<p><strong>Noun</strong></p><ul><li>An inert, gaseous element present in the sun's atmosphere and in natural gas, and also occurring as a radioactive decomposition product, used as a substitute for flammable gases in dirigible balloons.</li><li>Symbol: He</li><li>Atomic Number: 2</li><li>Density: 0.1785 grams/liter at 0°C and 760 millimeters pressure</li></ul>