The Periodic Spiral, like traditional iterations of the periodic table, arranges the chemical elements in blocks, corresponding to the outermost electron shell configuration of each element. There are four such blocks in the Periodic Spiral. The s-block includes hydrogen and helium, with the respective configurations of 1s and 2s, together with the alkali metals and alkaline earth metals that constitute groups 1 and 2. The p-block elements include the noble gases of group 18, each of which exhibits the complete filling of its p-shell, together with the metals, metalloids and non-metals of groups 13 through 17. The d-block elements begin with the fourth-period rare-earth metal scandium, which represents the first instance of a partially filled d-electron shell, and includes the metals of groups 4 through 12, as well as scandium's group 3 homologs yttrium, lanthanum, and actinium. (The rare earths cerium, gadolinium, and lutetium, all members of the f-block group, each have a single d electron, as do the actinons protactinium, uranium, neptunium, curium, and lawrencium. The actinon thorium has two d electrons and no f electrons. Despite these unusual configurations, the foregoing are considered f-block elements.) The f-block elements include the lanthanons and actinons of group 3.