You learned early in the course that valence electrons were key to the physical

Question

You learned early in the course that valence electrons were key to the physical properties of atoms. This is true for chemical species as well. Lewis/VSEPR diagrams inform chemists of the overall shape of a molecule and the charge distribution ) in 3D space. These are key to understanding a chemical species physical properties. Which of the following statement(s) about the knowledge you gained from Lewis/VSEPR diagrams is/are true? There may be more than one true statements select all true statements for full marks. Molecules with a more overall inear"shape should be able to interact more effectively with each other. Looking at the position of functional groups along a chain of carbon atoms may be important in determining overall molecular polarity. Significant bond polarities in a Lewis/NSEPR diagram are usually observed in the region of functional groups. Molecules with tetrahedral electronic geometries are always non-polar as the bond dipoles will be equally distributed in 3D space. Looking at the bond polarities in a Lewis diagram allows one to determine the direction but not magnitude (strength) of molecular polarity for a chemical species.

Explanation / Answer

The first statement is not entirely correct; its not always true that linear molecules are more effective at reactions. Carbonyl compounds are planer and are highly reactive. On the other hand, acetylene, a linear alkyne is quite stable and not as reactive as carbonyl compounds.

The overall molecular polarity depends on the arrangement of the atoms in the molecule. Functional groups often contain heteroatoms and heteroatoms (like N, O, S, X, X = Cl, Br, I) are more electronegative than carbon and hence, the C-(heteroatom) bond is polar. It is important to note the position as well as the number of functional groups in the molecule to determine the overall polarity of the molecule. Hence, the statement is partly correct.

Functional groups usually contain a carbon-(heteroatom) bond. Heteroatoms are more electronegative than carbon and hence the carbon-(heteroatom) bond is usually polar. Hence, the statement is true.

Tetrahedral molecules are not always non-polar. Tetrahedral molecules like CH4 or CCl4 where all the four groups are equivalent are non-polar. However, methanol, CH3OH is a tetrahedral molecule and is polar. Hence, the statement is wrong.

In order to determine the magnitude of the bond polarities, we need to do calculations and such calculations require us to know the electronegativity of the atoms involved in the bond formation as well as the bond distance. However, looking at the Lewis diagrams, we can predict the direction of bond polarity. The polarity of a bond is always from the less electronegative atom to the more electronegative atom. Hence, the statement is true.

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