If hydrogen gas were monatomic and oxygen diatomic, the gas volume ratio would be 4&colon;1. This is why helium would prefer to be monatomic over diatomic. So, hydrogen prefers to exist as a diatomic, and not monatomic, molecule. For example, calcium carbonate consists of the monatomic ion Ca and the polyatomic ion CO. Oxidizing species include singlet oxygen, hydroxyl, monatomic oxygen, and hydroperoxyl. According to this definition, noble gas atoms are considered molecules as they are monatomic molecules. Ion pairs of monatomic anions, such as halide ions, cannot be studied by this technique. Iodide is one of the largest monatomic anions. Such systems approximate the monatomic gases, helium and the other noble gases. Bonding orbitals lead to a more stable species than when the two hydrogens are monatomic. It is chemically inert and monatomic in all standard conditions. A monatomic ion is an ion consisting of exactly one atom. Thus helium molecules (monatomic molecules) travel faster in a sound wave and transmit sound faster. For an ideal monatomic gas, the adiabatic index is 5/3. The specific heats of monatomic gases, such as helium, are nearly constant with temperature.