Hybridisation of PH₃

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What is Hybridization?

Orbital Hybridization is one of the most important topics in modern Physical Chemistry. Orbital Hybridization generally referred to as Hybridization in chemistry, is a concept which narrates the combining of mixing of atomic orbitals to form new orbitals. These new orbitals are different in shape and energies when compared to the orbitals that are combining to form these orbitals. The new orbitals are thus called the hybrid orbitals. The hybrid orbitals are suitable for the pairing of electrons in valence bond theory to form chemical bonds. 

Cause for Hybridization

Hybridization is a phenomenon that occurs when an atom makes a bond with the other atom with the help of the electrons that are from both ‘s’ and ‘p’ orbitals. This kind of chemical bonding creates an imbalance in the energy levels of the two electrons. To stabilize this variation in energy levels of the electrons from two different orbitals, the orbitals that hold the electrons involved in bond formation combine to form a hybrid orbital. 

Structure of PH3 

The common name of PH3 is phosphine. Phosphine does not have any characteristic colour. However, it is an inflammable and toxic gas. It is identified as a pnictogen hydride. Though phosphine in its purest form does not have any characteristic odour, the technical grade samples of phosphine stink with an unpleasant odour of garlic or rotten fish. It is found that the presence of substituted phosphine and diphosphane induces this unpleasant odour. The molecular formula of Phosphine is PH3. This indicates that the structure of phosphine should include one phosphorus and three hydrogen atoms bound together. The bond angle in PH3 is 930 C. The geometry of its structure describes phosphine as a trigonal pyramidal molecule. Since phosphine is gaseous at room temperature, its boiling and melting points are comparatively low. The melting point of phosphine is -132.80 C and its boiling point is -87.70 C. Phosphine has a molar mass of 33.99758 g/mol and is highly soluble in water. 

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Hybridization in Phosphine

It is quite surprising to describe hybridization in Phosphine. This is because it is a known fact that Phosphine has a well defined orbital structure and electron distribution. In other words, we can simply say that the process of hybridization is not valid in case of a phosphine molecule. The subsequent sections of this page will give a brief overview of the absence of hybridization in Phosphine molecules. 

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The detailed analysis of the structure and formation of the phosphine molecule gives an understanding that the electrons in pure ‘p’ orbital will take part in the formation of chemical bonds. This acts as a resistance for the orbitals to get hybridized. The lone pair of electrons is mainly in ‘s’ orbital and ‘s’ orbital is the lone pair orbital. Phosphorus will thus have three bond pairs and one lone pair of electrons. The detailed explanation of the absence of PH3 hybridization is given by the Drago’s rule. PH3 is regarded as a Drago molecule. For the pure ‘p’ orbitals that hold the electrons involved in bond formation, the bond angle is nearly 900. The Lewis dot structure of Phosphine enables us to understand that the Phosphine is trigonal and pyramidal. 

Drago’s Rule and Hybridization of Phosphine

Drago’s rule states that there is no need for considering the hybridization of an element in the following cases:

Case 1: At least one lone pair of electrons is present on the central atom of the molecule.

Case 2: Any of the elements from group 13, 14, 15, 16 or from 3rd to 7th period forms the central atom.

Case 3: The central atom has an electronegativity less than or equal to 2.5.

Case 4: Sigma bonds are absent and 4 lone pairs are there.

Let us consider the phosphine molecule. In this molecule, the central element is Phosphorus. Phosphorus is an element which belongs to the 15th group and the third period of the modern periodic table. The electronegativity of phosphorus is 2.19. Also, the phosphine molecule has one lone pair of electrons on the phosphorus atom. Considering all these facts and figures, the hybridization is absent in the phosphine molecule according to the Drago’s rule. However, atomic orbitals in phosphine overlap on one another to form chemical bonds.

Let’s have further insight on hybridization in Phosphine. It can be calculated that, in the P - H bonds, only 6% of the s - character will be recorded. Considering that there are three P - H bonds in the phosphine molecule, the s - character taking all the three P - H bonds together is 6 x 3 = 18 %. With this calculation, we can infer that the lone pair of electrons is not in this orbital and it is present in the orbital which has 100 - 18 = 82% ‘s’ character.  However, it is proven that none of the hybridized orbitals will have such a higher percentage of s- character. This clearly indicates that the lone pair of electrons in the PH3 molecule is not in any of the hybridized orbitals. It is present in the pure s - orbital. 

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Fun Facts

  • Orbital hybridization does not happen in the case of phosphine molecules. 

  • The bond angle PH3 is approximately equal to 900.

  • Pure ‘p’ orbital electrons are involved in P - H bond formation of phosphine molecules.

FAQ (Frequently Asked Questions)

Q1. What are the different types of Hybridization?

Ans: Orbital hybridization is the process of mixing or combining unstable atomic orbitals to form hybrid orbitals to balance the energy level of electrons in different orbitals. The different types of hybridization are:

  • sp hybridization

  • sp2 hybridization

  • sp3 hybridization

  • sp3d hybridization

  • sp3d2 hybridization

  • sp3d3 hybridization

Q2. How do you find whether the hybridization is sp2 or sp3 in hydrocarbons?

Ans: All the alkanes have central carbon atoms that are sp3 hybridized with tetrahedral geometry. However, in the case of alkenes and alkynes, the carbon atoms with double and triple bonds respectively are sp2 and sp hybridized respectively. The alkenes have a trigonal planar geometry.