Drawing Lewis Structures [ENDORSED]

[Marc Farah 3D]

How do we know which element to put as the central atom when drawing lewis structures? Is it the one with the lowest electron affinity or electronegativity?

[Scott_Anderson_3J]

In my discussion section, my TA said that there is no hard and fast rule that will tell you. However, it is usually pretty evident. For example, if there is one of a certain element and three of another, the lone element is usually in the center. For example, in H2O, oxygen is in the center. I read that there is a general trend that less electronegative elements are usually in the center, but that is by no means absolute (as is evident with H2O).

[Da_Rhee_1O]

Just something to remember is that carbon tends to always be the central atom whereas hydrogen is always bonding to another atom. And when I went to a PFL session, the UA said that in molecules with more than one central atom, just looking at the chemical composition (for example: CH3SH) will help to determine which atoms will bond together (such that C and the 3 H are grouped together and S and H will be grouped near each other as well).