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Atoms — Physics STD 12 Science — Question

Rajasthan BoardEnglish MediumSTD 12 SciencePhysicsAtoms1 Mark
MCQ
For question two statements are given $-$ one labelled Assertion $(A)$ and the other labelled Reason $(R)$. Select the correct answer to these questions from the codes $(a), (b), (c)$ and $(d)$ as given below.
Assertion $(A)$ : Hydrogen atom consists of only one electron but its emission spectrum has many lines
Reason $(R)$ : Only Lyman series is found in the absorption spectrum of hydrogen atom whereas in the emission spectrum, all the series are found.
  • A
    Both $A$ and $R$ are true, and $R$ is the correct explanation of $A$.
  • ✓
    Both $A$ and $R$ are true, but $R$ is $\text{NOT}$ the correct explanation of $A$.
  • C
    $A$ is true, but $R$ is false.
  • D
    $A$ is false and $R$ is also false.
✓

Answer

Correct option: B.
Both $A$ and $R$ are true, but $R$ is $\text{NOT}$ the correct explanation of $A$.
Every atom has certain definite energy level. In the normal state, the electron in the hydrogen atom stays in lowest energy level.
When the atom gets appropriate energy from outside, then this electron rises to some higher energy level i.e., atom is excited.
Within nearly $10^{-8} \sec$, the electron leaves the higher energy level.
Now, it can return either directly to the lowest energy level $($or the ground state$)$ or come to the ground state after passing through other lower energy levels.
Since there are a large number of atoms in a light $-$ source $($hydrogen lamp$),$ hence all possible transitions take place in the source and many lines are seen in the spectrum.
The slit gives the shape of the spectrum and the large number of lines are obtained because a large number of atoms are getting excited and de $-$ excited to different energy levels.

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