The noble gases make a group of chemical elements with similar properties: under standard conditions, they are all odorless, colorless, monatomic gases with very low chemical reactivity.

There is some difference in the heat capacity of monatomic vs. polyatomic gasses, and also gas heat capacity is temperature-dependent in many ranges for polyatomic gases ; these factors act to modestly ( up to the discussed factor of 2 ) increase heat capacity per atom in polyatomic gases, as compared with monatomic gases.

If ununoctium forms a gas under standard conditions nevertheless, it would be one of the densest gaseous substances at standard conditions ( even if it is monatomic like the other noble gases ).

In low molecular weight gases, such as helium, sound propagates faster compared to heavier gases, such as xenon ( for monatomic gases the speed of sound is about 75 % of the mean speed that molecules move in the gas ).

In a thermodynamic system composed of two monatomic ideal gases, a and b, if the two gases are mixed, the final temperature T is

The noble gases do not form stable compounds and occur as monatomic gases, whilst nitrogen, although it does not have a stable configuration for its individual atoms, forms a diatomic molecule so strong that all oxides of nitrogen are thermodynamically unstable with respect to nitrogen and oxygen.

In ideal monatomic gases, thermal energy is entirely kinetic energy.

For gaseous systems, the factor f, the number of degrees of freedom, commonly has the value 3 in the case of the monatomic gas, 5 for many diatomic gases, and 7 for larger molecules at ambient temperatures.

This result is also valid if the gas is not monatomic, as the volume dependence of the entropy is the same as for all ideal gases.

But Dalton and other chemists of the time had not considered that some elements occur in monatomic form ( like Helium ) and others in diatomic form ( like Hydrogen ), or that water was H < sub > 2 </ sub > O, not the simpler and more intuitive HO — thus the atomic weights presented at the time were varied and often incorrect.

The values for are for diatomic gasses like air and its major components, and for monatomic gasses like the noble gasses.

* Single-charged monatomic ions like Na < sup >+</ sup >, K < sup >+</ sup >, and Cl < sup >−</ sup >

* Double-charged monatomic ions like Ca < sup > 2 +</ sup > and Mg < sup > 2 +</ sup >

Typically, monatomic ( single atom ) oxygen plasma is created by exposing oxygen gas at a low pressure ( O < sub > 2 </ sub >) to high power radio waves, which ionise it.

At standard temperature and pressure, radon forms a monatomic gas with a density of 9. 73 kg / m < sup > 3 </ sup >, about 8 times the density of the Earth's atmosphere at sea level, 1. 217 kg / m < sup > 3 </ sup >.

:* is the dimensionless specific heat capacity at constant volume, ≈ 3 / 2 for monatomic gas, 5 / 2 for diatomic gas and 3 for more complex molecules.

Thermal energy is most easily defined in the context of the ideal gas, which is well approximated by a monatomic gas at low pressure.

In 1876 at Strasbourg in collaboration with Emil Warburg, Kundt proved that mercury vapor is a monatomic gas.

The temperature of an ideal monatomic gas is a measure of the average kinetic energy of its atoms.

being the specific heat for constant volume, is the adiabatic index, and is the number of degrees of freedom ( 3 for monatomic gas, 5 for diatomic gas ).

where is the ratio of specific heats (= 5 / 3 for a monatomic gas ), the universal gravitational constant, the mass of the central star, the mass of the Sun,

A related situation occurs in hypersonic aerodynamics, where dissociation causes an increase in the “ notational ” molar volume, because a mole of oxygen, as O < sub > 2 </ sub >, becomes 2 moles of monatomic oxygen and N < sub > 2 </ sub > similarly dissociates to 2N.

For an ideal monatomic gas, the entropy as a function of the internal energy U, volume V, and number of moles n is given by the Sackur – Tetrode equation:

For example, the melting point, the first three ionization energies, and the hydration energy are greater than those of neodymium and lower than those of samarium ; similarly, the estimate for the boiling point, ionic ( Pm < sup > 3 +</ sup >) radius, and standard heat of formation of monatomic gas are greater than those of samarium and less those of neodymium.

Monatomic gas heat capacities per atom ( not per molecule ) are decreased by a factor of 2 with regard to solids, due to loss of half of the potential degrees of freedom per atom for storing energy in a monatomic gas, as compared with regard to an ideal solid.

For a monatomic ideal gas, when the convection is adiabatic, meaning that the convective gas bubbles don't exchange heat with their surroundings, mixing length theory yields

Since the specific heat of a monatomic gas is 3 / 2, this is the same as the thermodynamic formula for the entropy.

In everyday life on Earth, isolated hydrogen atoms ( usually called " atomic hydrogen " or, more precisely, " monatomic hydrogen ") are extremely rare.

John Dalton's original atomic hypothesis assumed that all elements were monatomic and that the atoms in compounds would normally have the simplest atomic ratios with respect to one another.

Helium is the least reactive noble gas after neon and thus the second least reactive of all elements ; it is inert and monatomic in all standard conditions.

Helium is the least water soluble monatomic gas, and one of the least water soluble of any gas ( CF < sub > 4 </ sub >, SF < sub > 6 </ sub >, and C < sub > 4 </ sub > F < sub > 8 </ sub > have lower mole fraction solubilities: 0. 3802, 0. 4394, and 0. 2372 x < sub > 2 </ sub >/ 10 < sup >− 5 </ sup >, respectively, versus helium's 0. 70797 x < sub > 2 </ sub >/ 10 < sup >− 5 </ sup >), and helium's index of refraction is closer to unity than that of any other gas.

To date, the critical density is estimated to be approximately five atoms ( of monatomic hydrogen ) per cubic metre, whereas the average density of ordinary matter in the Universe is believed to be 0. 2 atoms per cubic metre. Estimated relative distribution for components of the energy density of the Universe.

For a monatomic ideal gas,, and for a diatomic gas ( such as nitrogen and oxygen, the main components of air ).

Oxidizing species include singlet oxygen, hydroxyl, monatomic oxygen, and hydroperoxyl.

With an average atomic weight of ( for hydrogen-1 ), hydrogen is the lightest element and its monatomic form ( H < sub > 1 </ sub >) is the most abundant chemical substance, constituting roughly 75 % of the Universe's baryonic mass.

It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table.

In 1910, Lord Rayleigh discovered that an electrical discharge in nitrogen gas produced " active nitrogen ", an allotrope considered to be monatomic.

Neon is a colorless, odorless monatomic gas under standard conditions, with about two-thirds the density of air.

Neon is monatomic, making it lighter than the molecules of diatomic nitrogen and oxygen which form the bulk of Earth's atmosphere ; a balloon filled with neon will rise in air, albeit more slowly than a helium balloon.

Using a plasma source, a monatomic reactive species is generated.

Structure of a classical monatomic liquid.

# For monatomic ions, the OS is the same as the charge of the ion.