The number of following statement/s which is/are incorrect is ___________
(A) Line emission spectra are used to study the electronic structure
(B) The emission spectra of atoms in the gas phase show a continuous spread of wavelength from red to violet
(C) An absorption spectrum is like the photographic negative of an emission spectrum
(D) The element helium was discovered in the sun by spectroscopic method

Solution

Chemistry ma’am · 2 steps

Atoms in a gas give sharp lines, not a rainbow, so only one statement here is wrong.

1. Line spectrum idea. An isolated gaseous atom can only jump between fixed energy levels. So it emits or absorbs only certain wavelengths, and you see separate bright lines with dark gaps between them.

   Statement	Verdict	Reason
   (A) Line spectra study electronic structure	correct	The line positions reveal the energy gaps between electron levels.
   (B) Gas-phase atoms give a continuous spread	incorrect	Gaseous atoms give a line spectrum. A continuous red-to-violet spread comes from white light or hot solids.
   (C) Absorption is the negative of emission	correct	Dark lines appear exactly where the emission spectrum has bright lines.
   (D) Helium found in the sun by spectroscopy	correct	Lockyer saw an unknown yellow line in sunlight in 1868, before helium was found on Earth.

2. So. Only statement (B) is incorrect, so the number of incorrect statements is 1.

The common slip: 2. Marking (C) as incorrect as well gives 2. An absorption spectrum really does show dark lines exactly where the emission spectrum shows bright ones.

The idea to keep. Free gaseous atoms give line spectra, each element its own fingerprint. Continuous spectra come from white light or hot dense matter, never from isolated atoms.

Answer from the NTA final answer key. Solution by PrepWiser.