Lone Pairs on the Central Atom

[Ryan Danis 1J]

I am a bit confused about how/why the lone pairs on a central atom affect (or don’t affect) the VSEPR model. The book says that if lone pairs are present, only the electron arrangement is affected, but not the shape. Does this mean that a Lewis structure be an example of an electron arrangement of a molecule, since it includes the lone pairs? And VSEPR model is an example of the shape of a molecule, since it does not include the lone pairs?

[taline_n]

Lone pairs do affect the shape of the VSEPR model. Since lone pairs count as a region of electron density, they contribute to the formation of the structure, therefore to the overall molecular shape as well (lone pair electrons repel the atoms further away and change the shape and bond angles).

However, when naming the VSEPR shape, only the atoms are taken into consideration. If the molecule has 4 regions of electron density with 1 lone pair on the central atom (meaning there are 3 other atoms present along with the central atom), it has a tetrahedral arrangement but the VSEPR shape is trigonal pyramidal (since the lone pairs count toward the arrangement of the molecule but not the shape).

I'm not too sure about the Lewis structures being an electron arrangement, but the VSEPR models definitely do include lone pairs (VSEPR AXE formulas), just not in the process of naming the shape based on the atoms present.

[Ryan Danis 1J]

Your explanation makes a lot of sense. So the lone pairs actually do affect the molecular shape by influencing the arrangement of the atoms, while only the atoms are used to then name the shape. The way the book explained it tripped me up. Thankyou!!!

[Tuong Nguyen 2I]

It's also to keep in mind how lone pair repulsions are the strongest of them all, overpowering a lone pair-bonded pair and bond-bond repulsion. So lone pairs have a significant impact on the 3-D shape of the molecule.

[Yixiao Hu 3C]

because the substance wants its structure to be energy minimized; but the lone pairs and the electrons bonds will have repulsive forced which require more energy. in order to reduce energy, the bond electrons will go away from the lone pairs, affecting the shape.

[Katelyn Phan 2A]

Lone pairs do a play a major role in determining the shape. The repulsion of lone pairs is even stronger than those of bonded pairs. It will push the bonded pairs away. However, the actual shape does not take lone pairs in consideration. You name the shape based off the location of bonded pairs that were affected by the lone pairs.