For the test vsepr and lewis

[Adam Enomoto 1L]

For the Chapter 4 test should we be comfortable with naming the lewis structure as well as the VSEPR name. For example, for CH4 do we need to know that it's tetrahedral and trigonal pyramidal?

[Amanda Mac 1C]

The VSEPR name of CH4 is tetrahedral where there are all bonded pairs. Trigonal pyramidal would be three bonded pairs and one lone pair of electrons such as NH3. A lewis structure is a representation of the structure but I do not think they have names if that's what you're asking. Hope this helped!

[804899546]

How would we know when to call it trigonal pyramidal and when to call it tetrahedral?

[Amanda Mac 1C]

Tetrahedral named molecules do not have lone pairs on the central atom which means there are four electron density regions and all four of those regions are occupied by atoms. In trigonal pyramidal molecules, there is one lone pair on the central atom which means only 3 out of the 4 regions are occupied by atoms. This changes the tetrahedral shape and kind of pulls the central atom upwards so the shape “scrunches” together a little to form a pyramid with three corners kind of shape.

The naming of the shape really depends on the electron density regions and the lone pairs, if any.