[chem]Lewis Structure-Expanded Valence Shell

[Carolyn]

I am currently learning how to draw lewis structures and find it to be really confusing sometimes.

For example, for the molecule S03, the correct structure on the textbook is three resonance structures, each with one of the O's forming a double bond with the central atom S.

However, why can't we just put double bonds between S and all of the three O's? Then the formal charges for each atom wil be zero. Wouldn't be a more stable structure and therefore a perferable choice? Also since S is in the third period of the periodic table, so it can have expanded valence shells.

If anybody can provide some trick or method of knowing when to have expanded valence shells and when to have resonance structures, as well as how to identify other exceptions to the octet rule, I will be very grateful.

 

[Ester]

it's better that it fills its octet before making the charges be zero on a central atom. that's why there is resinance structures.

 

[Blueshift5]

I understand your confusion about drawing Lewis structures. It can be a challenging concept to grasp, especially when dealing with molecules that have expanded valence shells. In the case of SO3, it is true that we can draw a structure with double bonds between S and all three O atoms, and this would result in a formal charge of zero for each atom. However, this structure is not the most stable option for SO3.

The concept of resonance structures is used to represent the delocalization of electrons within a molecule. In the case of SO3, the electrons are delocalized over all three O atoms, resulting in a more stable overall structure. This is why the textbook shows three resonance structures for SO3, each with a double bond between S and one of the O atoms. The actual structure of SO3 is a combination of these resonance structures.

In terms of when to use expanded valence shells, this is typically seen in elements in the third row or beyond of the periodic table. These elements have d-orbitals available for bonding, which allows them to exceed the octet rule. However, not all molecules with elements in the third row or beyond will have expanded valence shells. It is important to follow the octet rule as a general guideline, but also be aware of exceptions and use resonance structures when necessary.

As for other exceptions to the octet rule, these can occur when dealing with odd-electron molecules (such as NO) or when molecules have an odd number of valence electrons (such as ClO2). In these cases, the central atom may not have a complete octet, but the overall structure is still stable due to the delocalization of electrons.

In summary, drawing Lewis structures can be complex and it is important to understand the concept of resonance structures and expanded valence shells. As you continue to learn and practice, you will become more comfortable with identifying when these concepts apply and when they do not. Keep in mind that there may be multiple valid Lewis structures for a molecule, and the actual structure is a combination of these resonance structures.