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Molecular Shapes, Polarity, and Intermolecular Bonding: A Guide with Exercises, Exams of Acting

Stanford UniversityActing

Ammonia boils at a temperature of -33.34 oC while NF3, which is a bigger molecule, boils at -129 oC. Explain why.

Typology: Exams

2022/2023

Uploaded on 03/01/2023

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Molecular shapes
The shapes of molecules can be derived from their Lewis dot structures by using the VSEPR (Valence Shell
Electron Pair Repulsion) model. Simply put, this model states that electron pairs will repel each other in 3D
space so as to be as far from each other as possible.
Step 1 Draw an electron dot diagram of all the bonds in the molecule.
Step 2 Identify the central atom by drawing a Lewis dot diagram.
Step 3 Count the number of bonding and non-bonding pairs of electrons around the central atom.
The table below gives an indication on how to predict the shape.
Step 4 Separate all the pairs of electrons surrounding the central atom as far apart from each other as possible.
Don't forget we are working in 3 dimensional space. Treat a double bond as a single bond for the purposes of
repulsion.
Step 5 Identify the shape of the molecule looking only at the location of the atoms.
Number of atoms around
the central atom.
Number of electron pairs around the
central atom
Shape
2
2
Linear
2
3
Bent or “V”shape
2
4
Bent or “V”shape
3
3
Trigonal planar
3
4
Triangular pyramid
4
4
Tetrahedral
pf3
pf4
pf5
pf8
pf9

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Molecular shapes

The shapes of molecules can be derived from their Lewis dot structures by using the VSEPR (Valence Shell Electron Pair Repulsion) model. Simply put, this model states that electron pairs will repel each other in 3D space so as to be as far from each other as possible. Step 1 Draw an electron dot diagram of all the bonds in the molecule. Step 2 Identify the central atom by drawing a Lewis dot diagram. Step 3 Count the number of bonding and non-bonding pairs of electrons around the central atom. The table below gives an indication on how to predict the shape. Step 4 Separate all the pairs of electrons surrounding the central atom as far apart from each other as possible. Don't forget we are working in 3 dimensional space. Treat a double bond as a single bond for the purposes of repulsion. Step 5 Identify the shape of the molecule looking only at the location of the atoms.

Number of atoms around

the central atom.

Number of electron pairs around the

central atom

Shape

2 2 Linear

2 3 Bent^ or “V”shape

2 4 Bent or “V”shape

3 3 Trigonal planar

3 4 Triangular pyramid

4 4 Tetrahedral

Now another thing that we must consider about molecules is the question of symmetry. Symmetrical molecules are said to be non- polar. What is a polar molecule? It is a molecule where the bonding electrons are not evenly distributed within the molecule due to electronegativity difference between the bonding atoms. This creates poles with a slight positive or negative charge as shown on the right. The molecule is said to have a dipole moment, as shown on the right. Small, negative charges occur on the surface of the molecule where the more electronegative atoms are. In the case of water, the oxygen carries the slight negative charge while the end with the hydrogen atoms is has a slight positive charge. These slight charges are written as δ+ or δ-. symmetrical molecules are said to be non-polar and asymmetrical molecules are said to be polar. The table below shows some examples of non-polar molecules. CH 4 CCl 4 SiF 4 or any other molecule with four identical atoms around the central atom in a tetrahedral arrangement CO 2 SO 2 The two identical atoms on either side of the central atom form a symmetrical arrangement. O 2 Cl 2 N 2 etc. SO 3 BF 3 Hydrocarbons

Formula Lewis dot structure Intra-molecular bonding Indicate the polarity of the polar molecules H 2 O Polar covalent O 2 CH 4 Non-polar CH 2 O 2 PH 3 SH 2 COH 2 NH 3 CCl 4 CH 3 COOH NOH

Predicting the strength of intermolecular bonding

The boiling point of a pure molecular substance is a good indicator of the strength of the intermolecular bonds. On the right is a diagram that may be useful in sorting similar sized molecules according to boiling temperature. The diagram on the right attempts to show that:

  • the intermolecular forces acting between symmetrical molecules are dispersion forces only. These forces however can be very strong if the molecule is large enough and that is why it extends all the way to the top. So a large symmetrical molecule can have a higher boiling temperature than a smaller molecule that exhibits hydrogen bonding.
  • similar sized asymmetrical molecules have intermolecular forces composed of dipole-dipole and dispersion forces. The red coloured bar, representing dispersion forces extend all the way to all type of molecules indicating the presence of dispersion forces in all molecules no matter their symmetry.
  • Dispersion forces can be significant and can produce strong intermolecular forces that can rival hydrogen bonding for very large molecules. Use the diagram to place the following molecules in increasing order of melting point. Give your reason for the selection, the first one is done for you as an example. SO 2 CO 2 SH 2 H 2 O CH 4 C 2 H 6

Dispersion forces, dipole-dipole and hydrogen

bonding.

  1. Consider the image on the right of a hydrogen molecule showing the two electrons in red and the two nuclei in green. a. Label the diagram using the following symbols. δ- or δ+ b. In the diagram on the right, label any changes that will take place on molecule “B” if the hydrogen molecule depicted above was next to it. Explain your reason for the changes and show the location of electrons and dipoles that may or may not form.
  2. Which of the following forms of intermolecular bonding rely solely on instantaneous dipoles? Circle the correct response and give a reason a) dipole-dipole Yes/No Explain b) hydrogen bonding Yes/No Explain c) dispersion forces Yes/No Explain
  3. Ammonia has a boiling temperature of - 33 oC whereas water boils at 100 oC. a. Describe the intramolecular bonding of each molecule NH 3 ________________________________________________ H 2 o ________________________________________________ b. Circle the correct response. What type of molecule is Ammonia Polar / non-polar Water Polar / non-polar c. Describe the intermolecular bonding of each molecule. NH 3 ________________________________________________ H 2 o ________________________________________________ d. Explain the difference in boiling temperature between the two molecules.
  1. Consider the table on the right which shows the electronegativity of some elements in the periodic table and the diagram below of the HCl and H 2 O molecules. a. Consider the image on the right, the dotted lines show the bonds formed between molecules of HCl and the bonds formed between molecules of H 2 O. i. Place the following symbols δ+ or δ- in the appropriate place on each molecule. ii. What type of intermolecular bonding exists between the molecules of: - HCl _____________________________ - H 2 O _____________________________ iii. Explain why the boiling temperature of HCl is - 85 oC while the boiling temperature of H 2 O is 100oC with reference to the table of the electronegativity of elements. b. Consider the diagram on the right. The dotted lines show the bonds formed between molecules of H 2 S and the bonds formed between molecules of NH 3. i. Place the following symbols δ+ or δ- in the appropriate place on each molecule. ii. What type of intermolecular bonding exists between the molecules of:
    • NH 3 _____________________________
    • H 2 S _____________________________ iii. Which molecule will have the highest boiling temperature?