Before spectroscopy was invented, Korner's absolute method had been used to determine whether a disubstituted benzene derivative was the ortho, meta, or para isomer. Korner's method involves adding a third group (often a nitro group) and determining how many isomers are formed. A turn-of-the-20th-century chemist isolated an aromatic compound of molecular formula C6H4Br2. He carefully nitrated this compound and purified three isomers of formula C6H3Br2NO2, one of which was obtained in very small amounts. Propose a structure for the original compound.

Answer is explained in the explanation section below.

Explanation:

Definitely, before the invention of spectroscopy, Korner's absolute method had been in use. So, the proposed structure is a diagram, that we can not draw here. Therefore, I have drawn it separately and attached in the attachment below. Please for this answer, refer to the diagram which is a proposed structure attached in the attachment below.

Proposed Structure is:

1,3-Dibromo Benzene.

In the Korner's method, 1,3- Dibromo benzene undergoes nitration process in the presence of HNO3/H2SO4.

The original compound that undergoes nitration is a meta disubstituted aromatic compound.

After nitration, 1,3-Dibromo benzene yields three having chemical formula C6H3Br2NO2.

Yes it would be a physical change.

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.