What Is the Molecular Geometry of Pf3?


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When we talk about phosphorus trifluoride, we know that it is one of those structures that contain the infamous electronegative atoms from the p-block. These are halogen atoms and they have a unique characteristic to bind with other molecules to create salts. 

Phosphorus is a unique metal that is used in many places related to ignition as it is a good combustion element. When united with three molecules of fluorine, it becomes an odourless and colourless gas that you can’t even detect with anything. 

And yet, it is as deadly as CO. So much that it can kill any person without even them knowing that they are slowly being poisoned by this harmful gas. It is used in gas chambers to execute prisoners and care must be taken while handling the same. 

However, when we study inorganic chemistry and the orbital arrangement of this compound in space, we see that it has a well-defined structure due to the presence of covalent bonds. These bonds define them as they are present in the hybridized state. Let’s dig into them further with questions and answers:-

What is the chemical nature of PF3? 

Ans. If you look at the structure of PF3, you would find three fluorine atoms bonded with phosphorus which is a metal. Since fluorine being a non-metal and a member of the 17th group of elements that are the complete opposite of hydrogen atoms, they are highly electronegative. 

This makes the molecule a very strong nucleophile as it would be attracted to a positive charge considering the number of fluorine atoms present in the structure. Also, this molecule would only react to those which have low negative charge density and high positive charge density to ensure that the bonds are formed perfectly. 

It is a Lewis pair donor because it donates a pair of an electron to the atom that is being bonded to. Since it has fluorine atoms in its primary state, it has many electrons that can be donated. 

How is the PF3 molecule stable? 

Ans. This question should be the first thing that you should ask yourself while studying orbital geometry because this is where the basics of chemistry are. Any compound mixed cannot be attached to each other just because they need some protons or electrons. It is done with the help of a rule known as the Octet rule.

The octet rule states that the number of electrons required in the outermost orbit for atoms that do not have the perfect number of electrons is the valency of an atom. The valency starts from 0 to 4 and then jumps back from 4 to 0. 

Since phosphorus has 5 electrons in the outermost shell, it needs 3 more. But due to its ultra valency, it is quite stable as compared to fluorine which has 7 electrons in the outermost shell and needs one more to form the octet.

Thus, phosphorus donates its three electrons to fluorine to form three bonds with three fluorine molecules and have 2 electrons remaining in the outermost shell. This is what charges the atom to have an oxidation number of minus two. 

How is the PF3 molecular structure concerning geometrical aspects?

Ans. When we talk about structures with a particular geometry that can be seen with the case of many organic and inorganic compounds, we see that they are studied concerning the number of bonds it forms with its donor atoms or the shape of the orbital form after the hybridization of the orbit. 

The Valence Shell Electron Pair Repulsion or the VSEPR theory is what is used to know the orbital structure of the PF3 molecule. When we talk about the spatial arrangement of the molecule, we see that it is made up of various types of rules which makes the molecule stable.

One such rule is the hybridization rule. Since the molecule has three bonds and not four, the molecule has an sp3 orbital and its geometry would be Trigonal pyramidal rather than tetrahedral. The bond angle between the phosphorus atoms and the fluorine atoms is 97°. 

Why is there such an anomaly in the structure?

Ans. This is such a great question to answer. You should be quick to realise the fact that I mentioned here that if it has single bonds everywhere within the structure, the orbital hybridization of the molecule must be sp3. 

Now, since the orbital hybridization of the molecule is sp3, it should have a bond angle of 109.5° right? This is what is seen in structures like methane and other carbon derivatives so why have I mentioned 97°?. 

This is because there are two electrons that remain in the molecule and are considered as lone pairs of electrons. These lone pairs of electrons combined with fluorine, which is such a small atom in comparison with phosphorus creates such an effect. 

Also, if you want to know in detail about this, you must realise that the charge of the lone pair of electrons pushes the molecule together and this charge is so strong that it can easily break down the stereotypical angle which is 109.5° and make it 97°. The repulsion force between the lone pair is stronger than the bond angle itself. 

What is the hybridization and the effect of reduced bond angle in PF3 molecules?

Ans. Due to the reduction in bond angle of the molecule, we can see that it distorts the whole structure by combining the fluorine atoms on one side and the lone pair on the other side. As to why this happens, it is because of the lone pair repulsion which is stronger than the bonds itself. 

Since the lone pair creates one orbital and the rest of the fluorine atoms creates another orbital, the molecule can form four new orbitals concerning the theory. This development of 4 hybrid orbitals is what is seen in sp3 hybridization. 

Also, the steric number of the molecule is 4. 4 means, it has three sigma bonds and 1 lone pair of electrons which contributes to the structure. This makes it possible to form 4 bonds which are why there are 4 hybrid orbitals in the structure. 

One major thing to note here is that since the molecule has a lone pair of electrons on one side and the fluorine atoms on the other side, it creates a density of negative charge around the lone pair-end and a positive charge around the fluorine atom end. This creates a sort of polarity in the molecule and it can thus be dissolved in water.