Phosphine or AKA Phosphorus Tri-hydrate (PH3) is the most misunderstood chemical compound in chemistry and the reason is it’s a polar molecule with non-polar bonds. Hence, the compound is crushed to the core and gains the ability to heat a talk to a debate.
Its pure form is odorless, and other forms have an unpleasant odor like a rotten fish, or garlic, due to the presence of substituted Phosphine and Diphosphane. Now that you know, a brief history of this toxic compound, let’s come back to its polarity!!
So, is PH3 Polar or Nonpolar? PH3 is a polar compound due to the presence of a lone pair of electrons with an electron-electron repulsion which gives rise to an overall “bent” structure. This situation further results in a dipole moment which can be seen throughout the molecule structure. PH3 (Phosphine) is also polar because of the rule that all polar molecules must contain polar bonds which are formed due to a difference in electronegativity found between the bonded atoms of the chemical compound.
Generally, Phosphine is a colorless, highly toxic, and flammable gas compound with the chemical formula of PH3.
It is generally classed as a pnictogen hydride in chemistry. Its IUPAC name is phosphane and it is slightly soluble in water and hence is considered to be insoluble which we will discuss later!
Before we dive deeper, let’s recall a few terms.
How to differentiate a polar or a non-polar molecule?
We can answer this based on the following terms:
1. Polarity is the electric charge distribution around atoms, molecules, or chemical groups.
2. A compound becomes Polar when an electronegativity difference is found between the bonded atoms.
3. When the charge distribution is equal between the atoms of a diatomic molecule, or when the polar bonds in a large molecule are able to cancel out each other, a nonpolar molecule is formed.
Why is PH3 a polar compound?
In the PH3 molecule, the Phosphorus atom has 5 valance electrons. Here, three hydrogens give 3 electrons to the central atom and thus, satisfy the octet rule for (P).
Therefore, a lone pair of electrons is left behind.
This lone pair of electrons becomes a negative region and the hydrogen forms a positive region as each hydrogen’s electrons will most likely occupy the bonds occurring between the hydrogen and phosphorus excluding the hydrogen’s positively exposed nucleus.
And as a rule of polarity, any molecule with a positive along with a negative region will be considered polar.
Hence, the Phosphine compound is a polar molecule.
Its dipole moment is 0.58 D. Dipole moment is the value of the measurement of the polarity extent of a compound.
Why PH3 is Polar if NCl3 is Nonpolar?
Let’s clear the mist behind this, the molecule NCl3 has the same structural behavior as PH3 but PH3 has a negatively charged lone pair on along with 3 positive hydrogens present in the lewis structure giving rise to unequal charges in the compound which we’ve discussed earlier!
Hence, PH3 has an electronegativity difference forming polar bonds. But in NCl3, Nitrogen, and Chlorine, both have the same electronegativity of 3.0, so the molecule NCl3 is Non-Polar.
For more understanding, check out the article for the polarity of NCl3.
Although, both compounds are Trigonal Pyramidal!
The geometrical structure of PH3
Let’s take a look at the molecular geometry of ph3 (phosphorus tri-hydrate) closely!
So, in the lewis structure of PH3 above, it’s visible that we have three hydrogens along with a lone pair that is formed on the top of it.
If we start off counting the number of groups or the number of atoms attached to the phosphorus, we get one, two, three, and the lone pair which is four that gives us the term, what’s called the steric number which is four in this case.
When a steric number four comes along with a lone pair we have a Trigonal Pyramidal forming just as shown in the VSEPR table below.
So, the molecular geometry for ph3 is Trigonal Pyramidal. This chart is very important hence, it helps to answer such questions. Memorize it ASAP!
Ammonia is also one of the compounds having the same geometrical structure ie; trigonal pyramidal. Check out the article regarding the polarity of NH3.
Lewis structure of Phosphine or PH3
The lewis structure of a chemical compound depicts its electronic arrangement around the atoms present in the molecule.
The PH3 Lewis structure has 8 valence electrons. Note that hydrogen (H) only requires two valence electrons to acquire a fuller outer shell.
The Lewis structure for PH3 and NH3 are quite similar in the structure since both P and N fall in the same group in the Periodic table hence. giving space for confusion to breed due to nitrogen!
Hybridization of Phosphine
In the formation of the chemical compound phosphine, pure p orbitals take part in bonding and avoid getting hybridized. The lone pair orbital is the s orbital here.
With three (H) bond pairs and one lone pair of electrons in PH3, Hybridization doesn’t occur. As, according to Drago’s Rule, which states that hybridization will not take place if;
- The central atom belongs to the third or higher period.
- The central atom has one lone pair.
- The electronegativity value of the central atom is less than carbon.
In essence, PH3 is a Drago molecule and thus its bond angle shows that the p-orbitals have an angle of 90°.
Hence, the molecular geometry of PH3 is trigonal pyramidal but there is no hybridization due to PH3 being a Drago molecule.
- The pure p orbitals take part in bonding.
- In phosphine, orthogonal 3p orbitals overlap with 1s orbital of H.
Are the bonds in phosphine nonpolar or polar covalent bonds?
Phosphine is the best example of a polar molecule having non-polar bonds. As with three hydrogen bonds and a lone pair, hydrogen and phosphorus are equal in electronegativity values.
This means they attract shared pairs of electrons in the same range. Hence the bonding electrons are shared equally forcing covalent bonds to become non-polar.
The lone pair is responsible for asymmetrical charge distribution and hence, PH3 is a polar molecule with non-polar covalent bonds.
How is Phosphine (PH3) formed and why it is Insoluble?
Phosphine formation starts when a strong base or hot boiling water reacts with the white phosphorus, or when a reaction takes place between water and calcium phosphide (Ca3P2).
Phosphine structurally looks like ammonia (NH3), but on the contrary PH3 acts as a poor solvent than ammonia and hence is much less soluble in water, to the extent that it is called insoluble with 31.2 mg/100 ml (17 °C) solubility in water.
Where is PH3 used commercially?
PH3 or Phosphine’s popular use is as a fumigant.
Commercially, it is used in semiconductors to introduce phosphorus into the silicon crystals, in plastic industries as a polymerization initiator, and the production of flame retardants, whereas, it is used as a pesticide in storing grains.
Phosphine (PH3) is also used in chemical processing, and more commonly in fumigation of grain, tobacco, and other food products, before an international export.
Properties of PH3
- Its IUPAC name is Phosphane.
- The compound PH3 has its molecular mass of 33.997 g/mol.
- It exists as a gas with a colorless appearance.
- It has a pungent odor like a fish-like smell.
- The melting point of PH3 is around −132.8 °C or −207.0 °F.
- The boiling point of PH3 is −87.7 °C or −125.9 °F.
- Being a polar molecule, PH3 has its dipole moment 0.58 D.
Hence, we can pridefully claim that PH3 is a polar molecule with nonpolar covalent bonds as we’ve proved above. This chemical compound is highly toxic and is capable of taking the lives of people due to its nature.
Even after, it’s so dangerous, it acts as a boon as a polymerization initiator, and is used as a pesticide in storing grains. This compound is soluble in non-polar solutions and as water is polar it’s insoluble in it!
I hope, you’re clear that there is no hybridization in PH3! Hence, Phosphine is a Polar molecule.