<p><strong>Noun</strong></p><ul><li>A rare, highly reactive, soft, metallic element of the alkali metal group, used chiefly in photoelectric cells.</li><li>Symbol: Cs</li><li>Atomic mass: 132.905</li><li>Atomic number: 55</li><li>Density: 1.9 at 20°C</li><li>Melting point: 28.5°C</li></ul>