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
Periodic table of topological invariants dimensions can be labeled by the elements of the formula_46-th homotopy group of the symmetric space. These groups are displayed in this table, called the periodic table of topological insulators: There may also exist weak topological invariants (associated to the fact that the suspension of the Brillouin zone is in fact equivalent to a formula_66 sphere wedged with lower-dimensional spheres), which are not included in this table. Furthermore, the table assumes the limit of an infinite number of bands, i.e. involves formula_67 Hamiltonians for formula_68. The table also is periodic in the sense that the group of invariants in formula_46