18. Argon

Name: Argon
Symbol: Ar
Atomic Number: 18
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)
Number of Protons/Electrons: 18
Number of Neutrons: 22
Classification: Noble Gas
Crystal Structure: Cubic
Density(293 K): 1.784 g/cm3
Color: Colorless Gas

Argon is colourless and odourless. Argon is very inert and is not known to form true chemical compounds. It makes a good atmosphere for working with air-sensitive materials since it is heavier than air and less reactive than N2.

Although no chemical compounds of argon are presently confirmed, argon can form clathrates with water when atoms of it are trapped in a lattice of the water molecules. Theoretical calculations on computers have shown several Argon compounds that should be stable but for which no synthesis routes are currently known.

This gas is isolated through liquid air fractionation since the atmosphere contains only 0.94% volume of argon (1.29% mass). The Martian atmosphere in contrast contains 1.6% of Ar-40 and 5 ppm Ar-36. In 2005, the Huygens probe also discoverd the presence of Ar-40 on Titan, the largest moon of Saturn.

Argon (Greek argos meaning "inactive") was suspected to be present in air by Henry Cavendish in 1785 but was not discovered until 1894 by Lord Rayleigh and Sir William Ramsay.

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Non-metals H, C, N, O, P, S, Se
Halogens F, Cl, Br, I, At
Alkali metals Li, Na, K, Rb, Cs, Fr
Alkaline Earth Be, Mg, Ca, Sr, Ba, Ra
Transition metals Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Rf, Db, Sg, Bh, Hs, Mt, Ds, Rg, Uub
Metalloids B, Si, Ge, As, Sb, Te, Po
Other metals Al, Ga, In, Sn, Tl, Pb, Bi
Rare Earth Lu, Lr, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Ac, Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No
Noble gases He, Ne, Ar, Kr, Xe, Rn

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