Question

The elements $Na, Mg, Al, Si, P, S, Cl\,and\,Ar$ are arranged in the periodic table in the increasing order of their atomic numbers.
Which element is least reactive?

Answer 1

In the periodic table the elements which have the same properties are arranged in the same group. Groups are the vertical columns with the same number of electrons in the outermost shell. Elements only react with other elements to complete their octet to acquire stable configuration.

Complete answer:
All the elements of the periodic table want to attain stability. Elements attain stability when their octet is complete i.e. outermost shell of the element is complete.
In the periodic table all the Noble gases are having the complete outermost shell so they do not react with any other element to attain stability because they are already stable elements.
In the given statement $Ar$ is the Noble gas or Inert gas. It is present in the zero group which contains the inert gases. So $Ar$ is the least reactive among others. Its atomic no. is $18$ and mass no is $40$
Periodicity occurs in the properties of elements which have similar electron configurations i.e. same number of electrons in their outermost orbit.

Note: $He, Ar, Kr, Xe$ etc. are the Noble gases. As we know that Noble gases do not react with any other metals, but $Xe$ is capable of forming varieties of compound like $Xe{{F}_{2}},Xe{{F}_{4}}, Xe{{F}_{6}}$ etc. Inert gases show very less participation in any chemical reactions. They have their outermost orbits filled so their tendency to gain or lose electrons is very less. Inert gases are mostly non-toxic in nature but they can be proved harmful if they replace higher levels of oxygen. Gold is also said to be chemically inert. Helium is considered one of the most inert out of all others.