<p><strong>Noun</strong></p><ul><li>A silver-white, metallic, active element resembling potassium, used in photoelectric cells and radio vacuum tubes.</li><li>Symbol: Rb</li><li>Atomic weight: 85.47</li><li>Atomic number: 37</li><li>Density: 1.53 at 20°C.</li></ul><p><strong>End of Response</strong></p>