ATOMS
Very Short Question Answers
Q1. What is the angular momentum of electron in the second orbit of Bohr’s model of hydrogen atom?
Ans. Angular momentum $L = mvr = nh/2π $
$if \:\:n =2$
$L = 2h/2π = h/π$
Q2. What is the expression for fine structure constant and what is its value?
Ans. Fine structure constant $\alpha= e^{2}/(2\epsilon_{o} Ch) $ Value of $\alpha= 1/137$
Q3. What is the physical meaning of ‘negative energy of electron?
Ans. The negative sign for energy of an electron in an orbit means that
The electron is bound to the positive nucleus.
Q4. Sharp lines are present in the spectrum of a gas. What does this indicate?
Ans. Sharp lines in the spectrum of a gas indicate the electronic transitions in the atoms.
Q5. Name a physical quantity whose dimensions are the same as those of angular momentum.
Ans. Plank’s constant, Dimensional formula$ [ML^{2} T^{-1} ]$
Q6. What is the difference between $\alpha$-particle and helium atom?
Ans. $\alpha $- Particle is the nucleus of helium atom.
The doubly ionized helium atom is$\alpha $-particle. $\alpha $-particle is denoted as $ _{2}^{4}\textrm{He}$.
Q7. How is impact parameter related to angle of scattering?
Ans. Impact parameter is the perpendicular distance of velocity vector of $\alpha $ -particle from the centre of the nucleus.
If the impact parameter (b)is small, the angle of scattering $ (θ)$ is high.
For large impact parameter, the $\alpha $ -particle goes un-deviated.i.e,$θ=0$
For head-on collision, b is minimum and $\alpha $-particle rebounds back i.e, $θ=π$
Q8. Among alpha, beta and gamma radiations, which get affected by the electric field.
Ans. Alpha $\alpha $ and beta $\beta $ radiations are deflected by electric field.
Q9. What do you understand by the phrase ground state atom?
Ans. In a ground state atom i) Electrons are in the lowest possible energy levels.
ii)Electrons revolves in the orbit of smallest radius.
Q10. Why does the mass of the nucleus not have any significance in scattering in Rutherford’s experiment?
Ans. Scattering occurs due to electrostatic repulsion between the positively charged nucleus and $\alpha $ -particles and not due to their masses. Hence, there is no significance for mass of nucleus.
Q11. The Lyman series of hydrogen spectrum lies in the ultraviolet region. Why?
Ans. Spectral emission occurs when and electron jumps from a higher energy state $E_{2}$ to a lower energy state $E_{1}$. The wave length $\lambda $ of emitted photon is given by $E_{2}- E_{1} = hc/\lambda $
Q12. The wavelength of some of the spectral lines obtained in hydrogen spectrum are 1216$A^{\circ}$ ,6463 $A^{\circ}$ , and 9546$A^{\circ}$.Which of these wavelengths belongs to the paschen series?
Ans. The line of 9546 $A^{\circ}$ belongs to pachen series.
Q13. Give two drawbacks of Rutherford’s atomic model.
Ans. i) It cannot Explain the stability of an atom
ii) It failed to explain the radiation spectrum of an atom.