By observing the values of different ionisation energies, I1​,I2​ and I3​ for atomic number (n+2), it is observed that there is very large difference between the second ionisation energy and first ionisation energy (I2​>I1​).This indicates that number of valence shell electrons is 1 and atomic number (n+2) should be an alkali metal.Also for atomic number (n+3),I3​>>I2​.This indicates that it will be an alkaline earth metal which suggests that atomic number (n+1) should be a noble gas and atomic number (n) should belong to halogen family. Since, n<10, hence, n=9( F atom)