a term introduced by D. I. Mendeleev in 1871 for chemical elements that were predicted by him but still unknown in his time; the elements were arranged in the periodic table under the corresponding known elements. All the eka-ele-ments were later discovered: eka-aluminum (1875) was called gallium; eka-boron (1879), scandium; and eka-silicon (1886), germanium. Mendeleev also predicted the existence of analogues of manganese (future Tc and Re), and tellurium (Po), iodine (At), cesium (Fr), barium (Ra), and tantalum (Pa).
The term “eka-element” is sometimes used to designate transuranium elements that have not yet been discovered and synthesized; for example, element no. 118 is called eka-radon. (See alsoPERIODIC SYSTEM OF ELEMENTS.)