- A soft silvery-white metallic element whose atomic number is 11. It is an alkali metal, extremely reactive, oxidizes rapidly in air, and reacts vigorously with water to produce sodium hydroxide. It is less dense than water, and has over 15 known isotopes, of which one is stable. Its applications in electronics include its use in sodium lamps, as a heat transfer agent, in solar power, and as a coolant in nuclear reactors. Its chemical symbol, Na, is taken from the Latin word for sodium: natrium.
- chemical symbol Na
- Abbreviation of numerical aperture 2. Same as N/A.
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