For example, silicon (Si)
has a metallic luster but is brittle and a poor conductor. The reactivities of the metalloids are
dependent on the elements with which they are reacting. Boron (B), for example, behaves like a
nonmetal when reacting with sodium (Na) and like a metal when reacting with fluorine (F). The
elements classified as metalloids form a "staircase" on the Periodic Table and include boron, silicon,
germanium (Ge), arsenic (As), antimony (Sb), tellurium (Te), polonium (Po), and astatine (At).
As we move down a group, the increasing principal quantum number implies that the valence
electrons will be found farther away from the nucleus because the number of inner shells is
increasing, separating the valence shell from the nucleus. Although the Z remains essentially
constant, the atomic radius increases down a group. Within each group, the largest atom will be at the
bottom, and within each period, the largest atom will be in Group IA (Group 1). For reference, the
largest atomic radius in the Periodic Table belongs to cesium (Cs, 260 pm), and the smallest belongs
to helium (He, 25 pm). Francium is typically not considered because it is exceptionally rare in nature.