Final answer:
Oxygen is most abundant as a solid within compounds in the Earth's crust, while it is a gas in the atmosphere, making up about 21 percent of its volume.
Step-by-step explanation:
Oxygen is indeed the most abundant element in the Earth's crust and is integral to the oxygen cycle. It is primarily found combined with other elements such as silicon, making up compounds like silicates which are solid at Earth's surface temperatures. In the Earth's atmosphere, oxygen exists as a gas predominantly in the form of O₂ molecules, constituting about 21 percent of the atmosphere's volume, and as ozone (O₃) to a lesser extent. Additionally, oxygen is vital in constituting about 89% of water by mass, making it also abundant in the hydrosphere. Therefore, although oxygen is a gas in the atmosphere, in the Earth's crust, it is primarily found as a solid within various compounds.