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energy of chemical bonds. The internal energy of a system can be changed
by heating the system or by doing work on it;
The internal energy is a state function of a system, because its value
depends only on the current state of the system and not on the path taken

or process undergone to arrive at this state. It is an extensive quantity. The
SI unit of energy is the joule (J). Some authors use a corresponding
intensive thermodynamic property called specific internal energy which

is internal energy per unit of mass (kilogram) of the system in question.
The SI unit of specific internal energy is J/kg. If intensive internal energy
is expressed relative to units of amount of substance (mol), then it is
referred to as molar internal energy and the unit is J/mol.

Thermodynamics often uses the concept of the ideal gas for teaching
purposes, and as an approximation for working systems. The ideal gas is a
gas of particles considered as point objects that interact only by elastic

collisions and fill a volume such that their free mean path between
collisions is much larger than their diameter. Such systems are
approximated by the monatomic gases, helium and the other noble gases.

Here the kinetic energy consists only of the translational energy of the
individual atoms. Monatomic particles do not rotate or vibrate, their
motion is purely translational. Therefore, they posses three numbers of

freedom i 3 , three coordinates XYZ (figure 2.2.1a) . Degree of
freedom is the number of variables required to describe the motion of a
particle completely. .
An ordinary gas like hydrogen or oxygen is in general diatomic

molecular gas. In this case, as seen in the following figure 2.2.1b , there
are 5 degrees of freedom; 3 of them are those of the center-of-mass motion
in the directions of the x-, y-, and z- axes, and the rest 2 are those of the

rotational motion around the center-of-mass, i.e. the angular degrees of
freedom  and  . For a linear molecule however, rotation around its
x
y
own axis is not considered to be rotation because it leaves the molecule
unchanged. So there are only 2 rotational degrees of freedom for any

linear molecule Therefore, the total number of degrees of freedom is i 5.
For a linear molecule however, rotation around its own axis is not
supposed to be rotation because it leaves the molecule unchanged. So,
there are only 2 rotational degrees of freedom for any linear molecule

A polyatomic gas (more then two atoms per molecule ,fig.2.2.1c)
generally has three possible axes of rotation (unless the three atoms lie in
straight line).Therefore, the total number of degrees of freedom i 6.

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