13712167.html
Given that the electronegativities of C, Si, N, P, S, O are approx. 2.5, 1.8, 3.0, 2.1, 2.5, , 3.5, resp., one of these rules has to give.
Thanks.

 

[docnet]

Whenever you have the elements C, Si, N, P, S and O, follow rule (b). Otherwise, follow rule (a).

 

[nomadreid]

Thanks, docnet. You are right, I am interested in determining the central atom, not the name. I forgot to mention that if you scroll in the link, you come to "How to Determine Which Atom to Use As the Central Atom". That is the section I am asking about.

(The primary rule in that link is to find the atom with the lowest subscript. Is it correct that this rule takes precedence over all others? If so, then my question only concerns atoms with certain pairs in equal numbers in the atom or ion with covalent bonds. )

I tried to find an appropriate example of a molecule or ion with more than two atoms with any pair in equal amounts:
from (b)-----electronegativities
C, Si---------(2.5, 1.8)
C, P---------(2.5, 2.1)
N, P---------(3.0, 2.1)
N, S---------(3.0, 2.1)
in equal amounts, but did not find any. Do they exist? (If the "least subscript" rule did not take precedence, it would be easier to find examples.)

If such a molecule or ion does exist, then you would have a conflict among the two rules mentioned. Are you saying that, in such a case, I would follow the order of the list (b) and ignore the electronegativity order (a)?

Thanks for your patience.

 

[docnet]

The 'least subscript rule' isn't so much a rule as a trend that appears in formulas of simple molecules with a low number of atoms. What ultimately determines the central atom is physics, which you can do by drawing lewis structures and comparing electronegativities, and following rule (b).

Do they exist? (If the "least subscript" rule did not take precedence, it would be easier to find examples.)

The rule doesn't work when you have an even number of candidates, like HCN. Hence it shouldn't be used as a primary method of determining the lewis structure.

Are you saying that, in such a case, I would follow the order of the list (b) and ignore the electronegativity order (a)?

Yes, it seems that (b) always overrules (a).

I have a question though.. how do you define the central atom of a cyclic compound like Benzene? or a molecule with two cyclic parts? or a huge 40 kDa protein? or even a simple molecule like $H-C\equiv C-H$?

This leads me to have a criticism of the website and of undergraduate organic chemistry courses in general, putting importance on identifying the 'central' atom. 'central' is a superficial label, a mere convention, that people use to describe small molecules in a general way. It does not in any way rigorously describe the high-level physics of molecules, and it only encourages thinking of molecules as 'sticks and spheres', and certain properties that you must memorize about specific elements instead of understanding the physics that allows the elements to exist and to be different from one another. For example, it is important to understand there are subatomic particles that interact via strong nuclear forces, weak nuclear forces, and electromagnetism, and that those forces are described in terms of the high level mathematics of quantum field theory. I remember learning the 'sticks and spheres' level of science in any organic chemistry class and memorizing reactions, which was tedious hell.

 

[nomadreid]

Thanks, docnet. A very good critique. I myself have very little chemistry background (more mathematics and physics) and tend to think more in terms of fields than particles, but was asked to help a student in her beginning chemistry class in secondary school. At this level the subject appears to be a collection of rules of thumb with lots of exceptions. Somewhat messy, but my student needs to start with the simple molecules, mostly inorganic, and I am trying to walk a tightrope between what will make sense to a young teenager and a representation closer to what actually happens. Thus my questions. Thanks very much for the clarification.