Cesium (element)
銫 is a rare kanji used specifically for the chemical element cesium. Its meaning is narrowly tied to that element and has no broader conceptual center.
銫 is a modern coined character for the element cesium, combining the metal radical 金 with the phonetic component 色 (ショク). The exact historical development is limited to its creation as a chemical term.
The metal radical 金 hints at a metallic element, and 色 (color) might remind you of the blue color cesium compounds can produce in a flame test. Together, 銫 is the metal cesium.
For ショク, imagine a shock of blue color when cesium is heated: shock -> ショク, and the metal 金 with color 色 produces a vivid shock of light.