# Atoms | class 12 important short questions with answers

### Important short previous year questions, frequently asked in CBSE board exams.

Atoms | class 12 important short questions with answers, this article contains the frequently asked important questions of CBSE board exams. these questions have been asked in previous 10 years so frequently. They are also important from an examination point of view. Practice these questions for better preparation. Good luck guys!

#### Atoms Short questions with answers:

Question 1:Thomson’s model of the atom is also called a plum pudding model . Why?

Solution. Thomson’s model is also called plum pudding model because the electrons are assumed to be uniformly embedded in a sphere of positively charged matter like the plums are distributed in a pudding.

Question 2. Why was wrong with Thomson’s atomic model ? (Punjab 041)

Solution. Thomson model was rejected because it was not able to explain the scattering of alpha particles through large angles in Rutherford’s gold foil experiment.

Question 3. Write any two important conclusions drawn from Rutherford’s alpha particle scattering experiment. ICE3SE OD 05C]

Solution. (i) Most of the mass and the positive charge of an atom is concentrated in a very small volume of the atom, which is called nucleus.

(ii) The radius of nucleus is about 1/10,000 of the atomic radius.

Question 4. Why does the mass of the nucleus does not affect the formula for impact parameter, but its charge does ?

Solution. It is because the scattering occurs due to the electrostatic field of the nucleus. which is due to the positive charge of nucleus. That is why charge of nucleus enters the expression for the impact parameter.

Problem 5. Why do we use a very thin gold foil in Rutherford’s alpha particle scattering experiment ?

Solution. If we use a thick foil, the kinetic energy of the a-particles will be absorbed and then the alpha particles will not be able to pass through the foil.

Question 6. If the kinetic energy of alpha particle incident on gold foil is doubled. How will it impact the distance of closest approach ? (CEISE OD 121)

Solution. Since the distance of closest approach is inversely proportional to the kinetic energy of the incident alpha particle, so the distance of closest approach will be halved if the kinetic energy of alpha particle is doubled.

Question 7 why do we get large angle scattering only due to collision with nucleus?

Solution. It is because, the alpha particles can be scattered through large angles only if they collide with a positively charged particle such as a nucleus.

Question 8. In The Rutherford’s alpha particle scattering experiment we see that most of the alpha particles pass through the foil without much deflection. while some of them get deflected through large angles. What does this tells about the structure of atom ?

Solution. (i) Since most of alpha particles pass straight, this indicated that most of the atom’s space is empty.

(ii) The large deflections indicates that most of the mass and charge of an atom is concentrated in a very small volume of the atom, which is called nucleus.

Problem 9. Why does electron revolve around the nucleus of an atom ? (Haryana 111)

Solution. If the electrons do not revolve, they would fall into the nucleus due to the electrostatic force of attraction and the atom would be unstable.

Question 10. In the Rutherford’s alpha particle scattering experiment the distance of closest approach for an $\alpha$-particle is $d_{0}$. If $\alpha$-particle is replaced by a proton, how much kinetic energy in comparison to $\alpha$-particle will it require to have the same distance of closest approach $d_{0}$?

Solution.

At the distance of closest approach,
$\therefore \quad K_{p}=\frac{1}{2} K_{\alpha}$. Thus, a proton would need half the initial K.E. of that

\begin{aligned} &K_{\alpha}=\frac{k Z e .2 e}{d_{0}} \ &K_{p}=\frac{k Z e . e}{d_{0}} \ &K_{p}=\frac{1}{2} K_{\alpha} . \end{aligned}
and of an $\alpha$-particle for the distance $d_{0}$.

Question 11

Question 11 What does the negative energy of an electron signifies?

Solution. The negative energy of an electron signifies that the electron is attracted to the nucleus. This is due to the electrostatic force of attraction between electron and protons of nucleus, the Potential energy is negative and also greater than Kinetic energy of electron. Which results in the total energy of electron to be negative. so, It cannot escape from the atom.

Question 12. What are the stationary orbits of an electron in bohr’s atomic model?

Solution. According to the Bohr’s postulate, the electrons can revolve around the nucleus in specific discrete, non-radiating orbits such that the angular momentum of electron should be the integral multiple of $h / 2 \pi$. Such orbits are known as stationary orbits.

Question 13. State the conditions of Bohr’s quantization in terms of de-Broglie wavelength.

Solution. The condition of Bohr’s quantization is that only those circular orbits are stationary orbits of an electron which are integral multiple of de-Broglie wavelengths.

Question 14. State Bohr’s postulate of quantization of angular momentum for an electron in hydrogen atom. [CBSE D 09C]

Solution. According to Bohr’s postulates only such orbits are allowed for the revolution of an electron around the nucleus where the angular momentum of an electron is an integral multiple of

$h / 2 \pi$. mathematically
$L=m v r=\frac{n h}{2 \pi}, \quad n=1,2,3, \ldots$

Here n is the principal quantum number.

Question 15. If an electron falls from a higher energy level to a lower energy level, the change in the energies appears in the form of electromagnetic radiation. Why is it not emitted as any other form of energy ? [Exemplar Problem]

Solution. This is because electrons can interact only electro-magnetically.

#### Summary

In this article, we discussed 15 questions on the class 12 physics chapter atoms. these questions are important for any exam. If you have any doubt regarding any question please let us know in the comment section. contact us if you like us to publish some content of your interest, we are waiting for your suggestions. Thank you!