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Monatomic
 
 
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In physics
Physics

Physics is the natural science which examines basic concepts such as energy, force, and spacetime and all that derives from these, such as mass, charge, matter and its Motion ....
 and chemistry
Chemistry

Chemistry is the science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions....
, monatomic is a combination of the words "mono" and "atomic," and means "single atom
Atom

|-! bgcolor=gray | Properties|-||}The atom is a basic unit of matter consisting of a dense, central atomic nucleus surrounded by a electron cloud of electric charge electrons....
." It is usually applied to gas
Gas

In physics, a gas is a state of matter, consisting of a collection of particles without a definite shape or volume that are in more or less random motion....
es: a monatomic gas is one in which atoms are not bound to each other.

At standard temperature and pressure (STP), all of the noble gas
Noble gas

|}The noble gases are a group of chemical elements with very similar properties: under standard conditions, they are all odorless, colorless, monatomic gases, with a very low chemical reactivity....
es are monatomic. These are helium
Helium

Helium is a colorless, odorless, tasteless, non-toxic, inert monatomic chemical element that heads the noble gas group in the periodic table and whose atomic number is 2....
,neon
Neon

Neon is the chemical element that has the symbol Ne and atomic number 10. Although a very common element in the universe, it is rare on Earth....
, argon
Argon

Argon is a chemical element designated by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table ....
, krypton
KRYPTON

KRYPTON is a frame language computer programming language."An Essential Hybrid Reasoning System: Knowledge and Symbol Level Accounts of KRYPTON", R.J. Brachman et al, Proc IJCAI-85, 1985....
, xenon
Xenon

Xenon is a chemical element represented by the chemical symbol Xe. Its atomic number is 54. A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts....
 and radon
Radon

Radon is a chemical element with symbol Rn and atomic number 86. Radon is a colorless, odorless, tasteless, naturally occurring, radioactive noble gas that is formed from the decay of radium....
. The heavier noble gases can form compounds, but the lighter ones are unreactive. All elements will be monatomic in the gas phase at sufficiently high temperatures.

The only mode of motion of a monatomic gas is translation (electronic excitation is not important at room temperature).






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In physics
Physics

Physics is the natural science which examines basic concepts such as energy, force, and spacetime and all that derives from these, such as mass, charge, matter and its Motion ....
 and chemistry
Chemistry

Chemistry is the science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions....
, monatomic is a combination of the words "mono" and "atomic," and means "single atom
Atom

|-! bgcolor=gray | Properties|-||}The atom is a basic unit of matter consisting of a dense, central atomic nucleus surrounded by a electron cloud of electric charge electrons....
." It is usually applied to gas
Gas

In physics, a gas is a state of matter, consisting of a collection of particles without a definite shape or volume that are in more or less random motion....
es: a monatomic gas is one in which atoms are not bound to each other.

At standard temperature and pressure (STP), all of the noble gas
Noble gas

|}The noble gases are a group of chemical elements with very similar properties: under standard conditions, they are all odorless, colorless, monatomic gases, with a very low chemical reactivity....
es are monatomic. These are helium
Helium

Helium is a colorless, odorless, tasteless, non-toxic, inert monatomic chemical element that heads the noble gas group in the periodic table and whose atomic number is 2....
,neon
Neon

Neon is the chemical element that has the symbol Ne and atomic number 10. Although a very common element in the universe, it is rare on Earth....
, argon
Argon

Argon is a chemical element designated by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table ....
, krypton
KRYPTON

KRYPTON is a frame language computer programming language."An Essential Hybrid Reasoning System: Knowledge and Symbol Level Accounts of KRYPTON", R.J. Brachman et al, Proc IJCAI-85, 1985....
, xenon
Xenon

Xenon is a chemical element represented by the chemical symbol Xe. Its atomic number is 54. A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts....
 and radon
Radon

Radon is a chemical element with symbol Rn and atomic number 86. Radon is a colorless, odorless, tasteless, naturally occurring, radioactive noble gas that is formed from the decay of radium....
. The heavier noble gases can form compounds, but the lighter ones are unreactive. All elements will be monatomic in the gas phase at sufficiently high temperatures.

The only mode of motion of a monatomic gas is translation (electronic excitation is not important at room temperature). Thus in an adiabatic process
Adiabatic process

In thermodynamics, an adiabatic process or an isocaloric process is a thermodynamic process in which no heat is transferred to or from the working fluid....
, monatomic gases have an idealised ?-factor
Heat capacity ratio

The heat capacity ratio or adiabatic index or ratio of specific heats, is the ratio of the heat capacity at constant pressure to heat capacity at constant volume ....
 (Cp/Cv) of 5/3, as opposed to 7/5 for ideal diatomic
Diatomic

Diatomic molecules are molecules made only of two atoms, of either the same or different chemical elements. The prefix di- means two in Greek....
 gases where rotation (but not vibration at room temperature) also contributes. Also, for ideal monatomic gases:
the molar heat capacity at constant pressure (Cp) is 2.5 R = 20.8 J K-1 mol-1 (4.97 cal
Calorie

The calorie is a pre-SI metric system unit of energy. The unit was first defined by Professor Nicolas Cl?ment in 1824 as a unit of heat. This definition entered French and English dictionaries between 1841 and 1867....
 K-1 mol-1);
the molar heat capacity at constant volume (Cv) is 1.5 R = 12.5 J K-1 mol-1 (2.98 cal K-1 mol-1);
where R is the gas constant
Gas constant

The gas constant is a physical constant which is featured in a large number of fundamental equations in the physical sciences, such as the ideal gas law and the Nernst equation....
.