the lanthanide contraction is attributed to the relativistic mass of high-velocity electrons and the smaller Bohr radius that results. See also Ionization energy Electronegativity Electron affinity Quantum mechanics Relativistic quantum mechanics References Further reading P. A. Christiansen; W. C. Ermler; K. S. Pitzer. Relativistic Effects in Chemical Systems. Annual Review of Physical Chemistry 1985, 36, 407–432. Quantum chemistry Special relativity