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Lewis Structure of BF3 (and hybridization)

BF3, boron trifluoride, is a tricky molecule to draw because Boron is an exception to the octet rule. It does not need eight electrons in its outer shell, although it can hold eight just like most other non-metals.

The Lewis Structure of BF3, boron trifluoride, has one boron atom in the centre, and three fluorine atoms surrounding it. Each of the fluorine atoms shares ONE electron with the boron, and the boron in turn shares ONE electron with each of the fluorines.

This gives it a trigonal planar shape, and its hybridization looks like it’s sp2:

This is the simplest Lewis Structure for Boron Trifluoride and it’s probably the one your teacher is looking for you to draw.

You can watch this structure get drawn here:

HOWEVER, in the real world things are not so simple.

At low temperatures, boron trifluoride forms a solid, and the arrangement of the atoms change. One fluorine from a BF3 molecule will donate one of its lone pairs to give a different Boron atom a full octet .. and then a fluorine connect to that Boron will complete the octet on another… etcetera.

In the end, each boron has fluorine atoms tetrahedrally arranged around it. This would imply its hybridization has changed to sp3, but also makes the substance seem more like a covalent network (also known as giant covalent) … or even like an ionic compound, if you wanted to bold.

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Lewis Structure of Al2O3, Aluminum Oxide

Aluminium oxide is a solid ionic compound, made from atoms of one metal (Aluminum) that have lost three electrons each to become +3 cations, and atoms of a non-metal (oxygen) which have gained two electrons each to become -2 anions.

The numbers 3 and 2 have a lowest common multiple of 6. This means you need TWO aluminum atoms (giving away 2×3=6 electrons total) and THREE oxygen atoms (accepting 3×2=6 electrons total).

Because the electrons are LOST by the metal, and GAINED by the non-metal, there has been a transfer of electrons and this makes it an ionic compound by definition.

Here, two Aluminum atoms are giving away 3 electrons each, and three oxygen atoms are accepting two electrons each.

You can watch this Lewis Structure get drawn below.

I created a video for this post! You’re welcome !