Application of Einstein special theory of relativity in chemistry seems to be superfluous; energies are too low. The average velocity of electron in hydrogen atom (1 s 1) is 1/135 c , making its actual mass only 26,6 ppm bigger than the rest mass. However, for heavier elements (about Z > 60) relativistic effects have to be taken into account and, more, many phenomena cannot be explained without ascribing new mass to electrons, in accordance with Einstein theory. In this paper such phenomena are described: color of metallic gold and Bi and Pb compounds, contraction of Ln-X bond of lanthanide trihalides, voltage of lead-acid and Zn/HgO battery, and the shape of gold clusters. Besides, essentials of Einstein theory and quantum chemistry were problems concerning the validity of Lavoisier law.