Why is argon gas inert

Gases such as helium, neon and argon are so unreactive that we call them the inert gases. They form no chemical compounds, and their only interactions are of the London dispersion force type. They cannot form hydrogen bonds, since they are not able to bond with hydrogen and are not electronegative. 

The outer shell of the helium atom is full and complete the shell can only accept two electrons and, indeed, is occupied by two electrons. Similarly, argon has a complete octet of electrons in its outer shell. Further reaction would increase the number of electrons if argon were to undergo a covalent bond or become an anion, or would decrease the number of electrons below the perfect eight if a cation were to form. There is no impetus for reaction because the monatomic argon is already at its position of lowest energy, and we recall that bonds form in order to decrease the energy. 

But if we look at an element like magnesium, there are several ionization processes possible  

The energy change in reaction (1) is called the first ionization energy and the energy associated with reaction (2) is the second ionization energy. We symbolize the two processes as I( ) and 1(2) respectively. 

It is clear from Table 2.8 that each ionization energy is larger than the one before. Also note that the last two ionization energies of an element are always larger than the others. The sudden rise follows because the last two energies represent the removal of the two 1 s electrons removal of electrons from the 2s and 2p orbitals is easier. 

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