Block (periodic table) form compounds. The first period elements in this block, however, are nonmetals. Hydrogen is highly chemically reactive, like the other s-block elements, but helium is a virtually unreactive noble gas. S-block elements are unified by the fact that their valence electrons (outermost electrons) are in the s orbital. The s-orbital is a single spherical cloud which can contain only one pair of electrons; hence, the s-block consists of only two columns in the periodic table. Elements in column 1, with a single s-orbital valence electron, are the most reactive of the block. Elements in the second column have two s-orbital
Extended periodic table momentarily give an effective "Z" of 174; uranium with uranium gives effective "Z" = 184 and uranium with californium gives effective "Z" = 190). Supercritical atoms are hence expected to pose no difficulties with their electronic structure, so that the end of the periodic table may be determined instead by nuclear instability rather than electron shell instabilities. The stability of nuclei decreases greatly with the increase in atomic number after plutonium, the heaviest primordial element, so that all isotopes with an atomic number above 101 decay radioactively with a half-life under a day, with an exception of dubnium-268. No elements