Scotoma is the technical term for the blind spot in our field of vision. Our brains interpolate the missing data so we don't perceive a black spot, but in fact we have two scotomas, caused by the lack of retinal cells where the optic nerve joins the retina (see link).
The Periodic Table is a reticulated array of data. In the mid 1930's, element 43 was still conspicuously lacking. Dmitri Mendeleev, that great Russian seer of visions and father of the Periodic Table, foresaw its existence and called it eka-manganese, meaning "one-after-manganese." Here's what the family of transition metals looked like in the mid 1930's:
Chemists sought eka-manganese unsuccessfully for 75 years after Mendeleev's prediction: their efforts are nicely summarized by van der Krogt. Of the fruitless efforts, those of Noddack et al. came closest, and they proposed the name masurium in 1925. Several Periodic Tables from that era even included Ma beneath manganese.
Unequivocal proof for element 43 appeared in 1937, after an Italian team led by Emilio G. Segrè isolated it from radioactive samples of next door molybdenum which had been bombarded with deuterium nuclei at Berkeley. They named the new element technetium from the Greek τεχνητος, meaning artificial.
*Gnomonclature is an homage to James Joyce who invented a literary device called gnomon to accentuate character or story element. In the words of his character Stephen in A Portrait of the Artist as a Young Man:
Absence is the highest form of presence.The absence of eka-manganese drove generations of chemists to search for it because it was there--somewhere.