Animated Solution for Chemistry - s and p-Block Elements: A xenon compound 'A' upon partial hydrolysis gives ${\text{XeO}}_2{\text{F}}_2$. The number of lone pair of electrons present in compound A is ......... (Round off to the nearest integer).

Have you ever wondered why noble gases, the introverts of the periodic table, suddenly decide to form compounds? Xenon, whose name literally means 'stranger', is the most famous exception. In this problem, we are acting as chemical detectives to uncover the hidden electrons in a mysterious Xenon compound.

The first clue given to us is a chemical reaction: partial hydrolysis. Hydrolysis is essentially breaking a compound using water. Xenon fluorides react with water in fascinating, step-wise ways.

Let's look at Xenon hexafluoride, ${\text{XeF}}_6$. When it reacts with a small amount of water, it doesn't break down completely. Instead, it swaps two fluorine atoms for one oxygen atom in a highly controlled manner.

This leads us to the master equation:

By matching the product ${\text{XeO}}_2{\text{F}}_2$ with our given clue, we have successfully unmasked compound A as ${\text{XeF}}_6$.

Now that we know our compound is ${\text{XeF}}_6$, the real counting begins. VSEPR theory is our best friend here. Xenon is located in Group 18 of the periodic table, which means it inherently possesses 8 valence electrons.

In ${\text{XeF}}_6$, Xenon forms 6 single covalent bonds with 6 fluorine atoms. That uses up exactly 6 of its valence electrons.

The remaining 2 electrons pair up to form 1 lone pair on the central Xenon atom. This lone pair pushes the fluorine atoms around, giving the molecule a distorted octahedral shape.

Lone Pairs

Here is where the trap is set! Many students find the 1 lone pair on Xenon and immediately write down '1' as their final answer. But read the question carefully: it asks for the lone pairs present in the compound, not just on the central atom.

Let's zoom in on the fluorine atoms. Fluorine is a halogen from Group 17, meaning it has 7 valence electrons.

One of these electrons is shared with Xenon to form the single bond. The other 6 electrons sit quietly on the periphery as 3 lone pairs per fluorine atom.

Since there are 6 fluorine atoms in the molecule, we have a massive reservoir of lone pairs just on the outside:

Now, we bring it all together. We add the 1 lone pair from the central Xenon atom to the 18 lone pairs from the surrounding fluorine atoms.

The total number of lone pairs of electrons present in compound A is 19. By paying attention to the details and not falling for the central-atom trap, we arrive at the perfect answer.