Radiation
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.
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.