A monatomic element is an element that is stable as an individual atom. The term combines the words “mono” and “atomic” and means “single atom.” Usually, a monatomic element refers to a monatomic gas. Under standard conditions, the noble gases are monatomic elements. At high temperatures, other elements exist in monatomic form. In fact, the most abundant monatomic element is atomic hydrogen.
If you’re a student asked to list the monatomic elements, keep it simple and list helium, neon, argon, krypton, xenon, and radon.
List of the Monatomic Elements
- Oganesson (unknown)
Under standard temperature and pressure conditions, the monatomic elements are the noble gases helium, neon, argon, krypton, xenon, and radon. All of these elements are gases. Oganesson may (or may not) exist as a monatomic element, but it’s not a gas. It’s likely a solid at room temperature and pressure.
Other elements exist as monatomic elements at low pressure or high temperatures. For example, monatomic oxygen (called atomic oxygen to distinguish it from single and molecular oxygen) accounts for 96% of the oxygen in low Earth orbit. Monatomic oxygen also occurs on Mars. In this case, ultraviolet light provides the energy to break any bonds that might form between oxygen atoms. Similarly, atomic hydrogen exists. While rare on earth, electrically neutral monatomic hydrogen accounts for about 75% of the mass of the universe!
Why Monatomic Elements Exist
The monatomic noble gases have filled valence electron shells. In other words, they satisfy the octet rule. Because they are stable, monatomic elements don’t readily form chemical bonds and compounds with other elements. However, scientists have synthesized compound of all of the noble gases except helium, neon, and oganesson.
Monatomic oxygen, hydrogen, and other elements exist in near-vacuum because the atoms don’t get close enough to other atoms to form chemical bonds. Nearly all elements form monatomic gases at extremely high temperatures or when exposed to enough radiation because there’s too much kinetic energy for bonds to form or the energy breaks chemical bonds between atoms.
Monatomic elements behave differently from diatomic and triatomic elements or compounds because they only have translational energy and not rotational or vibrational energy. This makes their thermodynamic behavior easier to predict.
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