Atoms(11th And 12th > Physics ) Questions and answers for exam Preparation

11th And 12th > Physics

ATOMS MCQs

Total Questions : 30 | Page 1 of 3 pages

In the Bohr's hydrogen atom model, the radius of the stationary orbit is directly proportional to (n = principle quantum number)

$n^{- 1}$
$n$
$n^{- 2}$
$n^{2}$

Discuss Question

Answer: Option D. ->

n^{2}

:
D
Bohr radius

r = \frac{\sum_{0} n^{2} h^{2}}{π Z m e^{2}}; ∴ r \infty n^{2}

Question 2.

In the $n^{t h}$ orbit, the energy of an electron $E_{n} = - \frac{13.6}{n^{2}} e V$ for hydrogen atom. The energy required to take the electron from first orbit to second orbit will be

10.2 eV
12.1 eV
13.6 eV
3.4 eV

Discuss Question

Answer: Option A. -> 10.2 eV
:
A
The energy in the first orbit is - 13.6eV

n = 2

---------------

E_{2} = - \frac{13.6}{(2)^{2}} = - 3.4 e V

E_{1 \to 2} = - 3.4 - (13.6) = + 10.2 e V

Question 3.

In the following atoms and molecules for the transition from n = 2 to n = 1, the spectral line of minimum wavelength will be produced by

Hydrogen atom
Deuterium atom
Uni-ionized helium
double -ionized lithium

Discuss Question

Answer: Option D. -> double -ionized lithium
:
D

\frac{1}{λ} = R Z^{2} (\frac{1}{1^{2}} - \frac{1}{2^{2}})

For double-ionised lithium the value of Z is maximum.

Question 4.

Hydrogen atom in its ground state is excited by radiation of wavelength $975 \circ A$ . How many lines will be there in
the emission spectrum

Discuss Question

Answer: Option C. -> 6
:
C
Using

\frac{1}{λ} = R [\frac{1}{n_{1}^{2}} - \frac{1}{n_{2}^{2}}] \Rightarrow \frac{1}{975 \times 10^{- 10}} = 1.097 \times 10^{7} (\frac{1}{1^{2}} - \frac{1}{n^{2}}) \Rightarrow n = 4

Now number of spectral lines

N = \frac{n (n - 1)}{2} = \frac{4 (4 - 1)}{2} = 6

Question 5.

A photon of energy 12.4 eV is completely absorbed by a hydrogen atom initially in the ground state so that it is excited. The quantum number of the excited state is

n = 1
n = 3
n = 4
n = $\infty$

Discuss Question

Answer: Option C. -> n = 4
:
C
Let electron absorbing the photon energy reaches to the excited state n. Then using energy conservation

\Rightarrow - \frac{13.6}{n^{2}} = - 13.6 + 12.4 \Rightarrow - \frac{13.6}{n^{2}} = - 1.2 \Rightarrow n^{2} = \frac{13.6}{1.2} = 12 \Rightarrow n = 3.46 ≃ 4

Question 6.

The wave number of the energy emitted when electron comes from fourth orbit to second orbit in hydrogen is $20, 397 c m^{- 1}$ . The wave number of the energy for the same transition in $H e^{+}$ is

$5, 099 c m^{- 1}$
$20, 497 c m^{- 1}$
$40, 994 c m^{- 1}$
$81, 588 c m^{- 1}$

Discuss Question

Answer: Option D. ->

81, 588 c m^{- 1}

:
D
Using

\frac{1}{λ} = ¯ v = R Z^{2} (\frac{1}{n_{1}^{2}} - \frac{1}{n_{2}^{2}}) \Rightarrow ¯ v \infty Z^{2} \Rightarrow \frac{_{2}}{_{1}} = {(\frac{Z_{2}}{Z_{1}})}^{2} = {(\frac{Z}{1})}^{2} = 4 \Rightarrow_{2} = ¯ v \times 4 = 81588 c m^{- 1}

Question 7.

In an atom, the two electrons move round the nucleus in circular orbits of radii R and 4R. The ratio of the time taken by them to complete one revolution is

$\frac{1}{4}$
$\frac{4}{1}$
$\frac{8}{1}$
$\frac{1}{8}$

Discuss Question

Answer: Option D. ->

\frac{1}{8}

:
D
Time period

T \propto \frac{n^{3}}{Z^{2}}

For a given atom (Z = constant) So

T \propto n^{3}

......... (i) and radius

R \propto n^{2}

......... (ii)

∴

From equation (i) and (ii)

T \propto R^{3 / 2} \Rightarrow \frac{T_{1}}{T_{2}} = {(\frac{R_{1}}{R_{2}})}^{3 / 2} = {(\frac{R}{4 R})}^{3 / 2} = \frac{1}{8}

Question 8.

Ionisation energy for hydrogen atom in the ground state is E. What is the ionisation energy of $L i^{+ +}$ atom in the $2^{n d}$ excited state

Discuss Question

Answer: Option A. -> E
:
A
Ionisation energy of atom in nth state

E_{n} α \frac{Z^{2}}{n^{2}}

For hydrogen atom in ground state (n = 1) and Z = 1

\Rightarrow E = E_{0}

....... (i)
For

L i^{+ +}

atom in

2^{n d}

excited state n = 3 and Z = 3, hence

E^{^{'}} = \frac{E_{0}}{3^{2}} \times 3^{2} = E_{0}

........ (ii)
From equation (i) and (ii)

E^{^{'}} = E

Question 9.

An electron in a hydrogen atom makes a transition $n_{1} \to n_{2}$ where $n_{1}$ and $n_{2}$ are principal quantum numbers of the states. Assume the Bhor's model to be valid. The time period of the electron in the initial states is eight times to that of final state. What is ratio of $\frac{n_{1}}{n_{2}}$

8 : 1
4 : 1
2 : 1
1 : 2

Discuss Question

Answer: Option C. -> 2 : 1
:
C

T_{n} \propto n^{3}

n \propto T^{\frac{1}{3}}

\frac{n_{1}}{n_{2}} = {[\frac{T_{1}}{T_{2}}]}^{\frac{1}{3}} = {[\frac{8}{1}]}^{\frac{1}{3}}

n_{1} : n_{2} = 2 : 1

Question 10.

Any radiation in the ultra violet region of Hydrogen spectrum is able to eject photoelectrons from a metal. Then the threshold frequency of the metal for a threshold $λ = \frac{1}{R} = 911 ˙ A$ ,is nearly