-Illium. comes in 3 isotopes: stable, mildly radioactive(half-time of thousands of years), and heavily radioactive(half-life of hours to days) liquid, relatively low melting and boiling point. the stable form releases the least energy. higher abundance than Naquahdah, releases less energy.
-Xerium. comes in just 1 stable isotope. higher atomic number than Illium. non-radioactive, non-fissionable, extraordinarily chemically reactive. on the Periodic table, it would be under Lithium, sodium, and francium etc.
Xerium salts are numerous and have wildly varying material qualities. solid at room temperature.
-Valarium. noble gas. radioactive, half-life varies wildly between isotopes. the most stable has a half-life of many millions of years. fissionable, highest energy density of all. there are no Valarium salts, the material is difficult to work with. hard to find.
-Naquahdah. i believe it should have a lower energy density.
-Naquahdriah. element of it's own, radioactive, unstable, but higher energy density than Valarium, which in turn has a highe energy density than Naquahdah.
-Trinium. stable, non-radioactive, non-fissionable material with extremely high strenght and incredible material properties. the same Exotic Particle Field that keeps these superatoms from degrading (the same exotic particles that Naquahdriah releases on fission) massively increases the Material Properties of Trinium
-Neutronium. highest atomic number of all. ultra-hard to find. incredible material properties. it's incredibility is only matched by it's rarity and impossibility to be processed. only advanced races with extensive knowledge of this stuff can actually use it for construction.
uses: (radioactive) Illium takes over the use of Liquid Naquadhah, as the material in itself never made a whole lot of sense to me.
power density and material incredibility are inversely related to their abundance: the more incredible, the less abundant. the higher the powerdensity, the lower the abundance. Naquahdriah holds the lowest abundance among the Power dense, Neutronium among the Material Incredible.
Naquahdriah should be the rarest: it's radioactive, and as it's name implies, comes from Naquahdah. natural Naquahdriah has already decayed, artificial is possible. *or recently formed*
Neutronium is the hardest to find. with it's density and all, on planets it'll be located inside the core.