Is PH3 polar or non-polar? - Polarity of PH3

PH₃ is the chemical formula of phosphorus trihydride, aka phosphine. It is a colorless, flammable, highly toxic gaseous compound. Pure phosphine is odorless and has a molar mass of 33.99 g/mol.

In this article, we will discuss the polarity of phosphorus trihydride PH₃ molecule. So, is PH₃ polar or non-polar? Continue reading to find that out.  

Is PH₃ polar or non-polar?

Phosphorus trihydride (PH₃) is a polar molecule. It consists of one phosphorus (P) atom and three hydrogens (H) atoms. The phosphorus is kept at the central position, while the three hydrogen atoms occupy surrounding positions, making a trigonal pyramidal molecular shape.

A very small electronegativity difference of 0.01 units exists between the bonded atoms of phosphorus and hydrogen in each P-H bond in the PH₃ molecule.

Thus, each P-H bond is very weakly polar or almost non-polar in PH₃ and possesses a very small dipole moment value (symbol µ).

However, it is due to a lone pair of electrons present on the central phosphorus atom in PH₃, which leads to distortion in the shape and symmetry of the molecule.

This molecular distortion results in an overall non-uniform charged electron cloud distribution in the molecule. Thus PH₃ is overall polar (net µ = 0.58 Debye).

Name of molecule	Phosphorus trihydride or phosphine (PH3)
Bond type	Very weakly polar covalent
Molecular geometry	Trigonal pyramidal
Polar or Non-polar?	Polar molecule
Dipole moment	0.58 Debye
Bond angle	∠ H-P-H= 93.5°

What makes a molecule polar or non-polar?

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 trihydride (PH₃) is overall a polar molecule.

Factors affecting the polarity of PH₃

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 more electrons that are required so that the phosphorus atom can achieve a complete octet electronic configuration.

Conversely, hydrogen (H) belongs to Group I A (or 1) of the Periodic Table. The electronic configuration of a hydrogen atom is 1s¹. Hence, each H-atom in the PH₃ molecule has a deficiency of 1 valence electron to obtain a full outer shell i.e., a duplet.

The P-atom obtains a complete octet via chemical bonding in PH3, while the H-atoms obtain complete duplets. The Lewis dot structure of PH3 thus contains three P-H bonds. 3 valence electrons of phosphorus consumed in covalent bonding out of the 5 initially available leaves behind 2 valence electrons i.e., a lone pair on the central P-atom.

Due to the presence of one lone pair of electrons on the central P-atom in PH₃, there is distortion witnessed in the trigonal pyramidal shape of the molecule.

Atom	Electronic configuration	Valence electrons
Phosphorus (15P)	1s22s22p63s23p3	5
Hydrogen (1H)	1s1	1

The electronegativity difference in PH₃ between P-atom (E. N= 2.19) and H-atoms (E. N= 2.20) in each of the three P-H bonds is only 0.01 units.

As per Pauling’s electronegativity scale, this electronegativity difference lies well below what is required for a bond to be polar in nature.

However, a covalent bond is truly non-polar only if it is made up of two identical atoms, such as H-H. P-H bonds contain two dissimilar atoms.

Hydrogen being slightly more electronegative, attracts the P-H bonded electrons more strongly than the phosphorus atom. 3 H-atoms attracting P-H bonded electrons from a common P-atom at the center of the molecule enhance the polarity effect.

Hence, in PH₃, each of the three terminal hydrogen atoms gains a partial negative (H^δ-) charge while the central phosphorus atom obtains a partial positive (P^δ+) charge.

Thus each P-H bond is considered weakly polar in the PH₃ molecule.

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 PH₃, the small dipole moment of each P-H bond points from P^δ+ to H^δ- (as shown below).

Molecular geometry

According to the valence shell electron pair repulsion (VESPR) theory of chemical bonding, PH₃ is an AX₃E₁-type molecule. To one P-atom at the center (A), three bonded atoms are attached (three H-atoms), and the central P-atom contains one lone pair of electrons (E).

It is due to this lone pair of electrons that the molecular geometry or shape of PH₃ is trigonal pyramidal which is not identical to its tetrahedral electron pair geometry.

To minimize lone pair-bond pair and bond pair-bond pair repulsions and to attain stability in the trigonal pyramidal shape, the bonded atoms of the PH₃ molecule possess a mutual bond angle of (∠H-P-H) 93.5°.

Due to the presence of one lone pair of electrons on the central P-atom, the PH₃ molecule does not have an identical molecular geometry (trigonal pyramidal) and electron geometry (tetrahedral).

Relatively highly electronegative H-atoms attract the shared cloud of the P-H bonds to a greater extent. It is due to the asymmetric trigonal pyramidal molecular shape that the small dipole moments of three P-H bonds do not get canceled equally on each side of the molecule.

Hence, the charged electron cloud in the trigonal pyramidal molecular shape of PH₃ stays non-uniformly distributed overall. Consequently, phosphorus trihydride PH₃ is overall a polar molecule (net µ = 0.58 Debye).

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), phosphine (PH3), nitrogen trifluoride (NF3), etc.	Examples include oxygen (O2), nitrogen (N2), methane (CH4), carbon disulfide (CS2), ethane (C2H6), propane (C3H8), etc.

Also, check –

• PH₃ lewis structure, molecular geometry, bond angle, hybridization
• How to tell if a molecule is polar or nonpolar?
• Is CH₂Cl₂ polar or nonpolar?
• Is CH₃F polar or nonpolar?
• Is NCl₃ polar or nonpolar?
• Is NO₃^– polar or nonpolar?
• Is BrF₅ polar or nonpolar?
• Is SF₄ polar or nonpolar?
• Is CO₂ polar or nonpolar?
• Is NH₃ polar or nonpolar?
• Is SO₂ polar or nonpolar?
• Is SO₃ polar or nonpolar?
• Is H₂O polar or nonpolar?
• Is H₂S polar or nonpolar?
• Is HCN polar or nonpolar?
• Is CCl₄ polar or nonpolar?
• Is XeF₄ polar or nonpolar?
• Is CH₂O polar or nonpolar?
• Is CHCl₃ polar or nonpolar?
• Is SF₆ polar or nonpolar?
• Is BF₃ polar or nonpolar?
• Is PCl₅ polar or nonpolar?
• Is CH₃Cl polar or nonpolar?

FAQ

Why is PH3 a polar molecule?

PH3 has slightly polar covalent bonds(very weak) present due to a specific electronegativity difference of 0.01 units between the bonded P-atom (E. N= 2.19) and H-atom (E. N= 2.20) in each P-H bond. PH3 is an asymmetrical trigonal pyramidal molecule. So the dipole moment of individually polar P-H bonds does not get canceled overall, enhancing the polarity of the molecule. Thus, PH3 is overall polar with the net dipole moment of µ = 0.58 Debye.

Is NH3 more polar than PH3?

Both PH3 and NH3 possess an asymmetric trigonal pyramidal shape and molecular geometry. In such cases, the polarity of the molecule is largely governed by the difference in electronegativity between the bonded atoms. The greater the electronegativity difference, the higher the bond polarity, resulting in a high net dipole moment value. The electronegativity difference between a nitrogen (E.N = 3.04) and a hydrogen (E.N = 2.20) atom is 0.84 units in each N-H bond in NH3. It is significantly higher than that between a phosphorus (E.N = 2.19) and a hydrogen atom in each P-H bond in PH3., 0.01 units. Hence, NH3 (net µ= 1.4 D) is more polar than PH3 (net µ = 0.58 D).

Whose dipole moment is greater, PH3 or PCl3?

PCl3 (net µ = 0.97 D) possesses a greater dipole moment than PH3 (net µ = 0.58 D). Both PH3 and PCl3 are trigonal pyramidal molecules with a lone pair of electrons on the central P-atom. But in PCl3, three strongly electronegative Cl-atoms are attached to a P-atom. There is an electronegativity difference of 0.97 units between the bonded phosphorus (E.N = 2.19) and chlorine (E.N =3.16) atoms in each P-Cl bond in PCl3. It is significantly higher than the P-H electronegativity difference of only 0.01 units in PH3. Thus, PCl3 is more polar with a higher net dipole moment value.

Which of the following molecules are polar molecules? PH3 , CCl4 , HBr and H2O.

Out of the four molecules, only CCl4 is non-polar (net µ = 0), while all the others are polar molecules (net µ > 0). CCl4 has a symmetrical tetrahedral So the dipole moments of individually polar C-Cl bonds get canceled equally on each side of the molecule. It is, thus, overall non-polar. PH3 is a weakly polar trigonal pyramidal molecule with slightly polar P-H bonds. The small dipole moments of P-H bonds add up due to molecular distortion, enhancing the overall polarity effect. HBr is linear But it is strongly polar owing to the huge electronegativity difference between Br and H-atoms. H2O possesses a bent shape. It is strongly polar as the dipole moments of individually polar O-H bonds do not get canceled in the overall molecular shape.

Is there a formal charge on the bonded atoms in the PH3 molecule? Formal charge of an atom = [ valence electrons – non-bonding electrons- ½ (bonding electrons)] For the central P-atom Valence electrons = 5 Bonding electrons = 6 Non-bonding electrons = 2 ∴ Formal charge on the central P-atom = 5 – 2 – 6/2 = 5 – 2 – 3 = 5 – 5 = 0 For H-atoms Valence electrons = 1 Bonding electrons = 2 Non-bonding electrons = 0 ∴ Formal charge on each H-atom = 1 – 0 – 2/2 = 1 – 0 – 1 = 1 – 1 = 0 All atoms in the PH3 molecule including phosphorus and hydrogen obtain a formal charge equal to zero, hence overall charge present on the phosphorus trihydride molecule is zero.

Summary

• Phosphorus trihydride, commonly known as phosphine (PH₃), is a polar molecule.
• It consists of three P-H bonds.
• P-H bonds are slightly polar almost nonpolar according to Pauling scale, with an electronegativity difference of only 0.01 units between the bonded P-atom (E. N= 2.19) and H-atom (E. N= 2.20).
• PH₃ has an asymmetric trigonal pyramidal shape with a mutual bond angle of ∠ H-P-H = 93.5°.
• Due to the presence of one lone pair of electrons on the central P-atom, there is distortion present in the shape and geometry of the PH₃. This distortion leads to a non-uniformly spread electron cloud over the molecule.
• Thus, in PH_3, there is a permanent dipole moment μ = 0.58 Debye, so phosphorus trihydride is overall a polar molecule.