Removing Non-Axis Atoms First

(Polar molecules, Non-polar molecules, etc.)

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erica thompson 4I
Posts: 55
Joined: Wed Sep 18, 2019 12:21 am

Removing Non-Axis Atoms First

Postby erica thompson 4I » Fri Nov 22, 2019 10:50 am

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
Posts: 51
Joined: Sat Jul 20, 2019 12:15 am

Re: Removing Non-Axis Atoms First

Postby Victoria Li 1L » Fri Nov 22, 2019 12:19 pm

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
Posts: 101
Joined: Sun Sep 29, 2019 12:16 am

Re: Removing Non-Axis Atoms First

Postby Eva Zhao 4I » Fri Nov 22, 2019 12:22 pm

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
Posts: 107
Joined: Fri Aug 09, 2019 12:15 am

Re: Removing Non-Axis Atoms First

Postby AArmellini_1I » Fri Nov 22, 2019 12:25 pm

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

BryantChung_4B
Posts: 45
Joined: Sat Aug 24, 2019 12:17 am

Re: Removing Non-Axis Atoms First

Postby BryantChung_4B » Fri Nov 22, 2019 12:36 pm

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.


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