The Periodic Spiral, like most variations of the periodic table of the elements, arranges the chemical elements in periods, characterized by the fact that they have the same number of electron shells. The number of electrons in these shells, which equals the element's atomic number, increases from left to right within each period. In each period the lighter metals begin on the right, the heavier metals continue in the lower segment, and the nonmetals on the left end with the noble gases at the top. Elements on the borderline between metals and nonmetals are called metalloids.