Chlorine water on cooling deposits greenish-yellow crystals of formula $ C{l_2}.X{H_2}O $ .What is the value of X?

Hint: Chlorine is a chemical element with the atomic number 17 and the symbol Cl. It is the second-lightest of the halogens, appearing on the periodic table between fluorine and bromine, and its characteristics are largely midway between them. At room temperature, chlorine is a yellow-green gas. It is a highly reactive element and a powerful oxidizer; on the Pauling scale, it has the greatest electron affinity and third-highest electronegativity, behind only oxygen and fluorine.

Complete answer:
Chlorine is a green gas. This is due to the fact that the halogen family of elements are colourful elements on the periodic table. The colour darkens as they become heavier.
The lighter fluorine absorbs greater intensity UV light because its outer electrons are more firmly bonded. The light that hasn't been absorbed is seen as yellow. The electrons in larger atoms are more insulated from the nucleus and are less firmly bonded. They absorb longer wavelengths, and the light hues get increasingly more absorbed as the absorption band changes down into the visible zone, like a rainbow, from blue to red. In the p orbital of the chloride ion, all of the valence electrons are coupled.
 $ C{l_2} + 8{H_2}O \to C{l_2}.8{H_2}O $
Hence the value of x is 8.

Note:
All chlorine in the Earth's crust is in the form of ionic chloride compounds, which includes table salt, due to its high reactivity. It is the second most prevalent halogen in the Earth's crust (after fluorine) and the twenty-first most abundant chemical element. Despite this, the vast quantities of chloride in saltwater dwarf these crustal deposits.