intuition: electrons with lower principal quantum numbers will have a higher probability density of being nearer to the nucleus. A nucleus with a large charge will cause an electron to have a high velocity. A higher electron velocity means an increased electron relativistic mass, and as a result the electrons will be near the nucleus more of the time and thereby contract the radius for small principal quantum numbers. Periodic-table deviations The periodic table was constructed by scientists who noticed periodic trends in known elements of the time. Indeed, the patterns found in it are what give the periodic table its power. Many of the chemical and physical differences between the 5th period (Rb–Xe) and the 6th period (Cs–Rn) arise from the larger relativistic effects of the latter. These relativistic effects are particularly large for gold and its neighbors platinum and mercury. An important quantum relativistic effect is the van der Waals force. Mercury Mercury (Hg) is a liquid down to −39 °C (see the Melting point). Bonding forces are weaker for Hg–Hg bonds than for their immediate neighbors such as cadmium (m.p. 321 °C) and gold (m.p. 1064 °C). The lanthanide contraction only partially accounts for this anomaly. Mercury in the gas phase is alone among metals in that it is quite typically found in a monomeric form as Hg(g). Hg22+(g) also forms, and it is a stable species due to the relativistic shortening of the bond. Hg2(g) does not form because the 6s2 orbital is contracted by relativistic effects and may therefore only weakly contribute to any bonding; in fact, Hg–Hg bonding must be mostly the result of van der Waals forces, which explains why the bonding for Hg–Hg is weak enough to allow for Hg to be a liquid at room temperature. Au2(g) and Hg(g) are analogous with H2(g) and He(g) with regard to having the same nature of difference. The relativistic contraction of the 6s2 orbital leads to gaseous mercury sometimes being referred to as a pseudo noble gas. Color of gold and caesium The reflectivity of aluminium (Al), silver (Ag), and gold (Au) is shown in the graph to the right. The human eye sees electromagnetic radiation with a wavelength near 600 nm as yellow. Gold appears yellow because it absorbs blue light more than it absorbs other visible wavelengths of light; the reflected light reaching the eye is therefore lacking in blue compared with