PF3 is the chemical formula of phosphorus trifluoride. It is a colorless and odorless gas with a molar mass of 87.968 g/mol.
In chemistry, it is used as a ligand in metal complexes. PF3 is a highly toxic gas because of its affinity towards iron in blood hemoglobin.
To find out whether the phosphorous trifluoride (PF3) molecule is polar or non-polar, continue reading the article.
Is PF3 polar or non-polar?
Phosphorus trifluoride (PF3) is a polar molecule. It consists of one phosphorus (P) atom and three fluorine (F) atoms. The phosphorus is kept at the central position, and all fluorine atoms are at the surrounding positions.
An electronegativity difference of 1.79 units exists between the bonded atoms of phosphorus and fluorine in the P-F bond in PF3. Thus, each P-F bond is individually polar in PF3 and possesses a specific dipole moment value (symbol µ).
As a result, in the PF3 molecule, as compared to the central P-atom, the highly electronegative fluorine atoms more strongly attract the shared electron cloud from each P-F bond.
One lone pair of electrons is also present on the central phosphorus atom, which leads to distortion in the shape and geometry of the molecule, further enhancing the polarity effect. The overall electron cloud does not stay uniformly distributed in the molecule. Thus, the PF3 molecule is overall polar (net µ =1.03 D).
A molecule is polar if there is a non-uniform charge distribution present in it. If the charge distribution gets equally balanced in different parts, then that molecule or molecular ion is considered non-polar.
The following three factors mainly influence the polarity of a molecule:
The electronegativity difference between two or more covalently bonded atoms
Dipole moment
Molecular geometry or shape
Now, let us discuss the effect of the above three factors one by one to prove that the phosphorus trifluoride (PF3) is overall a polar molecule.
Factors affecting the polarity of PF3
Electronegativity
It is defined as the ability of an elemental atom to attract a shared pair of electrons from a covalent chemical bond.
Electronegativity increases across a period in the Periodic Table while it decreases down the group.
Greater the electronegativity difference between bonded atoms in a molecule, the higher the bond polarity.
Phosphorus (P) is present in Group VA (or 15) of the Periodic Table. The electronic configuration of phosphorus is 1s2 2s2 2p6 3s2 3p3. As per this electronic configuration, a P-atom has a total of 5 valence electrons; it is thus short of 3 valence electrons that are required so that the phosphorus atom can achieve a complete octet electronic configuration.
Conversely, fluorine (F) is a halogen present in Group VIIA (or 17) of the Periodic Table. The electronic configuration of a fluorine atom is 1s2 2s2 2p5. According to this electronic configuration, an F-atom has a total of 7 electrons in its valence shell. Therefore, it lacks only one electron to complete its octet electronic configuration.
In PF3, the central P-atom is bonded to three F-atoms, each via a single covalent bond. Thus, the Lewis dot structure contains three P-F bonds. 3 valence electrons of phosphorus used out of the 5 initially available leaves behind 2 i.e., a lone pair on the central P-atom in PF3.
Contrarily, each of the three F-atoms contains three lone pairs of electrons. In this way, all bonded atoms attain a complete octet shell via chemical bonding in PF3.
Due to the presence of one lone pair of electrons on the central P-atom inPF3, there is distortion in the shape or geometry of the molecule.
Atom
Electronic configuration
Valence electrons
Phosphorus (15P)
1s22s22p63s23p3
5
Fluorine (9F)
1s22s22p5
7
The electronegativity difference between a P-atom (E. N= 2.19) and an F-atom (E. N= 3.98) in each of the three P-F bonds is 1.79 units.
Hence, the fluorine atom with higher electronegativity gains a partial negative (Fδ-) charge while the central phosphorus atom obtains a partial positive (Pδ+) charge.
As a result, in PF3, all P-F bonds are individually polar with an electronegativity difference of 1.79 units.
Dipole Moment
Dipole moment (μ) is a vector quantity that points from the positive pole to the negative pole of a bond or a molecule.
It is mathematically calculated as a product of the magnitude of charge (Q) and charges separation (r). The dipole moment is expressed in a unit called Debye (D).
The dipole moment of a polar covalent bond conventionally points from the positive center to the center of the negative charge.
So in highly polar bonds of PF3, the dipole moment of each P-F bond points from Pδ+ to Fδ-(as shown below).
Molecular geometry
According to the valence shell electron pair repulsion (VSEPR) theory of chemical bonding, PF3 is an AX3E1-type molecule. To one P-atom at the center (A), three bonded atoms are attached (three F-atoms), and the central P-atom contains one lone pair (E).
The molecular geometry or shape of PF3 is trigonal pyramidal which is not identical to its ideal tetrahedral electron pair geometry. To minimize the repulsions and to attain stability in a trigonal pyramidal structure, the bonded atoms of the PF3 molecule possess a bond angle of (∠ F-P-F) 96°.
Due to the presence of one lone pair of electrons on the central P-atom, the PF3 molecule does not have an identical molecular geometry (trigonal pyramidal) and electron geometry (tetrahedral).
The dipole moments of individually polar P-F bonds do not get canceled in the molecule overall due to its asymmetric shape.
The overall charged electron cloud in the trigonal pyramidal molecular shape of PF3 stays non-uniformly distributed.
Consequently, phosphorus trifluoride (PF3) is a strongly polar molecule (net µ = 1.03 D).
Difference between polar and nonpolar?
Polar molecule
Non-polar molecule
Atoms must have a difference in
electronegativity
Atoms may have the same or different electronegativity values
Unequal charge distribution overall
Equal charge distribution overall
Net dipole moment greater than zero
Net dipole moment equals to zero
Examples include water (H2O), ethanol (CH3CH2OH), ammonia (NH3), sulfur dioxide (SO2), bromine trifluoride (BrF3), phosphorus trifluoride (PF3), nitrogen trifluoride (NF3), etc.
Examples include oxygen (O2), nitrogen (N2), methane (CH4), carbon disulfide (CS2), ethane (C2H6), propane (C3H8), etc.
PF3 has polar covalent bonds present due to a specific electronegativity difference of 1.79 units between the bonded P-atom (E.N= 2.19) and F-atom (E.N= 3.98) in each P-F bond.
PF3 has an asymmetrical trigonal pyramidal shape. So the dipole moments of individually polar P-F bonds do not get canceled overall, enhancing the polarity of the molecule.
Thus, PF3 is overall polar with a net dipole moment of µ = 1.03 Debye.
Which one is more polar, PF3 or NF3?
PF3 (net µ = 1.03 Debye) is more polar than NF3 (net µ = 0.24 Debye).
Both have a trigonal pyramidal shape, but it is due to a higher electronegativity difference between bonded atoms in each P-F bond (1.79 units) as compared to that between a nitrogen and a fluorine atom in each N-F bond (0.94 units) that PF3 is more polar than NF3.
Higher dipole moments of individually polar bonds lead to a higher net dipole moment of the molecule.
Both PF3 and BF3 have polar bonds present due to an electronegativity difference between the bonded atoms. But, PF3 is an asymmetrical trigonal pyramidal molecule while BF3 is a symmetrical trigonal planar.
Individual bond polarities stay intact in PF3 while they get canceled in opposite directions in the symmetrical BF3.
The net dipole moments of two downwards-pointing B-F bonds get neatly canceled with the dipole moment of an upwards-pointing B-F bond.
So, PF3 is a polar molecule (net dipole moment = 1.03 D), while BF3 is a non-polar molecule (net dipole moment = 0).
Is there a formal charge on the bonded atoms in the PF3 molecule?
Formal charge of an atom = [ valence electrons – non-bonding electrons- ½ (bonding electrons)]
All atoms in the PF3 molecule including phosphorus and fluorine obtain a formal charge equal to zero, hence overall charge present on the phosphorus trifluoride molecule is zero.
Thus, it is a stable molecule.
Summary
Phosphorus trifluoride (PF3) is a polar molecule.
It consists of three P-F bonds.
P-F bonds are highly polar, with an electronegativity difference of 1.79 units between the bonded P-atom (E. N= 2.19) and F-atom (E. N= 3.98).
Phosphorus trifluoride PF3 has a trigonal pyramidal molecular geometry and tetrahedral electron geometry with a bond angle of ∠F-P-F = 96°.
Due to the presence of one lone pair of electrons on the central P-atom, the PF3 molecule occupies an asymmetrical trigonal pyramidal shape.
The dipole moments of individually polar P-F bonds do not get canceled equally on each side of the molecule in this asymmetrical trigonal pyramidal shape.
As a result, in PF3, there is a permanent net dipole moment of 1.03 D, and it is overall polar.
Vishal Goyal is the founder of Topblogtenz, a comprehensive resource for students seeking guidance and support in their chemistry studies. He holds a degree in B.Tech (Chemical Engineering) and has four years of experience as a chemistry tutor. The team at Topblogtenz includes experts like experienced researchers, professors, and educators, with the goal of making complex subjects like chemistry accessible and understandable for all. A passion for sharing knowledge and a love for chemistry and science drives the team behind the website. Let's connect through LinkedIn: https://www.linkedin.com/in/vishal-goyal-2926a122b/
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