Physics NCERT Exemplar Solutions Class 12th Chapter Twelve

The simple Bohr model is not applicable to He⁴atom because
(a) He⁴ is an inert gas 
(b) He⁴ has neutrons in the nucleus
(c) He⁴ has one more electron
(d) Electrons are not subject to central forces

Let E_n= $\frac{-1\mathit{m}{\mathit{e}}^4}{8{\mathit{\epsilon }}_0^2{\mathit{n}}^2{\mathit{h}}^2}$ be the energy of the nth level of H atom. If all the H-atoms are in the ground state and radiation of frequency $\frac{{\mathit{E}}_2-{\mathit{E}}_1}{\mathit{h}}$  falls on it

(a) It will not be absorbed at all

(b) Some of atoms will move to first excited state

(c) All atoms will be excited to the n= 2 state

(d) No atoms will make a transition to then n=3 state

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

The Bohr model for the spectra of a H-atom
(a) Will not be applicable to hydrogen in the molecular form
(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

Consider aiming a beam of free electrons towards free protons. When-they scatter, an electron and a proton cannot combine to produce a H-atom.
(a) Because of energy conservation
(b) Without simultaneously releasing energy in the form of radiation
(c) Because of momentum conservation
(d) Because of angular momentum conservation

An ionised H-molecule consists of an electron and two protons. The protons are separated by a small distance of the order of angstrom. In the ground state,
(a) The electron would not move in circular orbits
(b) The energy would be (2)⁴times that of a H-atom
(c) The electrons, orbit would go around the protons
(d) The molecule will soon decay in a proton and a H-atom

A set of atoms in an excited state decays
(a) In general to any of the states with lower energy
(b) Into a lower state only when excited by an external electric field
(c) All together simultaneously into a lower state
(d) To emit photons only when they collide.

Two H atoms in the ground state collide inelastically. The maximum amount by which their combined kinetic energy is reduced is
(a) 10.20 eV  

(b) 20.40eV    

(c) 13.6 eV

0₂ molecule consists of two oxygen atoms. In the molecule, nuclear force between the nuclei of the two atoms
(a) Is not important because nuclear forces are short-ranged
(b) Is as important as electrostatic force for binding the two atoms
(c) Cancels the repulsive electrostatic force between the nuclei
(d) Is not important because oxygen nucleus have equal number of neutrons and protons

For the ground state, the electron in the H-atom has an angular .momentum = h, according to the simple Bohr model. Angular momentum is a vector and hence there will be infinitely many orbits with the vector pointing in all possible directions. In actuality, this is not true,
(a) Because Bohr model gives incorrect values of angular momentum
(b) Because only one of these Would Have a minimum energy
(c) Angular momentum must be in the direction of spin of electron
(d) Because electrons go around only in horizontal orbits