Final answer:
The symbol "X" stands for Oxygen, represented by O₂, which forms a double covalent bond between the two oxygen atoms. Oxygen is essential for life, affecting various chemical processes and necessary for respiration in aquatic and terrestrial organisms.
Step-by-step explanation:
The gas represented by the symbol "X" in the options you provided is Oxygen, denoted by the symbol O₂. A molecule of oxygen comprises two atoms of oxygen, which is indicated by the subscript 2 in the molecular formula O₂. This molecular form of oxygen is quite different from single oxygen atoms, as oxygen typically exists as a diatomic molecule in its most stable form under standard conditions on Earth.
Two atoms of oxygen form a double covalent bond (O=O), where each atom shares two pairs of electrons to achieve stability by filling its valence shell. Oxygen is essential to life as it is a key component of cellular respiration, and it is also a critical part of many other chemical processes on Earth. This odorless, colorless gas is slightly denser than air and is essential for the survival of aquatic and terrestrial life.