1. Shriver, D. F., Atkins, P. W., and Langford, C. H.Inorganic Chemistry;OUP:Oxford, 1990; pp28−30.
2. Mellor, J. W.A Comprehensive Treatise on Inorganic and Theoretical Chemistry;Longman:London, 1924; Vol.V, pp494−586.
3. Greenwood, N. N. and Earnshaw, A.Chemistry of the Elements;Pergamon:Oxford, 1984; p1429.
4. Ionization Energies of Atoms and Atomic Ions
5. Atomic energy levels - the rare-earth elements :