Physics · Atoms and nuclei · chapter 12 of 14

Class 12 Physics Chapter 12: Atoms Assertion-reason questions with answers

2 assertion-reason questions on Atoms, each with its answer and the reason, written from the NCERT chapter. For the CBSE board, JEE Main and NEET, which are set from the same NCERT chapter. Open a question's solution to check your working; then take the free test to find your next chapter. Each question gives an Assertion (A) and a Reason (R); choose how they stand to each other.

Question 1
Assertion (A): Bohr's model cannot be applied to the helium atom. Reason (R): In helium, the electric force between the two electrons is comparable to the force between each electron and the nucleus, and Bohr's model leaves it out.
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Answer: (a) Both A and R are true, and R is the correct explanation of A.

Bohr's model is limited to one-electron (hydrogenic) atoms. In helium the charges and distances involved are of the same order, so the electron-electron force is not small; the model has no place for it, which is exactly why it fails for helium. R explains A.

Question 2
Assertion (A): In the energy level diagram of hydrogen, the levels come closer together as n increases. Reason (R): The total energy of the electron in a hydrogen atom is negative, showing that the electron is bound to the nucleus.
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Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Both are true. The crowding follows from En ∝ 1/n²: the gaps En+1 − En shrink as n grows. The sign of the energy only says the electron is bound; a negative energy alone does not fix how the levels are spaced, so R does not explain A.

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