Answer:

cesium

AnswerCrossword Clue
CESIUMElectropositive element
CESIUMMetal in atomic clocks
CESIUMElement found in photoelectric cells
CESIUMElement used in photoelectric cells
CESIUMSoft, silvery-white element
CESIUMElement used in atomic clocks
CESIUMElement in atomic clocks
CESIUMPhotoelectric cell component
CESIUMPhotoelectric cell element
CESIUMHighly reactive metal
CESIUMElement in photoelectric cells
CESIUMAtomic number 55