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

Question 1. Hydrogen atom emits blue light when it jumps from n = 4 energy level to the n = 2 level. Which colour of light would the atom emit when it changes from the n = 5 level to the n = 2 level?

Red
Yellow
Green
Violet

Explanation:-

Answer: Option D. -> Violet
:
D

E_{5} - E_{1} = Δ E_{51}

Δ E_{51} > Δ E_{42}

Violet color has more energy than Blue color while all colors in the option have less energy than Blue color

∴

Violet

Question 2. After two hours, one- sixteenth of the starting amount of a certain radioactive isotope remained undecayed. The half life of the isotope is

15 minutes
30 minutes
45 minutes
1 hour

Explanation:-

Answer: Option B. -> 30 minutes
:
B

\frac{N}{N_{0}} = {(\frac{1}{2})}^{\frac{t}{T}} \Rightarrow (\frac{1}{16}) = {(\frac{1}{2})}^{\frac{2}{T}} \Rightarrow {(\frac{1}{2})}^{4} = {(\frac{1}{2})}^{\frac{2}{T}}

\Rightarrow

T = 0.5 hour = 30 minutes.

Question 3. If Avogadro's number is

6 \times 10^{23}

then the number of protons, neutrons and electrons in 14 g of

_{6} C^{14}

are respectively

36×1023,48×1023,36×1023
36×1023,36×1023,36×1021
48×1023,36×1023,48×1021
48×1023,48×1023,36×1021

Explanation:-

Answer: Option A. -> 36×1023,48×1023,36×1023
:
A
Since the number of protons, neutrons and electrons in an atom of

_{6} C^{14}

are 6, 8 and 6 respectively. As 14 gm of

_{6} C^{14}

contains

6 \times 10^{23}

atoms, therefore the numbers of protons, neutrons and electrons in 14 gm of

_{6} C^{14}

are

6 \times 6 \times 10^{23}, = 36 \times 10^{23}, 8 \times 6 \times 10^{23} = 48 \times 10^{23}, 6 \times 6 \times 10^{23} = 36 \times 10^{23}

.

Question 4. On the bombardment of a neutron with Boron.

α

-particle is emitted and product nuclei formed is

6C12
3Li6
3Li7
4Be9

Explanation:-

Answer: Option C. -> 3Li7
:
C

_{5} B^{10} +_{0} n^{1} \to_{3} L i^{7} +_{2} H e^{4}

.

Question 5. The radius of the first orbit of hydrogen in

0.528 ˙ A

. The radius of second orbit of hydrogen is

4.752˙A
2.112˙A
0.071˙A
0.142˙A

Explanation:-

Answer: Option B. -> 2.112˙A
:
B

r_{n} \propto n^{2}

\frac{r_{2}}{r_{1}} = \frac{2^{2}}{1^{2}} = 4

r_{2} = 4 r

= 4 \times 0.528

= 2.112 ˙ A

Question 6. An electron in ground state of hydrogen has an angular momentum

L_{1}

and an electron in the first excited state of Lithium has an angular momentum

L_{2}

. Then

L1=L2
L1=4L2
L1=L22
L1=2L2

Explanation:-

Answer: Option C. -> L1=L22
:
C

L = \frac{n h}{2 π}

Hence

L_{1} \propto n_{1}

L_{2} \propto n_{2}

\frac{L_{2}}{L_{1}} = \frac{n_{2}}{n_{1}} = \frac{2}{1}

L_{1} = \frac{L_{2}}{2}

Question 7. The circumference of first orbit of hydrogen atom is ‘s’. Then the Broglie wavelength of electron in that orbit is

S2
2S
S
3S

Explanation:-

Answer: Option C. -> S
:
C

2 π r_{n} = n λ

n = 1

2 π r_{1} = λ

λ = 2 π r_{1} = S

Question 8. The ratio of wave length of first line of Lyman series to the first line of Balmer series is

1 : 4
5 : 27
27 : 20
20 : 27

Explanation:-

Answer: Option B. -> 5 : 27
:
B

λ_{1} = \frac{4}{3 R}

λ_{2} = \frac{36}{5 R}

\frac{λ_{1}}{λ_{2}} = \frac{4}{3} \times \frac{5}{36} = \frac{5}{27}

Question 9. Energy levels A,B,C of an atom are shown below

Energy Levels A,B,C Of An Atom Are Shown BelowIf EA<EB<...

If

E_{A} < E_{B} < E_{C}

, the wavelength emitted due to transistion from C to A is

λ_{3}

then correct statements of the following is

λ3=λ1+λ2
λ3=λ1×λ2λ2+λ1
λ1+λ2+λ3=0
λ23=λ21+λ22

Explanation:-

Answer: Option B. -> λ3=λ1×λ2λ2+λ1
:
B

E_{C A} = E_{C B} + E_{B A}

\frac{h c}{λ_{3}} = \frac{h c}{λ_{1}} + \frac{h c}{λ_{2}}

λ_{3} = \frac{λ_{1} \times λ_{2}}{λ_{2} + λ_{1}}

Question 10. 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
n2

Explanation:-

Answer: Option D. -> n2
:
D
Bohr radius

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