Transuranium Elements are chemical elements of atomic number greater than 92, the atomic number of uranium in the periodic table (see Periodic Law). More than 20 such elements have been identified. These elements consist of more than 100 radioactive isotopes, which are characterized by radioactive instability (see Radioactivity). These radioisotopes are produced artificially by bombarding heavy atoms either with neutrons produced in nuclear reactors or with charged particles accelerated to high energy in particle accelerators. The first 10 transuranium elements, together with actinium, thorium, protactinium, and uranium, constitute the actinide elements, which are chemically analogous to the rare earth elements (see Actinide Series; Lanthanide Series). They are, in order, neptunium, plutonium, americium, curium, berkelium, californium, einsteinium, fermium, mendelevium, and nobelium.
Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.
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