Phosphorus trifluoride (PF3) is colorless as well as an odorless gas having similar toxicity as that of carbon monoxide. It binds to the iron present in the blood hemoglobin to spread throughout the body and prevents the blood from absorbing the oxygen.

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PF3 is a nucleophile where it donates a pair of electrons during a chemical reaction.

Lewis diagram is a pictorial presentation of the number of valence electrons present in an atom, which readily reacts with the valence electrons of another atom to form a bond.

The diagram is drawn with the help of eight dots around the atom, mostly in pairs. Here, the number eight has been selected as per the octet rule.

Moreover, the line represents the formation of a bond between the valence electrons of the two atoms. Counting these lines can help with determining how many bonds have been formed within a molecule.

Octet Rule

As per this rule, the maximum number of valence electrons an atom can have is eight. One phosphorus atom has five valence electrons, having a scarcity of three to complete its outermost shell or octet.

On the other hand, a single fluorine atom has seven valence electrons with a d of one to complete its octet and reach a stable condition.

Valence electrons in Phosphorus and Fluorine atom

The electrons present in the outermost shell of an atom are called valence electrons. Because they are present in the outermost shell, the hold of the nucleus is weak on them.

Moreover, uneven or unpaired electrons compel them to participate in the bond formation.

Here, it is essential to understand that a higher number of valence electrons will strengthen the ability of an atom to accept the electrons rather than donating.

So, fluorine accepts the electron whereas, the phosphorus atom tends to donate the electrons to complete their octet and reach a stable condition. The reason for the same is explained with the help of their electronic configuration.

The atomic number of phosphorus is fifteen which makes its electronic configuration 1s2 2s2 2p6 3s2 3p3. As we know, the p shell can hold a maximum of six electrons there is a scarcity of three electrons.

Whereas, on the other hand, the atomic number of fluorine is nine which makes its electronic configuration 1s2 2s2 2p5, having a scarcity of only one valence electron.

Lewis structure of Phosphorus Trifluoride (PF3)

The Lewis structure is drawn using eight dots of valence electrons around the symbols of the atom with lines showing bond formation.

PF3 is a tetra-atomic molecule where phosphorus donates three valence electrons, and three fluorine atoms accept one electron each to undergo a bond formation and reach a stable condition.

Below are the steps to draw the lewis structure of the PF3 molecule 

1. Find out the total number of valence electrons in PF3, which is 26.

2. Find out the number of valence electrons further needed of a single PF3 molecule to stabilize itself. It is six in total, where three valence electrons are needed by the phosphorus atom and one, each by three fluorine atoms.

3. Find the total number of bonds forming in a single PF3 molecule. It is three covalent single bonds, each between phosphorus and fluorine atom with the presence of no double or triple bonds.

4. Find the central atom to draw the Lewis structure, which is phosphorus in the case of phosphorus trifluoride (PF3).

5. Lastly, draw the lewis diagram as:

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Geometrical Structure of Phosphorus Trifluoride (PF3) molecule

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The geometrical structure of the tetra-atomic Phosphorus Trifluoride (PF3) molecule is studied with the help of the Valence Shell Electron Pair Repulsion (VSEPR) theory.

This theory explains that the bond angle between the fluorine-phosphorus-fluorine (F-P-F) is 97°. This angle makes the structure bent where the ideal bond angle for the bent, trigonal pyramidal structure is 109.5°.

This anomaly is due to the lone pair of electrons, and the smaller size of the fluorine atom. As the lone pair repulsion is stronger than the bond pair or bond pair-lone pair repulsion, it reduces the bond angle.

There exist a lone pair of electrons on the phosphorus which does not participate in the bond formation. Because they are highly stable, their repulsion is stronger than that of the bonding pair of electrons.

When any shared pair of electrons come in the near vicinity of lone pair of electrons, as do not want to bond.

This repulsion distorts the whole structure where the effect increases to many folds because of the smaller size of fluorine and shorter atomic radius distance.

Due to its original pyramidal shape, the PF3 molecule turns out to be polar. You can also check an article related to the polarity of PF3.

Hybridization in Phosphorus Trifluoride (PF3) molecule

Hybridization is a method of combining atomic orbitals of the same atom to produce new orbitals which are called hybrid orbitals.

To figure out the hybridization of the central atom, it is essential to determine the steric number in the phosphorus trifluoride (PF3) molecule.

The steric number is equal to the lone pairs and sigma bonds the central atom has.

As a single phosphorus trifluoride (PF3) molecule has three bonds (between phosphorus and fluorine) and one lone pair of electrons, the steric number is four.

It means the molecule will produce four new hybrid orbitals of equal energy which corresponds to sp3 hybridization.

As we know in a single covalent bond, the formation of only sigma (σ) bonds takes place with no pi (π) bonds. One sp3 hybrid orbital accommodates the lone pair of the electrons of the phosphorus.

Whereas, the other three sp3 hybrid orbitals formed from the covalent phosphorus-fluorine bonds, are used to accommodate the 2p orbitals of the fluorine.

The newly produced hybrid orbitals have 25% behavior of the s orbital and 75% behavior of the p orbital.

For more detailed knowledge, you must also check out the article PF3 Lewis Structure and Hybridization.

Molecular Orbitals diagram of Phosphorus Trifluoride (PF3) molecule

The molecular orbital diagram helps with determining how chemical bond formation is taking place. Also, it helps with figuring out how mixing and overlapping have taken place to produce four new hybrid orbitals.

The mixing and overlapping occur in the orbitals of similar energy, whereas the bonding electrons contribute to the formation of higher energy antibonding molecular orbitals.

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Is there back-bonding in phosphorus trifluoride (PF3) molecule?

Surprisingly, yes. Back bonding occurs between the atoms, where one has a lone pair of electrons while the other has a vacant orbital.

When they reach adjacent to one another, a compound is formed which shows pi-bonding characteristics where formed from sigma bond.

If you realize, a single lone pair of electrons in phosphorus is available to each one of the fluorine atoms. So, the phosphorus trifluoride (PF3) molecule undergoes back bonding.

Moreover, this further can be confirmed through the hybridization which says a single PF3 molecule shows 75% characteristics of p orbital.

Conclusion

The Lewis structure of the tetra-atomic phosphorus trifluoride (PF3) molecule shows three fluorine atoms bonded to a single phosphorus central atom. Three single covalent bonds are formed between the phosphorus and fluorine atoms which contributes to the presence of three strong sigma bonds and no pi bonds.

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Irrespective of it, the phosphorus trifluoride (PF3) shows pi bonding characteristics due to sp3 hybridization and back-bonding. The detail of how hybridization is taking place can be studied through the molecular orbital diagram of phosphorus trifluoride (PF3) molecule.