Rehenium halide
Halogen compound of thenium; examples are ReCI3.ReCI4.ReF4, and ReF6.

Rhodium
A soft silver white metal of the transition group. It generally occurs alloyed with platinum. Its alloy with platinum is employed in making scientific apparatus.

Rubidium halometallate
Halogen-metal-containing compounds of rubidium; examples are Rb2GeF6(rubidium hexafluorogermanate), Rb2PtCI6 (rubidium chloropl-atinate), and Rb2PdCI5 (rubidium palladium chloride).

Rust (Hydrated iron oxide)
The red-brown flaky solid which forms on iron and steel by the action of water and air; Fe2O3.xH2O is the formula of rust. This oxide is non-coherent and permeable. Consequently, it does not protect the metal from further reaction

Rydberg Spectrum
An absorption spectrum of a gas in the ultraviolet region, consisting of a series of lines that becomes closer together towards shorter wavelengths, merging into a continuous absorption region. The absorption lines correspond to electronic transitions to successively higher energy levels. The onset of the continuum corresponds to photoionization of the atom or molecule, and can thus be used to determine the ionization potential