When argon is placed in a container of neon, the argon spontaneously disperses throughout the neon because

Answer is: the dispersion of argon atoms produces an increase in disorder.
Argon and neon are gases, so they have high kinetic energy and move through space (gases move more disorderly. Entropy (amount of disorder) of system increases, the higher the entropy the greater is disorder.

The  argon    spontaneously  disperses   throughout   the  argon   because  dispersion  of  argon  atom   produces  an  increase  in  disorder.The  disorder  of  element  is  measured  by  use  of  entropy.According  to  the  second  law  of  thermodynamic  in  a  closed  system tend  to maximize entropy.

The density would be 1.622