Have you ever wondered why NH3, PH3, AsH3, SbH3, and other molecules in the same group have different bond angles? The bond angle of ammonia is 107 °, but for PH3, there is a difference of 13°, i.e.94°. Similarly, for AsH3 and SbH3, the corresponding bond angles are 93° and 92°, respectively.
Now let's discuss why NH3 and PH3 have a difference in bond angle of 13°. To explain this anomalous property, the Dragos rule can be used. In Dragos molecules, hybridisation does not occur for compounds of elements of the third period or higher, bonded to a less electronegative element like hydrogen.
If the central atom of a molecule belongs to the 15th or 16th group and is in the 3rd period or below, and is bonded to a substituent having electronegativity less than or equal to 2.5, then the lone pair present in the s-orbital doesn't participate in hybridisation and bonds are formed by pure p-orbitals. As pure p-orbitals are at 90 ͦ to each other, the angle between bond pairs is close to 90 ͦ. A lone pair of s-orbitals is found in a stereo chemically inactive s-orbital.
Dragos molecules have no hybridisation.
Hence, i = ∞.
The bond angle is nearly equal to 90 ͦ. It ranges between 90-92 ͦ.
As for molecules formed with elements of the 3rd period or above, they usually form weak bonds because of poor overlapping of atomic orbitals.
Conditions for Dragos Rule
The central atom should be large. That is, the third period or higher.
The side atom or the surrounding atom should be small and less electronegative, like hydrogen (EN≤2.5).
There should be a lone pair present. Lone pairs are usually present in s-orbitals and do not participate in hybridisation. Hence, these pairs of electrons are stereo-chemically inert. Or the s-orbital is said to be stereo-chemically inactive.
Molecules satisfying these conditions fall under the class of Dragos molecules.
Elements forming Dragos Molecule
Phosphorus (P), arsenic (As), antimony (Sb), sulphur (S), selenium (Se), and tellurium (Te) of the 15th and 16th groups, respectively effectively obey Dragos' rule. Hence, being bismuth and polonium radioactive, they show deviation from obeying Dragos' rule. Examples of Dragos molecules include: PH3, AsH3, SbH3, H2S, H2Se, H2Te etc.
Applications of Dragos Rule
The Dragos Rule can be used to predict the basicity of molecules. While considering NH3 and PH3, NH3 is more basic than PH3. It is because the lone pair of nitrogen atoms are present in their hybrid orbitals, whereas those of phosphorus are in unhybridised orbitals. Nitrogen is sp3 hybridised. Hence, these electrons are involved in hybridisation. In PH3, the lone pair is stereo-chemically inactive and not involved in hybridisation.
Thus, NH3 > PH3
Hybridization of PH3
2. Reducing Character
Dragos molecules act as reducing agents. The size of the atom increases as it moves down a group. Hence, the bond strength will decrease because of the poor overlapping of atomic orbitals.
∴ SbH3 > AsH3 > PH3 > NH3
3. Percentage s-Character
The Dragos molecule has no hybridisation, hence the percentage of s-characters is zero. Also, it is found that these molecules have a very small bond angle. The bond angle has an inverse relationship with the percentage s-character. Some other relations of percentage s-character are derived as follows:
% s-character ∝ Bond angle
∴ % s-character ∝ Bond strength
Phosphine is an example of a Dragos molecule. It has a chemical formula of PH3. In the phosphine molecule, phosphorus belongs to the third period and obeys all the conditions of the Dragos Rule. The phosphorus atom makes three sigma bonds with three hydrogen atoms. The s-orbital of phosphorus has a lone pair that is not involved in hybridisation.
Thus, the formation of PH3 is solely dependent on p-orbitals. Every hydrogen atom forms a 90-degree angle with a phosphorus atom. Thus, it takes on the shape of a trigonal pyramid.
The Dragos rule is limited to certain elements only.
This rule is set up such that only the third or higher period elements follow it. From this, it is clear that the Dragos rule is applicable only to p-block elements.
Not all elements of p-block obey this rule.
Dragos molecules have no hybridisation. It is because the lone pairs present in the s-orbital do not participate in hybridisation. Thus, only p-orbitals are involved in the formation of Dragos molecules.
The bond angle observed in Dragos molecules is considered relatively small, nearly equal to 90 degrees. Due to this, groups of 15 and 16 elements tend to form hydrides easily, as hydrogen is a less electronegative atom.
The solubility of Dragos molecules in water is slightly lower as compared to the compounds of the 2nd period.
The bond angles of hydrides in groups 15 and 16 or elements in the third period or higher are explained by Drago's rule.
It only applies to PH3, AsH3, SbH3, H2S, H2Se, and other related compounds.
Drago's rule states that when the following criteria are met, there will be a significant energy difference between the involved atomic orbitals, preventing any orbital mixing or hybridisation.
Only p-orbitals are involved in the formation of Dragos compounds.
The bond angle of the Dragos molecule is nearly 90 ˚.