Plutonium was discovered in 1941 by Dr. Glenn T. Seaborg and Edwin McMillan, Kennedy, and Wahl by deuteron bombardment of uranium in the 60-inch cyclotron of the Berkeley Radiation Laboratory at the University of California, Berkeley, but the discovery was kept secret. It was named after the planet Pluto, having been discovered directly after Neptunium. (Pluto is the next planet out after Neptune).
The most important isotope of plutonium is 239Pu, with a half-life of 24,200 years. Because of its short half-life, there are only extremely tiny trace amounts of plutonium naturally in uranium ores.
By far of greatest importance is the isotope Pu239, with a half-life of 24,100 years, produced in extensive quantities in nuclear reactors from natural uranium: 238U(n, gamma) --> 239U--(beta) --> 239Np--(beta) --> 239Pu. Fifteen isotopes of plutonium are known.
The plutonium isotope 238Pu is an alpha emitter with a half life of 87 years. These characteristics make it well suited for electrical power generation for devices which must function without direct maintenance for timescales approximating a human life time. It is therefore used in RTGss such as those powering the Galileo and Cassini space probes.
Plutonium also exhibits four ionic valence states in aqueous solutions: Pu+3 (blue lavender), Pu+4 (yellow brown), PuO+ (pink?), and PuO+2(pink-orange). The ion PuO+ is unstable in aqueous solutions, disproportionating into Pu+4 and PuO+2. The Pu+4 thus formed, however, oxidizes the PuO+ into PuO+2, itself being reduced to Pu+3, giving finally Pu+3 and PuO+2. Plutonium forms binary compounds with oxygen: PuO, PuO2, and intermediate oxides of variable composition; with the halides: PuF3, PuF4, PuCl3, PuBr3, PuI3; with carbon, nitrogen, and silicon: PuC, PuN, PuSi2. Oxyhalides are also well known: PuOCl, PuOBr, PuOI.
Plutonium is a key fissile component in modern nuclear weapons; care must be taken to avoid accumulation of amounts of plutonium which approach critical mass, the amount of plutonium which will self-generate a nuclear reaction. Despite not being confined by external pressure as is required for a nuclear weapon, it will nevertheless heat itself and break whatever confining environment it is in. Shape is relevant; compact shapes such as spheres are to be avoided.
Plutonium is also a fire hazard, especially finely divided material. It reacts chemically with oxygen and water which may result in an accumulation of plutonium hydride, a pyrophoric compound, that is, a material that will burn in air at room temperature. Plutonium expands considerably in size as it oxidizes and thus may break its container. Special precautions are necessary to store or handle plutonium in any form; generally a dry inert atmosphere is required. These are in addition to the hazards of radioactivity. Magnesium oxide sand is the most effective material for extinguishing a plutonium fire. It both cools the burning material, acting as a heat sink, but also blocks off oxygen. Water is also effective. There was a major fire at the Rocky Flats Plant near Boulder, Colorado in 1969 
Plutonium could also be used to manufacture radiological weapons or as a (not particularly deadly) poison. Large stockpiles of plutonium were accumulated by both the old Soviet Union and the United States which since the end of the Cold War have become a focus of nuclear proliferation concerns. In 2002, the United States Department of Energy took possession of 34 metric tons of excess weapons grade plutonium stockpiles from the United States Department of Defense, and as of early 2003 was considering converting several nuclear power plants in the US from enriched uranium fuel to MOX fuel as a means of disposing of these.
Plutonium is sometimes described in media reports as the most toxic substance known to man, although there is general agreement among experts in the field that this is incorrect. As of 2003, there has yet to be a single human death officially attributed to plutonium exposure. Naturally-occurring radium is about 200 times more radiotoxic than plutonium, and some organic toxins like Botulism toxin are billions of times more toxic than plutonium.
The chemical and radiological toxicity of plutonium should be distinguished from the danger of plutonium. Many, both in the anti-nuclear movement and in the continuing green politics movement, refer to plutonium as the most dangerous substance known to man because of its crucial role in the production of nuclear weapons.
Possibly it is the confusion of these two issues that has led to sensational exaggerations of plutonium toxicity. A 1989 paper by Bernard L. Cohen states, "Pu hazards are far better understood than [those from insecticides or food additives], and the one fatality per 300 years they may someday cause is truly trivial by comparison. In spite of the facts we have cited here, facts well known in the scientific community, the myth of Pu toxicity lingers on."  (html-ized version)
That said, there is no doubt that plutonium may be extremely dangerous when handled incorrectly. The alpha radiation it emits does not penetrate the skin, but can irradiate internal organs when plutonium is inhaled or ingested. Extremely small particles of plutonium on the order of micrograms can cause lung cancer if inhaled into the lungs. Considerably larger amounts may cause acute radiation poisoning and death if ingested or inhaled; however, so far, no human is known to have died because of inhaling or ingesting plutonium and many people have measurable amounts of plutonium in their bodies.
The metal has a silvery appearance and takes on a yellow tarnish when slightly oxidized. It is chemically reactive. A relatively large piece of plutonium is warm to the touch because of the energy given off in alpha decay. Larger pieces will produce enough heat to boil water. The metal readily dissolves in concentrated hydrochloric acid, hydroiodic acid, or perchloric acid. The metal exhibits six allotropic modifications having various crystalline structures. The densities of these vary from 16.00 to 19.86 g/cm3.