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10
MCQ (Multiple Correct Answer)

The Bohr model for the spectra of a H -atom

A
will not be applicable to hydrogen in the molecular from
B
will not be applicable as it is for a He -atom
C
is valid only at room temperature
D
predicts continuous as well as discrete spectral lines
11
MCQ (Multiple Correct Answer)

The Balmer series for the H -atom can be observed

A
if we measure the frequencies of light emitted when an excited atom falls to the ground state
B
if we measure the frequencies of light emitted due to transitions between excited states and the first excited state
C
in any transition in a H -atom
D
as a sequence of frequencies with the higher frequencies getting closely packed
12
MCQ (Multiple Correct Answer)

Let $E_n=\frac{-1 m e^4}{8 \varepsilon_0^2 n^2 h^2}$ be the energy of the $n$th level of H -atom. If all the H -atoms are in the ground state and radiation of frequency $\frac{\left(E_2-E_1\right)}{h}$ falls on it,

A
it will not be absorbed at all
B
some of atoms will move to the first excited state
C
all atoms will be excited to the $n=2$ state
D
no atoms will make a transition to the $n=3$ state
13
MCQ (Multiple Correct Answer)

The simple Bohr model is not applicable to $\mathrm{He}^4$ atom because

A
$\mathrm{He}^4$ is an inert gas
B
$\mathrm{He}^4$ has neutrons in the nucleus
C
$\mathrm{He}^4$ has one more electron
D
electrons are not subject to central forces
14
Subjective

The mass of a H -atom is less than the sum of the masses of a proton and electron. Why is this?

Explanation

Since, the difference in mass of a nucleus and its constituents, $\Delta M$, is called the mass defect and is given by

$$\Delta M=\left[Z m_p+(A-Z) m_n\right]-M$$

Also, the binding energy is given by $B=$ mass defect $(\Delta M) \times c^2$.

Thus, the mass of a H -atom is $m_p+m_e-\frac{B}{c^2}$, where $B \approx 13.6 \mathrm{eV}$ is the binding energy.

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