Role of lone pairs of electrons on molecular structure

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

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Kevin Chang 2I
Posts: 11
Joined: Mon Jan 04, 2016 9:16 am

Role of lone pairs of electrons on molecular structure

Postby Kevin Chang 2I » Fri Jul 08, 2016 5:59 pm

Why are shapes for molecules determined only by the location of atoms and not the electron pair? What exactly is the role of lone pairs in determining molecular structure and why do they cause atoms around the central atom to push closer together?

Jonah Lipel 1E
Posts: 13
Joined: Fri Jun 17, 2016 11:28 am

Re: Role of lone pairs of electrons on molecular structure

Postby Jonah Lipel 1E » Fri Jul 08, 2016 11:10 pm

Molecular shapes are only determined by the locations of atoms and not electron lone pairs because the electrons are so minuscule that they don't even contribute to the actually shape of the molecule and chemists disregard them for the shape. How ever as Doctor Lavelle said today they do repel each other. Dr. Lavelle mentioned the strength of repulsion today. He said that lone pair to lone pair is the strongest, followed by lone pair to bond, and finally bond to bond. The reason lone pair to lone pair repulsion is the strongest is because lone pairs are more autonomous then bonding pair electrons. In covalent bonds the electrons are pulled by two positively charged nuclei and which constrain them. However, lone pairs are only pulled by one nuclei which gives their region of election density a bigger volume, thus creating more repulsion. For example in H2O; oxygen has two lone pairs and is connected to two hydrogen atoms. The electron arrangement for this molecule would be tetrahedral, but the molecular shape is bent because there are only three atoms and the two lone pairs repel each other and the bonds pushing them closer together with bond angles less than 109.5. Hope this helps- Jonah Lipel

Chem_Mod
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Re: Role of lone pairs of electrons on molecular structure

Postby Chem_Mod » Mon Jul 11, 2016 9:48 am

Molecular shapes are determined by the atoms only because it describes the arrangement of atoms. However, electron lone pairs do affect its geometry since electrons repel each other and this repulsion must be minimized by arranging them in a particular manner in 3D space.

The strengths of electron repulsion is nicely described above and reiterated here: "lone pair to lone pair is the strongest, followed by lone pair to bond, and finally bond to bond. The reason lone pair to lone pair repulsion is the strongest is because lone pairs are more autonomous then bonding pair electrons."


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