CHEMISTRY COMMUNITY

When setting up Lewis structures, should we always ensure that the formal charge of our central atom is 0? Are there any exceptions?

You have to take into account all of the atoms in the Lewis structure and the overall charge of the molecule because it is more stable when the negative formal charge is on the more electronegative atom and the positive formal charge is on the less electronegative atom.

The formal charge of the central atom might not always be 0, but as long as the the sum of all the formal charges add up to the charge of the overall molecule, it's fine.

What the people of above me said :). The center atom's formal charge may not always be zero, if there is a choice between an outer atom with a formal charge of zero and the center atom with zero than it is best to have the center atom with zero.

Ideally, you'd want all the elements within the atom to have a formal charge of zero because it would make it extremely stable. However, this is not always the case, and it is best to make the overall charge neutral.

do resonance structures depend only on the FC of central atom or on all of the atoms

Formal charge of the central atom is not always zero. It is better to look at formal charges of all atoms inside a structure. With more atoms having a formal charge of zero the structure is the most stable. And in the case when the formal charges are not zero, the more electronegative atom should have a more negative formal charge.

When drawing lewis structures your goal should not be to have the central element have a formal charge of zero but rather have the most elements have a formal charge of zero and the most electronegative elements carry the negative formal charges.

When drawing a lewis structure I think the goal is to have the most elements with a formal charge of zero rather than trying to make the formal charge of the central element zero.