Ytterbium is a soft, malleable and ductile chemical element that displays a bright silvery luster when pure. It is a rare earth element, and it is readily dissolved by the strong mineral acids. It reacts slowly with cold water and it oxidizes slowly in air.
Ytterbium has three allotropes labeled by the Greek letters alpha, beta and gamma; their transformation temperatures are −13 °C and 795 °C, although the exact transformation temperature depends 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.
Ytterbium metal tarnishes slowly in air. 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 Yb (s) + 3 I2 (g) → 2 YbI3 (s) [white]
The ytterbium(III) ion absorbs light in the near infrared range of wavelengths, but not in visible light, so ytterbia, Yb2O3, is white in color and the salts of ytterbium are also colorless. Ytterbium dissolves readily in dilute sulfuric acid to form solutions that contain the colorless Yb(III) ions, which exist as nonahydrate complexes:
- 2 Yb (s) + 3 H2SO4 (aq) + 18 H
2O (l) → 2 [Yb(H2O)9]3+ (aq) + 3 SO2−
4 (aq) + 3 H2 (g)
Yb(II) vs. Yb(III)
Although usually trivalent, ytterbium readily forms divalent compounds. This behavior is unusual for lanthanides, which almost exclusively form compounds with an oxidation state of +3. The +2 state has a valence electron configuration of 4f14 because the fully filled f-shell gives more stability. The yellow-green ytterbium(II) ion is a very strong reducing agent and decomposes water, releasing hydrogen gas, and thus only the colorless ytterbium(III) ion occurs in aqueous solution. Samarium and thulium also behave this way in the +2 state, but europium(II) is stable in aqueous solution. Ytterbium metal behaves similarly to europium metal and the alkaline earth metals, dissolving in ammonia to form blue electride salts.
Natural ytterbium is composed of seven stable isotopes: 168Yb, 170Yb, 171Yb, 172Yb, 173Yb, 174Yb, and 176Yb, with 174Yb being the most common, at 31.8% of the natural abundance). 27 radioisotopes have been observed, with the most stable ones being 169Yb with a half-life of 32.0 days, 175Yb with a half-life of 4.18 days, and 166Yb with a half-life of 56.7 hours. All of the remaining radioactive isotopes have half-lives that are less than two hours, and most of these have half-lives under 20 minutes. Ytterbium also has 12 meta states, with the most stable being 169mYb (t1/2 46 seconds).
The isotopes of ytterbium range in atomic weight from 147.9674 atomic mass unit (u) for 148Yb to 180.9562 u for 181Yb. The primary decay mode of ytterbium isotopes lighter than the most abundant stable isotope, 174Yb, is electron capture, and the primary decay mode for those heavier than 174Yb is beta decay. The primary decay products of ytterbium isotopes lighter than 174Yb are thulium isotopes, and the primary decay products of ytterbium isotopes with heavier than 174Yb are lutetium isotopes.