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

(a) Helium, Neon, and Argon are the examples of the special group of elements called the noble gases.

(b) This is because of their completely filled valence shell. All these elements have a valency of zero because of their fully filled outermost shells.

(c) Their atomic numbers are 2, 8 and 18 respectively, by their electronic configuration Helium has 2 electrons in the K shell, Neon has 8 in the L shell and Argon has 8 in the M shell.

(d) The K shell in the Helium is filled and the L and M shells in Neon and Argon respectively have a complete octet of electrons which makes them stable, so they do not require sharing their electrons and have the valency of zero.