CBSE 2025, Set 1

Question

**Assertion (A):** The Balmer series in hydrogen atom spectrum is formed when the electron jumps from higher energy state to the ground state.

**Reason (R):** In Bohr's model of hydrogen atom, the electron can jump between successive orbits only. A). Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). B). Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). C). Assertion (A) is true, but Reason (R) is false. D). Both Assertion (A) and Reason (R) are false.

Edvaya · Accepted Answer

The correct answer is Both Assertion (A) and Reason (R) are false.. **Assertion (A) — FALSE:** The **Balmer series** corresponds to transitions where the electron jumps from a higher orbit ($n = 3, 4, 5, \ldots$) to the **second orbit ($n = 2$)**, NOT to the ground state ($n = 1$). The spectral series of hydrogen: | Series | Final orbit | Region | |---|---|---| | Lyman | $n = 1$ (ground state) | UV | | **Balmer** | **$n = 2$** | **Visible/UV** | | Paschen | $n = 3$ | Infrared | | Brackett | $n = 4$ | Far IR | So Assertion A is **incorrect** — Balmer transitions end at $n=2$, not $n=1$. **Reason (R) — FALSE:** In Bohr's model, electrons can jump between **any** two orbits — not just successive ones. The frequency of emitted radiation is determined by the energy difference between any two allowed orbits: $$h
u = E_i - E_f$$ The restriction is only that orbits must be quantised ($mvr = nh/2\pi$). Jumps can skip multiple orbits. **Answer: (D) — Both A and R are false.** > **Key revision:** Memorise the series names and their terminal orbits (n=1 for Lyman, n=2 for Balmer, n=3 for Paschen). The Balmer series is the only one partially in visible light.

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