Wikipedia, Sept. 8, 2012: Caesium (Caesium or cesium is a chemical element with symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal)

The element cesium, via It's elemental website, Sept. 10, 2012 (Cs; Cesium; Atomic number: 55; Element Classification: Metal; Period Number: 6; Group Number: 1; Group Name: Alkali Metal)