Monatomic gas

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In physics and chemistry, monatomic is a combination of the words "mono" and "atomic," and means "single atom." It is usually applied to gases: a monatomic gas is one in which atoms are not bound to each other. All chemical elements will be monatomic in the gas phase at sufficiently high temperatures.

Noble Gases[edit]

The only chemical elements which are stable single atom molecules at standard temperature and pressure (STP), are the noble gases. These are helium, neon, argon, krypton, xenon and radon. The heavier noble gases can form chemical compounds, but the lighter ones are non-reactive or inert. For example, Helium, being the simplest Noble Gas, has only two electrons, thus meaning it is satisfied with a complete outer shell, making it relatively non-reactive, hence it is a Noble Gas.

The noble gases, when grouped together with the homonuclear diatomic gases, such as nitrogen (N2), are called "elemental gases" or "molecular gases" to distinguish them from molecules that are also chemical compounds.

Other elements[edit]

Monatomic hydrogen comprises about 75% of the elemental mass of the universe.[1]

Chemists noticed very long ago that when some gases (e.g. hydrogen, oxygen, chlorine) are being created e.g. during electrolysis or in reactions of metals with acids (hydrogen), they are radically more reactive than normally. This state of gases has been named "in statu nascendi" (Latin, pron. nahstzendee), meaning "in the state of being born". That aggressive reactivity can be used to intensify some chemical reactions. The source of that high reactivity is the fact that such gases remain monatomic radicals for some time. The presence of a catalyst such as e.g. arsenic can prolong that monatomic state.[citation needed]

The motion of a monatomic gas is translation (electronic excitation is not important at room temperature). Thus in an adiabatic process, monatomic gases have an idealised γ-factor (Cp/Cv) of 5/3, as opposed to 7/5 for ideal diatomic gases where rotation (but not vibration at room temperature) also contributes. Also, for ideal monatomic gases:[2][3][4]

the molar heat capacity at constant pressure (Cp) is 5/2 R = 20.8 J K-1 mol-1 (4.97 cal K-1 mol-1);
the molar heat capacity at constant volume (Cv) is 3/2 R = 12.5 J K-1 mol-1 (2.98 cal K-1 mol-1);

where R is the gas constant.

In popular culture[edit]

The biblical, historical, and mythological references to, and scientific properties and applications of, m-state metals, particularly gold, are pivotal to the plot of James Rollins' second SIGMA Force novel Map of Bones (2005).[5][6][7] On page 1 that book's Author's Note, Rollins writes:[8]

As to the heart of this novel, it would take volumes to discuss the truth behind both m-state metals and the long trail they trace throughout history. Luckily, that volume has already been written, following in great detail the path from the Egyptians to modern times, including the strange effects of Meissner fields, superconductivity, and magnetism. I encourage anyone with even a slight interest in this topic to pick up Lost Secrets of the Sacred Ark by Sir Laurence Gardner.[9] It was my own personal bible for this novel.


  1. ^ Palmer, D. (13 September 1997). "Hydrogen in the Universe". NASA. Retrieved 2008-02-05. 
  2. ^ Heat Capacity of an Ideal Gas
  3. ^ Heat Capacity of Ideal Gases
  4. ^ Lecture 3: Thermodynamics of Ideal Gases & Calorimetry, p. 2
  5. ^ White, Claire E. (2005). "A Conversation With James Rollins". The Internet Writing Journal. 
  6. ^ "James Rollins on his new novel - Map of Bones". Backstory. April 30, 2005. 
  7. ^ "An Interview With James Rollins". BookBrowse. 2005. 
  8. ^ Rollins, James (2005). "Author's Note". Map of Bones (HarperCollins). p. 1. 
  9. ^ The book that Rollins is referring to is: Laurence Gardner (Mar 25, 2003). Lost Secrets of the Sacred Ark: Amazing Revelations of the Incredible Power of Gold. Thorsons. ISBN 978-0007142958.