Questions about molecular forces and shapes

In summary, the conversation is discussing multiple choice questions related to molecular structures and shapes, and how to determine the correct answers using the valence bond theory. The first question asks which molecule does not have a dipole moment, and the correct answer is CO2. The second question asks about the shape of PCl3, which is trigonal pyramidal, and the third question asks about the molecular structure of IF6+, which is octahedral. The conversation also mentions the use of textbooks and online forums as resources for help with general chemistry homework.
  • #1
sanjuro
10
0
Hi
I wonder if you could help me with the following multiple choice questions:

[1] Which of the following does not have a dipole moment

a) H20 b)NH3 c)CO2 D)SF4 E)BRCL5

Are we just suppose to know that the answer is CO2 or is
there some simple way of working it out?


[2] The Lewis structure of PCl3 indicates four regions of high electron density around the phosphorus atom: one lone pair and three single bonds. What is the shape of the PCl3 molecule?

[a] Tetrahedral (b)trigonal pyramidal c)trigonal planar e) T-shape

How do we know that the answer is a trigonal pyramidal shape?

3] Similar sort of question - what is the molecular structure of IF6+
(a) octahedral (b) trigonal bypyramidal (c) square pyramidal (d) trigonal planer

In this case the number 6 denotes that it is an octahedral but often the answer is not so straightforward when non bonding pairs are concerned. How do we know if there are non bonding pairs?
 
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  • #2
For question 1:

Use the valence bond theory to deduce the shapes of the molecules and from this you can deduce if it has a dipole moment. Carbon dioxide has 16 valence electrons; 4 from carbon, 12 from both oxygens. Now draw one possible structure; with the carbon in the middle.

-Connect the atoms through a single bond.

-Make sure that both carbon and oxygen has a full octet by distributing the lone pairs. As you can see, we do not have enough electrons. Through single bonds we will need more than 16 electrons.

-So make another proposal. Connect the oxygen to carbon through double bonds. Make sure that each atom has eight electrons around it. This structure satisfies the valence bond theory. Notice that this structure does not have any lone pairs. Thus the central carbon is sp hybridized; it is a pure linear molecule.

For question 2 study ammonia; look into a general chemistry textbook. Trigonal pyramidal is more specific in indicating an sp3 hybridized central atom with a lone pair.

For question 3 look into what I said for the question number 1.

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  • #3


Hi,

Thank you for reaching out. I am happy to assist you with your questions about molecular forces and shapes.

For the first question, the answer is CO2. In order to determine which molecule does not have a dipole moment, we need to look at the molecular geometry and the polarity of the bonds. CO2 has a linear geometry and has two polar bonds that cancel each other out, resulting in a nonpolar molecule. The other options all have a dipole moment due to the asymmetry in their molecular geometry.

For the second question, the answer is trigonal pyramidal. This can be determined by looking at the Lewis structure and the VSEPR theory. PCl3 has a lone pair and three bonding pairs, resulting in a trigonal pyramidal shape. The lone pair adds an extra electron density, causing the molecule to have a bent shape.

For the third question, the answer is (a) octahedral. In this case, the number 6 indicates that there are six regions of electron density around the central atom, which results in an octahedral shape. Nonbonding pairs do not affect the overall shape of the molecule, as they are not involved in the bonding and are considered as regions of electron density. In this case, the IF6+ molecule has six bonding pairs and no lone pairs, resulting in an octahedral shape.

I hope this helps clarify your questions. Let me know if you have any further questions or need any additional assistance. Good luck with your studies!
 

1. What are molecular forces?

Molecular forces are the attractive or repulsive interactions between atoms and molecules. These forces are responsible for holding molecules together and determining their physical properties.

2. What are the different types of molecular forces?

The three main types of molecular forces are Van der Waals forces, dipole-dipole interactions, and hydrogen bonding. Van der Waals forces are the weakest and occur between all molecules, while dipole-dipole interactions occur between polar molecules, and hydrogen bonding occurs between molecules with a hydrogen atom bonded to a highly electronegative atom.

3. How do molecular forces affect the properties of substances?

Molecular forces play a crucial role in determining the melting and boiling points, solubility, and viscosity of substances. Stronger molecular forces result in higher melting and boiling points, lower solubility in polar solvents, and higher viscosity.

4. What is molecular shape?

Molecular shape refers to the three-dimensional arrangement of atoms in a molecule. It is determined by the number of atoms and the types of bonds between them. Molecular shape is important because it affects the molecule's polarity and therefore its interactions with other molecules.

5. How are molecular forces and molecular shape related?

The type and strength of molecular forces are influenced by the molecular shape. For example, molecules with symmetrical shapes tend to have weaker molecular forces, while molecules with asymmetrical shapes have stronger molecular forces. This is because asymmetrical molecules have greater differences in electronegativity, resulting in stronger dipoles and therefore stronger molecular forces.

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