Answer:
Krypton is a noble gas, so it was difficult for chemists to find it, as though it was a secret
Step-by-step explanation:
Krypton is a noble gas and this demand it has full orbital electron shells (2.8.18.8 or [Ar].3d¹⁰.4s².4p⁶). They, therefore, do not have an affinity for gaining or losing an electron hence rarely react with other elements. They were therefore hard to discover by early chemists because there were no known compounds of this gas and other noble gases. This is why the name ‘hidden’.