Question

Atoms of elements other than noble gases are assumed to have unstable electronic configuration

Answer 1

An atom is supposed to contain shells for electrons.  The shells are energy states for the electrons.  The shells are  called K, L, M, N etc. They correspond to principal quantum number n=1,2,3,4,5 etc.

The maximum number of electrons in each shell is equal to 2 n².

The energy of electrons is associated with this value of n - quantum number.  The difference of energy between two successive shells is high. 

The atoms which have the maximum number of electrons in its outermost shells, are supposed to be stable.  As, the attractive forces of nucleus on electrons is stable.  Then those atoms do not attract electrons from another nearby atoms.  Hence they do not react chemically with other atoms of the same matter or other substance.

Noble gases have 2n² number of electrons in their outermost shell. Hence they are stable.  They remain as atoms.

Other elements, want to give up electrons to reduce to having outermost electrons being full in their lower quantum number shell.  Or, they want to get more electrons to make their orbitals & shell complete to 2n² capacity.  Hence they are reactive.  That is why they form molecules and compounds.  So they are called unstable.

Answer 2

Answer:

I am writing in short way

Explanation:

atoms of element other then noble gas are assumed to have unstable electronic configuration by losseing electron from its outer most shell to attain stability of nearest noble gas .

For example -

magnesium atomic no is 12

nearest noble gas is neon [ 2,8]

looses two electron and becomes magnesium ion . mg 2+

chlorine atomic no is 17

nearest noble gas is argon [ 2,8,8]

accepts one electron and become chloride ion

so ,magnesium atom combine With two chlorine atoms to give Mgcl2 .

i hope this will be helpful .