Gold.
untold
Nickel is an element that rhymes with tickle.
Tin is an element that rhymes with chin.
The element that rhymes with "keen" is "tin."
The closest rhymes for "untold" would be unfold and unsold. However, anything that rhymed with "old" would be acceptable, such as behold, bold, cold, gold, strolled, rolled, fold, hold, behold, foretold and controlled, to name a few.
phosphorous
antimony
Phosphorus
tin
The word "cobalt" rhymes with "proballt" and "snowballt."
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Fluorine, Chlorine