Question
Explain Bohr's atomic model.
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State three postulates of Bohr's atomic model.
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State three postulates of Bohr's atomic model.
✓
Answer
→Bohr combined classical and early quantum concepts and gave his theory in the form of three postulates. These are :
(i) Bohr's first postulate :
An electron in an atom could revolve in certain stable orbits without the emission of radiant energy.
→ According to this postulate, each atom has certain definite stable states in which it can exist, and each possible state has definite total energy. These are called the stationary states of the atom.
→This contrary to the predictions of electromagnetic theory.
(ii) Bohr's second postulate :
The electron revolves around the nucleus only in those orbits for which the angular momentum is in integral multiple of $\frac{h}{2 \pi}$.
→Where, $h$ is Planck's constant
$h=6.625 \times 10^{-34} J s \text {. }$
$L =\frac{n h}{2 \pi}$ Where, $n=1,2,3 \ldots$
(iii) Bohr's third postulate :
An electron makes a transition from one of its specified non-radiating orbits to another of lower energy. When it does so, a photon is emitted having energy equal to the energy difference between the initial and final states.
→The frequency of the emitted photon is then given by
$h v= E _i- E _f$
Where $E _i$ and $E _f$ are the energies of the initial and final states and $E _i> E _f$
(i) Bohr's first postulate :
An electron in an atom could revolve in certain stable orbits without the emission of radiant energy.
→ According to this postulate, each atom has certain definite stable states in which it can exist, and each possible state has definite total energy. These are called the stationary states of the atom.
→This contrary to the predictions of electromagnetic theory.
(ii) Bohr's second postulate :
The electron revolves around the nucleus only in those orbits for which the angular momentum is in integral multiple of $\frac{h}{2 \pi}$.
→Where, $h$ is Planck's constant
$h=6.625 \times 10^{-34} J s \text {. }$
$L =\frac{n h}{2 \pi}$ Where, $n=1,2,3 \ldots$
(iii) Bohr's third postulate :
An electron makes a transition from one of its specified non-radiating orbits to another of lower energy. When it does so, a photon is emitted having energy equal to the energy difference between the initial and final states.
→The frequency of the emitted photon is then given by
$h v= E _i- E _f$
Where $E _i$ and $E _f$ are the energies of the initial and final states and $E _i> E _f$
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(i) If the speed of the electron is now doubled to 2vo. The radius of the circle will change to
(A) $4 r_0$ (B) $2 r_0$ (C) $r _{ o }$ (D) $r _0 / 2$
(ii) If v = 2vo, then the time required for one revolution of the electron (To ) will change to
(A) $4 T_0$ (B) $2 T_{ O }$ (C) $T _{ o }$ (D) $T _{ d } / 2$
(iii) A charged particles is projected in a magnetic field . The acceleration of the particle is found to be. Find the value of x.
(A) $4 ms^{-2}$ (B) $-4 ms^{-2}$ (C) $-2 ms^{-2}$ (D) $2 ms^{-2}$
(iv) If the given electron has a velocity not perpendicular to B, then trajectory of the electron is
(A) straight line (B) circular (C) helical (D) zig-zag
OR
If this electron of charge (e) is moving parallel to uniform magnetic field with constant velocity v, the force acting on the electron is
(A) Bev (B) Be/v (C) B/ev (D) Zero
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→
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→
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- $\text{Zero}$
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