Why do helium, neon, and argon have zero valency?

Helium, neon, and argon are all noble gases, and they have zero valency because of their full outer electron shells. Here's a detailed explanation:

1. Full Outer Shell of Electrons:

   Helium (He), neon (Ne), and argon (Ar) are all located in Group 18 of the periodic table, also known as the noble gases.
   These gases have completely filled outer electron shells, which means they do not need to gain, lose, or share electrons to achieve a stable configuration.

   Helium has 2 electrons in its first shell (the maximum it can hold).
   Neon has 8 electrons in its second shell.
   Argon has 8 electrons in its third shell.

2. Stable Electron Configuration:

   The complete filling of the outer shell makes these gases chemically stable. Atoms tend to react in ways that lead to a stable electron configuration, typically by having 8 electrons in their outer shell (except for helium, which follows the duet rule with 2 electrons).
   Since helium, neon, and argon already have a stable electron arrangement, they do not need to form bonds with other elements to achieve stability.

3. Zero Valency:

   Valency is defined as the ability of an atom to combine with other atoms. Since these gases already have a stable electron configuration, they have no tendency to react or combine with other elements.
   This lack of reactivity is why they are said to have zero valency.

Helium, neon, and argon have zero valency because they have full outer electron shells, making them stable and chemically inert. Their stable electron configuration means they do not need to gain, lose, or share electrons, and therefore, they do not form chemical bonds with other elements.