Radiation is the emission of energy rays from an object. In particular the term is applied to the emission of X-rays.
Radioactive — Radioactive elements are those of high atomic weight (radium, thorium, uranium etc.) which spontaneously emit alpha, beta or gamma rays.
Radioactivity — Radioactivity is the spontaneous disintegration of an atomic nucleus to produce more stable nucleus and accompanied by the emission of charged particles and possibly also gamma rays.
Radium — Radium is a radioactive gaseous element with the symbol Ra.
Rain — Rain is condensed moisture from the atmosphere which falls to the ground in visible separate drops.
Red-lead — Red-lead is an oxide of lead used as a red pigment.
Rhenium — Rhenium is a metal element with the symbol Re.
Rhodium — Rhodium is a metal element with the symbol Rh.
Rose's Metal — Rose's metal is a fusible alloy consisting of 50 per cent bismuth, 25 percent lead and 25 per cent tin.
Rust — Rust is an iron oxide.
Ruthenium — Ruthenium is a metal element with the symbol Ru.
Rutherfordium — Rutherfordium is an artificial radioactive element with the symbol Rf and with a very short lifespan.