Figuring molecule shapes and polarity

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Discussion Overview

The discussion revolves around understanding molecular shapes, specifically trigonal planar and tetrahedral geometries, as well as the factors influencing molecular polarity. Participants explore the criteria for determining molecular shapes and the role of electronegativity and geometry in assessing polarity.

Discussion Character

  • Homework-related
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant questions how to determine if a shape is trigonal planar, suggesting it may relate to the number of bonds compared to tetrahedral structures.
  • Another participant explains that trigonal planar structures follow the AX3 form, characterized by three bonding pairs and no non-bonding pairs, using BF3 as an example.
  • There is a discussion on molecular polarity, with one participant asserting that polarity is influenced by both electronegativity and molecular geometry.
  • Examples are provided, such as CO being polar due to electronegativity differences, while CO2 is nonpolar because the polarities cancel out due to its geometry.
  • A later reply adds that non-bonding or lone pairs can also affect molecular geometry, using XeF4 as an example to illustrate how lone pairs influence the overall shape and bond angles.
  • This participant also notes that cis- molecules tend to be polar compared to trans- molecules, highlighting the impact of molecular arrangement on polarity.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and provide different insights into the concepts of molecular shape and polarity. No consensus is reached on the best approach to determine these properties, and multiple perspectives remain present.

Contextual Notes

Some assumptions regarding the definitions of molecular shapes and the influence of lone pairs on geometry are not fully explored. The discussion includes specific examples that may depend on additional context not provided.

touma
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I am having a lot of trouble understanding the following:

1) How do I determine if the shape is a trogonal planar? Is it because it would have more bonds than the tetrahedral?

2)Is figuring the polarity of a molecule only based on the electronegativity?

Please help! I have a big test tomarrow and I need ot understand this material!
 
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Seriously, it would be amazing if someone could help!
 
1) How do I determine if the shape is a trogonal planar? Is it because it would have more bonds than the tetrahedral?
Trigonal planar structures follow the form of AX3, they have 3 bonding pairs of electrons and no non-bondin pairs. An example would be BF3.

2)Is figuring the polarity of a molecule only based on the electronegativity?
No, it is also based on geometry. There, are instances when the atoms in molecules have large enough electronegativity differences to make them polar, but due to their geometry, it all cancels out and it is non polar. A good example of this would be Carbon Monoxide and Carbon Dioxide. CO is polar due to the electronegativity differece between C and O, but in CO2, since the Oxygen atoms are located opposite each other, the polarities cancel out and it is nonpolar.
 
Great! That was all I needed! Thanks!
 
Just to add on, non-bonding or lone pairs can also affect the geometry of the molecule. Take for instance XeF4. While one would initially think it would be tetrahedral, but when one counts up the electrons (36 in this case) and places them on Xenon (it can expand its octet, since it is period 3 or beyond). In actuality, this molecule is octahedral. The Fluorine atoms take on equatorial positions and the two lone pairs take up the axial positions. The bond angles for this one, since it is octahedral, are 90 degrees instead of 109.5 for tetrahedral.

In terms of AXn notation, XeF4 is AX4, but the hybridization is sp3d2 due to the lone pairs.

Also, just in case you didn't know, cis- molecules tend to be polar compared to trans- molecules. If you draw out a molecule that has a cis- or trans- distinction, you'll notice that there is a net dipole moment on the cis- molecule, as the placement of the atoms in the trans- molecule cancel any charge out.

Good luck for your exam. Hope that helps :)
 
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