Chemical Bonding — Molecular Geometry and Hybridization of Atomic Orbitals: Determine Electron Pair Geometry and Molecular Geometry

Question

What are the electron-pair geometry and the molecular geometry of each of the following molecules or ions?

  1. ClF5
  2. ClO2–
  3. PCl3
  4. SeF4
  5. PH2–

 

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  1. Electron-pair geometry: octahedral
    Molecular geometry: square pyramidal
  2. Electron-pair geometry: tetrahedral
    Molecular geometry: bent
  3. Electron-pair geometry: tetrahedral
    Molecular geometry: trigonal bipyramidal
  4. Electron-pair geometry: trigonal bipyramidal
    Molecular geometry: seesaw
  5. Electron-pair geometry: tetrahedral
    Molecular geometry: bent

Refer to Section 5.1: Molecular Structure and Polarity (1).

Strategy Map

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Table 1: Strategy Map
Strategy Map Steps 
1. Create a Lewis structure for the molecule.
2. Identify how many total electron domains are attached to the centre atom

Show/Hide Hint

Electron domains are all the bonding domains and lone pairs attached to the centre atom.

Example: PCl3 has 4 electron domains

In the centre of the image is the letter P indicating a phosphorus atom. Above the P are two small, filled circles, or dots which are circled and highlighted in red. Three lines branch off of the letter P with one to the right, one to the left and one to the bottom. Each of the three lines are connected to the letters "Cl" indicating a chlorine atom. Each "Cl" is circled and highlighted in red.
3. Identify how many of the electron domains are lone pairs. This will determine the molecules’ molecular geometry.

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Lone pairs will repel the other atoms and distort the molecules’ shape.

Example: In PCl3, the lone pairs attached to phosphorus push the chlorine atoms downwards.

The letter "P" is centred in the Image. Above the letter "P" is two small, filled circles, or dots. One downwards arrow is to the left of the letter "P" and another downwards arrow is to the right of the letter "P." To the left of the letter "P" and below the arrow are the letters "Cl" connected by a line. To the bottom of the letter "P" are the letters "Cl" the two atoms are connected by a solid black, filled triangle. To the right of the letter "P" and below the arrow are the letters "Cl" connected by two diagonal lines.

Solution

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a. ClF5

  • 42 valence electrons; Cl will be central atom.
  • 6 electron domains (The centre chlorine atom is attached to 5 fluorines and has 1 lone pair).

Answer:

  • Electron-pair geometry: octahedral
  • Molecular geometry: square pyramidal

b. ClO2–

  • 17 valence electrons; Cl will be central atom.
  • 4 electron domains (The centre chlorine atom is attached to 2 oxygens and has 2 lone pairs).

Answer:

  • Electron-pair geometry: tetrahedral
  • Molecular geometry: bent

c. PCl3

  • 42 valence electrons; P will be central atom.
  • 4 electron domains (The centre phosphorus atom is attached to 3 chorines and has 1 lone pair).

Answer:

  • Electron-pair geometry: tetrahedral
  • Molecular geometry: trigonal bipyramidal

d. SeF4

  • 44 valence electrons; Se will be central atom.
  • 5 electron domains (The centre selenium is attached to 4 fluorines and has 1 lone pair).

Answer:

  • Electron-pair geometry: trigonal bipyramidal
  • Molecular geometry: seesaw

e. PH2–

  • 8 valence electrons; P will be central atom
  • 4 electron domains (The centre phosphorus atom is attached to 2 hydrogens and has 2 lone pairs).

Answer:

  • Electron-pair geometry: tetrahedral
  • Molecular geometry: bent

Guided Solution

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The guided solution below will give you the reasoning for each step to get your answer, with reminders and hints.

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Table 2: Guided Solution
Guided Solution Ideas
This question is a theory problem where you are asked to determine both the electron-pair geometries (the name for the shape given the number of domains) and the molecular geometries (the name of the shape depending on the electron cloud distortions) for each of the given molecules.

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Refer to Section 5.1: Molecular Structure and Polarity (1).
To figure out how many electron domains are in the molecule, we must first draw the molecule with all lone pairs and bonds.

Show/Hide Think About This!

We can do this easily by first creating the molecule’s Lewis structure.

Example: The Lewis structure below shows that the molecule has 4 electron domains.

In the centre of the image is the letter P indicating a phosphorus atom. Above the P are two small, filled circles, or dots which are circled and highlighted in red. Three lines branch off of the letter P with one to the right, one to the left and one to the bottom. Each of the three lines are connected to the letters "Cl" indicating a chlorine atom. Each "Cl" is circled and highlighted in red.

 
Why is the electron-pair geometry different from the molecular geometry?

Show/Hide Think About This!

Recall that the electron-pair geometry is the basic shape with a certain number of electron domains.

The molecular geometry is the shape the molecule displays of the bonding domains, which is due to electron pair repulsions. If there are lone pairs attached to the central atom, they influence the shape of the molecule.

The molecular geometry can be the same as the electron-pair geometry if the central atom does not have any lone pairs.

Example:

The letter "P" is centred in the Image. Above the letter "P" is two small, filled circles, or dots. One downwards arrow is to the left of the letter "P" and another downwards arrow is to the right of the letter "P." To the left of the letter "P" and below the arrow are the letters "Cl" connected by a line. To the bottom of the letter "P" are the letters "Cl" the two atoms are connected by a solid black, filled triangle. To the right of the letter "P" and below the arrow are the letters "Cl" connected by two diagonal lines.
Table 3: Complete Solution
Complete Solution
a. ClF5

6 electron domains (The centre chlorine atom is attached to 5 fluorines and has 1 lone pair).

Lewis Structure: In the centre of the image are the letters "Cl." five lines branch from the central "Cl" atom. At the end of each line is the letter "F." To the bottom right of the letters "Cl" are two small, filled circles, or dots.

Molecular Geometry (with lone pairs): In the centre of the image are the letters "Cl." a line connects the "Cl" to the letter "F" from above. To the top left and to the top right of the letters "Cl" are the letters "F" which are both connected to the "Cl" by diagonal lines. To the bottom left and the bottom right of the letters "Cl" are the letters "F" which are both connected to the "Cl" by solid black, filled triangles. To the bottom of the "Cl" are two small. filled circles, or dots.

Answer:

  • Electron-pair geometry: octahedral
  • Molecular geometry: square pyramidal
b. ClO2–

4 electron domains (The centre chlorine atom is attached to 2 oxygens and has 2 lone pairs).

Lewis Structure:

In the centre of the image are the letters "Cl." To the left of the "Cl" are two horizontal lines connected to the letter "O." Both above and below the "Cl" are two pairs of small, filled circles , or dots with each pair containing two dots. To the right of the "Cl" is a line connected to the letter "O". To the right of the letter "O" is a short, horizontal line indicating a negative charge.

Molecular Geometry (with lone pairs):

In the centre of the image are the letters "Cl." To the bottom left of the "Cl" are two horizontal lines connected to the letter "O." Above the "Cl" are two pairs of small, filled circles , or dots with each pair containing two dots. To the bottom right of the "Cl" is a line connected to the letter "O". To the right of the letter "O" is a short, horizontal line indicating a negative charge.

Answer:

  • Electron-pair geometry: tetrahedral
  • Molecular geometry: bent
c. PCl3

4 electron domains (The centre phosphorus atom is attached to 3 chorines and has 1 lone pair).

Lewis Structure:

In the centre of the image is the letter "P." Above the "P" are two small, filled circles or dots. To the left, right, and below the "P" are the letters "Cl" and each "Cl" is connected by a line.

Molecular Geometry (with lone pairs):

In the centre of the image is the letter "P." Above the "P" are two small, filled circles or dots. To the bottom left, bottom right, and below the "P" are the letters "Cl." The bottom left Cl is connected to the "P" by a line, the bottom "Cl" is connected to the "P" by a solid, filled triangle and the bottom right "Cl" is connected to the "P" by two diagonal lines.

Answer:

  • Electron-pair geometry: tetrahedral
  • Molecular geometry: trigonal bipyramidal
d. SeF4

5 electron domains (The centre selenium is attached to 4 fluorines and has 1 lone pair).

In the centre of the image are the letters "Se". Four lines branches from the central "Se" atom. At the end of each line is the letter "F". To the left of the "Se" letters are two filled dots.

Molecular Geometry (with lone pairs):

In the centre of the image are the letters "Se". Directly above and below "Se", are two lines connected to the letter "F". To the top right of "Se", the letter "F" is connected by solid, filled triangle and the bottom right of "Se" is connected to "F" by a diagonal line.

Answer:

  • Electron-pair geometry: trigonal bipyramidal
  • Molecular geometry: seesaw
e. PH2–

4 electron domains (The centre phosphorus atom is attached to 2 hydrogens and has 2 lone pairs).

Lewis Structure:

In the centre of the image is the letter "P". To the right and left of "P" is a line connected to the letter "H". Directly above and below the letter "P" are two filled dots, and beside "P" is a short, horizontal line indicating a negative charge.

Molecular Geometry (with lone pairs):

In the centre of the image is the letter "P". To the bottom right and left of "P" are the letters "H" and each "H" is connected to "P" by a line. Above "P" are two pairs of filled dots with each pair containing two filled dots. Beside letter "P" is a short horizontal line representing a negative charge.

Answer:

  • Electron-pair geometry: tetrahedral
  • Molecular geometry: bent

Check Your Work

Electron pair geometry depends on the electron pair domains around a central atom. The molecular or species shape then describes how the bonds are arranged around the central atom.

Show/Hide Watch Out!

Making sure that your Lewis structure is correctly drawn is very important before determining shape. Pay attention to whether the species is neutral or charged. Make sure you have counted valence electrons properly.

Does your answer make chemical sense?

Show/Hide Answer

Previously, we had been looking at molecules 2 dimensionally, as if they were flat like a paper. With molecular geometry, we begin to see that molecules have 3-dimensional shapes that moves freely. They will move into positions that require the least amount of energy. This means that the outside atoms will move as far as they can from other negative charges (i.e., other atoms and electrons). Lone-paired electrons are very repulsive and, thus, will cause even more distortion within an atom, giving it a 3-dimensional shape.

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References

1. OpenStax. 5.1: Molecular Structure and Polarity. In CHEM 1500: Chemical Bonding and Organic Chemistry; LibreTexts, 2022. https://chem.libretexts.org/Courses/Thompson_Rivers_University/CHEM_1500%3A_Chemical_Bonding_and_Organic_Chemistry/05%3A_Chemical_Bonding_II-_Molecular_Geometry_and_Hybridization_of_Atomic_Orbitals/5.01%3A_Molecular_Structure_and_Polarity.

2. Blackstock, L.; Brewer, S.; Jensen, A. PASS Chemistry Book CHEM 1500; LibreTexts, 2023. https://chem.libretexts.org/Courses/Thompson_Rivers_University/PASS_Chemistry_Book_CHEM_1500.

3. Blackstock, L.; Brewer, S.; Jensen, A. 5.1: Question 5.E.09 PASS – Determine Electron Pair Geometry and Molecular Geometry. In PASS Chemistry Book CHEM 1500; LibreTexts, 2023. https://chem.libretexts.org/Courses/Thompson_Rivers_University/PASS_Chemistry_Book_CHEM_1500/05%3A_Chemical_Bonding_II_-_Molecular_Geometry_and_Hybridization_of_Atomic_Orbitals/5.01%3A_Question_5.E.09_PASS_-_determine_electron_pair_geometry_and_molecular_geometry.

4. OpenStax. 7.E: Chemical Bonding and Molecular Geometry (Exercises). In Chemistry 1e (OpenSTAX); LibreTexts, 2022. https://chem.libretexts.org/Bookshelves/General_Chemistry/Chemistry_1e_(OpenSTAX)/07%3A_Chemical_Bonding_and_Molecular_Geometry/7.E%3A_Chemical_Bonding_and_Molecular_Geometry_(Exercises).

5. Flowers, P.; Robinson, W. R.; Langley, R.; Theopold, K. Ch. 7 Exercises. In Chemistry; OpenStax, 2015. https://openstax.org/books/chemistry/pages/7-exercises.

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