Removing Non-Axis Atoms First

[erica thompson 4I]

Can someone remind me why we remove the atoms that are NOT on the axis first in order to determine molecular shape?

[Victoria Li 1L]

Axial atoms are removed first only for molecules with octahedral electron geometry i.e. 8 regions of electron density. This is because this particular arrangement minimizes electron-electron repulsion, especially if there are two lone pairs (they'd be on opposite sides of the molecule).

However, equatorial atoms are removed first for molecules with trigonal bipyramidal electron geometry for the same reason: optimizing the arrangement of atoms and lone pairs while minimizing electron-electron repulsion. It just happens that removing equatorial atoms is more stable that axial for this type of molecule.

[Eva Zhao 4I]

Referencing back to the seesaw shape, the lone pair in the equatorial plane is removed first to minimize repulsion. By doing so, the more repulsive lone pair interacts with only 2 bonds at 90° instead of three bonds if the axial lone pair was removed. Hence, removing the lone pair in the equatorial plane results in a more stable structure.

[AArmellini_1I]

It helps to picture the molecule and look at how many other angles a lone pair will affect in each region

[BryantChung_4B]

The goal of the VSEPR model is showing how each region of electron density repels others. You do not always remove non-axis electrons first, it depends on whatever creates the farthest distance between each electron density region.