Monatomic Gases

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A monatomic gas a gas composed of particles molecules that generally consist of single atoms such as helium or sodium vapour. They are different from diatomic triatomic or we can say in general polyatomic gases. The thermodynamic behaviour of a monatomic gas in the ordinary temperature range is extremely simple because it is free from the rotational and energy vibrational components characteristic of polyatomic gases. Thus its heat capacity is independent of temperature and molecular or atomic weight and its entropy (that is is a measure that of the disorder depends only on temperature and molecular weight).


Monatomic Gas Examples

Perfect gas is also known ideal gas is a gas that conforms in physical behaviour to a particular idealized relation between pressure and volume and temperature, known as the general gas law. This law which we know is a generalization containing both Boyle’s the law and the law of Charles’s as special cases and states that we already know for a specified quantity of gas. T the product of the volume denoted by letter V and pressure denoted by letter P is proportional to the absolute temperature T in equation form PV = kT. is in which letter k is a constant. We can here say that such is a relation for a substance known as its equation of state and is sufficient to describe its gross behaviour.

The general law of gas can be derived from the kinetic theory of gases and relies on the assumptions - 

  • The gas consists of a large number of molecules which we can say are in random motion and obey Newton’s laws of motion.

  • The volume of the molecules is negligibly small compared to the volume occupied by the gas No forces act. 

Although we can here note that no gas has these properties, the behaviour of real gases is described quite closely by the general gas law at sufficiently high temperatures A gas which we know already does not obey the equation when conditions are such that the gas or any of the component gases in a mixture near its condensation point the temperature at which it liquefies.


The general law is written in a form applicable to any gas according to the law of the Avogadro’s if the constant specifying the quantity of gas is expressed in terms of the number of molecules of gas. This is done by using as the mass the unit and the gram-mole, the molecular weight expressed in grams. The equation of state of letter n that denotes gram-moles of a perfect gas can then be written as pv/t = nR, in, R is known as the universal gas constant.


Noble Gases are Monatomic

In subjects like physics and chemistry  "monatomic" a combination of the words "mono" and "atomic", means "single atom". It is usually applied to the gases monatomic in nature, in which atoms are not bound to each other. We can take examples at standard conditions which generally include the noble gases that are listed as argon, krypton, and xenon. Though we can see that in all chemical elements which will be monatomic in the gas phase at sufficiently high temperatures. The behaviour thermodynamic of a gas monatomic is extremely simple when compared to polyatomic gases is because it is free of any rotational or vibrational energy.


The only chemical elements that we can say are stable for the single atom molecules that at standard temperature and pressure (STP) are the noble gases. Helium, neon, argon, krypton, xenon, and radon etc. the gases which are the noble gases have a full outer valence shell making them rather non-reactive species. While these elements have been described historically as completely inert elements in the branch of chemical compounds have been synthesized with all but neon and helium.


When these all are is grouped together with the homonuclear diatomic gases such as nitrogen denoted by N2 the noble gases are known as "elemental gases" or  the "molecular gases" to distinguish them from molecules that are also chemical compounds.


What is Monatomic Gas

The term ‘monatomic’ is is a combination of two words “mono” and “atomic” which generally means a single atom. This term is used in both Physics and Chemistry and is then is applied to the gases as a monatomic gas. In the gaseous phase that we already know is at sufficiently high temperatures all the chemical elements which are monatomic gases.


The noble gases are monatomic as they are unreactive in nature, a property of these gases. These glasses are to do find applications in daily life like:

  • The gas helium used in filling balloons as their density is lower than that of the air.

  • The gas neon generally used for creating advertising signs as they glow when electricity flows through it.

  • The gas argon is used in a light bulb to prevent burning of the filament as it is unreactive in nature.

The molecules that are diatomic are those molecules that are composed of only two atoms.

FAQ (Frequently Asked Questions)

Q1. Is Hydrogen a Monatomic Gas?

Ans: There are some of the elements which are monatomic, that means they are made of a single mon- atom -atomic in their molecular form. The gas helium is an example of a monatomic element. Hydrogen H2 and then the element oxygen O2 and chlorine Cl2 molecules for example that each contains two atoms. Thus, hydrogen is not monoatomic.

Q2. Explain What are Monatomic and Diatomic Gases.

Ans: In a monatomic gas,t it only has one molecule, the ways for it to have energy will be less than a diatomic gas since  a diatomic gas generally has more ways to have energy

Q3. How Do You Identify a Monatomic Gas?

Ans: Monatomic is a combination of two words that are “mono” and “atomic” which means a single atom. This term that we have seen is used in both subjects Physics and Chemistry and is applied to the gases which are as a monatomic gas.

Q4. Is Nitrogen Monatomic or Diatomic?

Ans: There are hundreds of monoatomic molecules that generally have been identified in the environment of the Earth in the laboratory and in interstellar space which we know already. About 99% of the planet earth atmosphere is composed of two species of diatomic molecules that are nitrogen 78% and oxygen 21%.