| hydrogen | | |
| n. (substance) | 1. atomic number 1, h, hydrogen | a nonmetallic univalent element that is normally a colorless and odorless highly flammable diatomic gas; the simplest and lightest and most abundant element in the universe. |
| ~ tritium | a radioactive isotope of hydrogen; atoms of tritium have three times the mass of ordinary hydrogen atoms. |
| ~ chemical element, element | any of the more than 100 known substances (of which 92 occur naturally) that cannot be separated into simpler substances and that singly or in combination constitute all matter. |
| ~ h2o, water | binary compound that occurs at room temperature as a clear colorless odorless tasteless liquid; freezes into ice below 0 degrees centigrade and boils above 100 degrees centigrade; widely used as a solvent. |
| ~ gas | a fluid in the gaseous state having neither independent shape nor volume and being able to expand indefinitely. |
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