Chemical Bonding — Molecular Geometry and Hybridization of Atomic Orbitals: Which Central Atom Makes a Polar Molecule?

Question

Molecule XF3 has a dipole moment. Is X boron or phosphorus?

 

Show/Hide Answer

X is phosphorus.

Refer to Section 5.1.4: Molecular Polarity and Dipole Moment (1).

Strategy Map

Do you need a little help to get started?

Check out the strategy map.

Show/Hide Strategy Map
Table 1: Strategy Map
Strategy Map Steps
1. Add up the valence electrons for each molecule (BF3 and PF3).
2. Using B and P as central atoms, draw the Lewis Structure for each molecule.

Recall your steps for drawing Lewis Structures.

Show/Hide Resource

Refer to Section 4.4 Lewis Symbols and Structures (2).

Show/Hide Hint
  1. Arrange and connect your atoms.
  2. Add lone pairs on terminal atoms until you have used them all.
  3. Recall that boron is an exception to the octet rule.
3. Evaluate bond dipole moments and determine the shape of each molecule.

Show/Hide Hint

Differences in electronegativities between atoms will cause a bond to be polar. Lone pairs on the central atom influence molecular shape.

When looking at the shape of a molecule:

  • If the bond dipole vectors cancel out, the molecule will be nonpolar.
  • If the bond dipole vectors do not cancel out, the molecule will have a net diploe moment and be polar.
4. Identify any differences between the 2 molecules that could cause 1 to be polar.

Show/Hide Hint

Which molecule has a lone pair on the central atom? What are the shapes of the 2 molecules?

Solution

Do you want to see the steps to reach the answer?

Check out this solution.

Show/Hide Solution

Boron as X, BF3, 24 valence electrons

Lewis structure:

Phosphorus is the central atom connected by a line representing a single bond to each of the 3 fluorines. In the centre of the image is the letter "B". To the right, left, and directly below "B" is a line connected to the letters "F". Each letter "F" is surrounded by three pairs of filled dots representing lone pairs. One pair is to the left, one is to the right, one is at the bottom. Each fluorine has 3 lone pairs. The phosphorus has 1 lone pair.

Molecular geometry, trigonal planar:

Boron is the central atom, connected by a line representing a single bond to each of the 3 flourines. The bonds to the flourines are an equal distance apart around the boron.

This molecule is nonpolar.

 

Phosphorus as X, PF3, 26 valence electrons

Lewis structure:

Phosphorus is the central atom connected by a line representing a single bond to each of the 3 fluorines. Each fluorine has 3 lone pairs. The phosphorus has 1 lone pair. In the centre of the image is the letter "P". To the right, left, and directly below "P" is a line connected to the letters "F". Each letter "F" is surrounded by three pairs of filled dots representing lone pairs. One pair is to the left, one is to the right, one is at the bottom. and each pair contains two filled dots. Above the letter "P" are two filled dots.

Molecular geometry, trigonal pyramid:

Phosphorus is the central atom, with 1 lone pair in the plane of the screen page. It is connected to 3 flourines. One fluorine bond is in the plane of the screen, 1 bond has a wedge that represents it coming out of the screen towards you, and 1 bond is a dashed line that represents it going into the screen away from you. This image indicates tetrahedral geometry.

This molecule is polar.

 

Answer: X is phosphorus!

 

Show/Hide Watch Out!

Recall that boron has a full octet with only 6 electrons; this is an exception to the octet rule.

Guided Solution

Do you want more help?

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

Show/Hide Guided Solution
Table 2: Guided Solution
Guided Solution Ideas
This question is a theory type problem that tests you on your ability to identify polarity within molecules by comparing 1 polar and 1 nonpolar molecule.

Show/Hide Resource

Refer to Section 5.1.4: Molecular Polarity and Dipole Moment (1).
How does polarity occur within a molecule?

Show/Hide Think About This!

 

Differences in electronegativity between atoms will cause a polar bond.

When looking at the shape of a molecule:

  • If the bond dipole vectors cancel out, the molecule will be nonpolar.
  • If the bond dipole vectors do not cancel out, the molecule will have a dipole moment and be polar.
  • To review how we determine bond and molecular polarity watch this video (3):

Jessicas second test – Direct URL link (pasted directly in editor, Did NOT use the add media button )
How do lone pairs on the central atom impact polarity?

Show/Hide Think About This!

Lone pairs on the central atom influence the molecular shape. The resulting positions of terminal atoms must be considered to see if the bond dipole moments cancel out.
Table 3: Complete Solution
Complete Solution
Boron as X, BF3

1. Count valence electrons.

Show/Hide Don’t Forget!

Valence electrons are the outer shell electrons.

  • B has 3 valence electrons.
  • Each F has 7.
  • Total = 3 + 3(7) = 24 valence electrons.

2. Connect your atoms:

Show/Hide Watch Out!

The atom that can form the most bonds is the central atom and is usually the least electronegative atom.

  • B is the central atom.
  • Each F is connected to B.

3. Draw the Lewis Structure.

Show/Hide Don’t Forget!

Recall that boron has a full octet with only 6 electrons; this is an exception to the octet rule.

  • Connect the B and each F by a single bond.
  • Place the remaining electrons as lone pairs around the terminal F atoms.

Doing this uses 24 electrons, which matches our count of valence electrons, so the Lewis Structure is complete.

Phosphorus is the central atom connected by a line representing a single bond to each of the 3 fluorines. In the centre of the image is the letter "B". To the right, left, and directly below "B" is a line connected to the letters "F". Each letter "F" is surrounded by three pairs of filled dots representing lone pairs. One pair is to the left, one is to the right, one is at the bottom. Each fluorine has 3 lone pairs. The phosphorus has 1 lone pair.

4. Determine the molecular geometry or shape, and decide if the molecule is polar or nonpolar.

Show/Hide Think About This!
  • Valence shell electron-pair repulsion theory (VSEPR theory) tells us that the electron pairs will arrange themselves to minimize repulsions by maximizing the distance between pairs.
  • The molecular geometry or shape describes the relative positions of the bonds around a central atom. Consider the bond dipoles in 3 dimensions to determine if they cancel out.

Since there are 3 single bonds of the central atom and no lone pairs, the single bonds arrange themselves equidistant apart in a trigonal planar shape. The bond dipole vectors will all cancel out, and the molecule will be nonpolar.

Boron is the central atom, connected by a line representing a single bond to each of the 3 flourines. The bonds to the flourines are an equal distance apart around the boron.
Phosphorus as X, PF3

1. Count valence electrons

  • P has 5 valence electrons.
  • Each F has 7.
  • Total = 5 + 3(7) = 26 valence electrons.

2. Connect your atoms

  • P is the central atom.
  • Each F is connected to P.

3. Draw the Lewis Structure

  • Connect the P and each F by a single bond.
  • Add electrons as lone pairs around the terminal F atoms.
  • Count how many valence electrons you have used.
  • At this point, we have used 24 valence electrons and have 2 left. Add a lone pair to P to complete its octet.

The Lewis Structure is complete.

Phosphorus is the central atom connected by a line representing a single bond to each of the 3 fluorines. Each fluorine has 3 lone pairs. The phosphorus has 1 lone pair. In the centre of the image is the letter "P". To the right, left, and directly below "P" is a line connected to the letters "F". Each letter "F" is surrounded by three pairs of filled dots representing lone pairs. One pair is to the left, one is to the right, one is at the bottom. and each pair contains two filled dots. Above the letter "P" are two filled dots.

4. Determine the molecular geometry or shape, and decide if the molecule is polar or nonpolar.

The lone pair and the 3 single bonds give a tetrahedral electron-pair geometry. Since 1 is a lone pair, the molecular geometry or shape of the resulting molecule will be a trigonal pyramid. With this molecular geometry, the bond dipole vectors cannot cancel out and the molecule will be polar.

Phosphorus is the central atom, with 1 lone pair in the plane of the screen page. It is connected to 3 flourines. One fluorine bond is in the plane of the screen, 1 bond has a wedge that represents it coming out of the screen towards you, and 1 bond is a dashed line that represents it going into the screen away from you.
Since the molecule XF3 has a dipole moment, the molecule must be PF3.

Answer: X is phosphorus!

Check Your Work

Summary of what we would expect based on the related chemistry theory

Show/Hide Watch Out!

BF3 is a trigonal planar molecule with no lone pairs and identical terminal atoms, so it will be nonpolar. PF3 has 1 lone pair on the central atom, a tetrahedral electron pair geometry, and a trigonal pyramid shape, so it will be polar.

If there are no lone pairs on the central atom and the terminal atoms are identical, the molecule will be nonpolar since the bond vectors cancel out.

If there is a lone pair on the central atom and the bond vectors cannot cancel out, the molecule will be polar.

Does your answer make chemical sense?

Show/Hide Answer

For a molecule to be nonpolar, the net dipole moment must be 0, meaning the bond dipole moments cancel each other out when the molecular shape is considered.

In the case where atom “X” is boron, there are no lone pairs on the central atom and the 3 bond dipole moments are equal in magnitude and in opposing directions when molecular shape is considered. The bond vectors cancel out, and the molecule is nonpolar.

Due to phosphorus having a lone pair on the central atom, even though the bond dipole moments are equal, they do not cancel each other out. This means the entire molecule has a dipole-moment and is polar.

PASS Attribution

Media Attribution

All figures are by Ashlynn Jensen, from LibreTexts PASS Chemistry Book CHEM 1500 (4) by Blackstock et al., and are used/updated under a CC BY-NC 4.0 license.

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. OpenStax. 4.4: Lewis Symbols and Structures. 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/04%3A_Chemical_Bonding_I-_Basic_Concepts/4.04%3A_Lewis_Symbols_and_Structures.

3. TRUChemOnline. Help Video: Polarity. YouTube, June 8, 2022. https://youtu.be/KSEkba0O43M?si=MHhiVWgNFohRF9u_ (accessed July 14, 2025).

4. 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.

5. Blackstock, L.; Brewer, S.; Jensen, A. 5.2: Question 5.E.17 PASS – Which  Central Atom Makes a Polar Molecule? 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/5.02%3A_Question_5.E.17_PASS_-_which_central_atom_makes_a_polar_molecule.

6. OpenStax. 4.11: Exercises. In Chemistry – Atoms First 2e (OpenStax); LibreTexts, 2023. https://chem.libretexts.org/Bookshelves/General_Chemistry/Chemistry_-_Atoms_First_2e_(OpenStax)/04%3A_Chemical_Bonding_and_Molecular_Geometry/4.11%3A_Exercises.

7. Flowers, P.; Robinson, W. R.; Langley, R.; Theopold, K. 7.1 Ionic Bonding. In Chemistry; OpenStax, 2015. https://openstax.org/books/chemistry/pages/7-exercises.

License

Icon for the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License

PASSchem Copyright © 2025 by Sharon Brewer, Lindsay Blackstock, TRU Open Press is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

Share This Book