12 These investigators also described the effects of hydration and wetting on the CaQ - Ca(OH}o perticies. Beczuse of the derse low porosity nature of these particles, their atmospheric hydration was dependent on the aqueous environment encountered during "fallout." Complete hydration, observed over several weeks time, was accompanied by a 100% increase in particle volume and the development of a crumbly, fluffy structure. When CaQ/Ca(0H)> particles were wet with seawater, they beaan to dissolve slowly. The freed calcium fons reacted with sulphate ions in the seawater to form calcium sulphate-dihydrate (gypsum) while the hydroxy] ions reacted to form insoluble Mg(OH)2. A hard shell of Mg(OH)> formed around the particle, which, during the period of observation, apparently stopped any further intrusion of seawater as a region of Ca(OH)> remained on the inner surfaces of the often hollow spherical particles. The remaining radioactivity was associated with the Ca(OH)> in the center of the sphere. Some of the freed Ca ions in the spheres also formed CaC03 by the reaction with the bicarbonate ions in seawater. 3.3 Chemical Properties of Plutonium and Americium in Aqueous Environments 3.3-1 General Considerations Plutonium (element 94) and americium (element 95) are revvers of the innter transition series of elements (from actinium (Z = 89] to lawrencium [Z = 103] known as the actinide elements. These elements, like their rare earth homologs, the lanthanides, and the main ("d") group of transition elements, are defined as those elements having partly filled d or f quantum shells. The chemical bonding and complexing properties of trans- ‘tion elements are determined by the electronic environment around their valence (bonding) electrons. Many chemical reactions possible for actinide elements are understood by analogy of the similarities and differences between