on the pressure and stress. The beta allotrope (6.966 g/cm3) exists at room temperature, and it has a face-centered cubic crystal structure. The high-temperature gamma allotrope (6.57 g/cm3) has a body-centered cubic crystalline structure. The alpha allotrope (6.903 g/cm3) has a hexagonal crystalline structure and is stable at low temperatures. The beta allotrope has a metallic electrical conductivity at normal atmospheric pressure, but it becomes a semiconductor when exposed to a pressure of about 16,000 atmospheres (1.6 GPa). Its electrical resistivity increases ten times upon compression to 39,000 atmospheres (3.9 GPa), but then drops to about 10% of its room-temperature resistivity at about 40,000 atm (4.0 GPa). In contrast with the other rare-earth metals, which usually have antiferromagnetic and/or ferromagnetic properties at low temperatures, ytterbium is paramagnetic at temperatures above 1.0 kelvin. However, the alpha allotrope is diamagnetic. With a melting point of 824 °C and a boiling point of 1196 °C, ytterbium has the smallest liquid range of all the metals. Contrary to most other lanthanides, which have a close-packed hexagonal lattice, ytterbium crystallizes in the face-centered cubic system. Ytterbium has a density of 6.973 g/cm3, which is significantly lower than those of the neighboring lanthanides, thulium (9.32 g/cm3) and lutetium (9.841 g/cm3). Its melting and boiling points are also significantly lower than those of thulium and lutetium. This is due to the closed-shell electron configuration of ytterbium ([Xe] 4f14 6s2), which causes only the two 6s electrons to be available for metallic bonding (in contrast to the other lanthanides where three electrons are available) and increases ytterbium's metallic radius. Chemical properties Ytterbium metal tarnishes slowly in air, taking on a golden or brown hue. Finely dispersed ytterbium readily oxidizes in air and under oxygen. Mixtures of powdered ytterbium with polytetrafluoroethylene or hexachloroethane burn with a luminous emerald-green flame. Ytterbium reacts with hydrogen to form various non-stoichiometric hydrides. Ytterbium dissolves slowly in water, but quickly in acids, liberating hydrogen gas. Ytterbium is quite electropositive, and it reacts slowly with cold water and quite quickly with hot water to form ytterbium(III) hydroxide: 2 Yb (s) + 6 H2O (l) → 2 Yb(OH)3 (aq) + 3 H2 (g) Ytterbium reacts with all the halogens: 2 Yb (s) + 3 F2 (g) → 2 YbF3 (s) [white] 2 Yb (s) + 3 Cl2 (g) → 2 YbCl3 (s) [white] 2 Yb (s) + 3 Br2 (g) → 2 YbBr3 (s) [white] 2