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At normal atmospheric pressure oxygen melts at 54.36 [[Kelvin]] (K) and boils at 90.2 K; on Earth it essentially exists only as a gas. It readily dissolves in water, which makes aquatic life possible. Under normal conditions oxygen forms molecules consisting of two oxygen atoms each sharing two electrons in the 2p subshell; this form of oxygen is known as O<sub>2</sub>. O<sub>2</sub> is a pale blue odourless gas.
At normal atmospheric pressure oxygen melts at 54.36 [[Kelvin]] (K) and boils at 90.2 K; on Earth it essentially exists only as a gas. It readily dissolves in water, which makes aquatic life possible. Under normal conditions oxygen forms molecules consisting of two oxygen atoms each sharing two electrons in the 2p subshell; this form of oxygen is known as O<sub>2</sub>. O<sub>2</sub> is a pale blue odourless gas.
==Isotopes==
There are three stable and 14 radioactive isotopes of oxygen. The stable isotopes are <sup>16</sup>O, <sup>17</sup>O and <sup>18</sup>O, with <sup>16O</sup> being by far the most common. Radioisotopes range from <sup>10</sup>O to <sup>26</sup>O, with the two most stable being <sup>15</sup>O and <sup>14</sup>O. This gives oxygen an atomic weight of slightly below 16.