#18 Argon

Argon is a chemical element with symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. ... The complete octet (eight electrons) in the outer atomic shell makes argon stable and resistant to bonding with other elements.

Atomic mass:39.948 amu

Melting point:-189.3 C (83.85 K, -308.74 F)

Boiling point:-186.0 C (87.15 K, -302.8 F)

Density:293 K: 1.784 g/cm3