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Atoms and Nuclei question

2022 · 24 Jun · Shift 2 · Q64
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Atoms and Nuclei question

2022 · 24 Jun · Shift 2 · Q64

JEE MainPhysicsAtoms and NucleiMCQ+4 / −1
In Bohr's atomic model of hydrogen, let K, P and E are the kinetic energy, potential energy and total energy of the electron respectively. Choose the correct option when the electron undergoes transitions to a higher level :
  1. A
    All K, P and E increase.
  2. B
    K decreases, P and E increase.
  3. C
    P decreases, K and E increase.
  4. D
    K increases, P and E decrease.
View written solutionFree

Correct answer: B

  1. Write the Bohr model energy relations

For the electron in the nnnth orbit of hydrogen:

En=−13.6n2 eVE_n=-\frac{13.6}{n^2}\,\text{eV}En​=−n213.6​eV

Using the Coulomb-force/virial relation for Bohr orbit:

Kn=−En=13.6n2 eVK_n=-E_n=\frac{13.6}{n^2}\,\text{eV}Kn​=−En​=n213.6​eV

and

Pn=2En=−27.2n2 eVP_n=2E_n=-\frac{27.2}{n^2}\,\text{eV}Pn​=2En​=−n227.2​eV

So:

  • KnK_nKn​ is positive and proportional to 1n2\dfrac{1}{n^2}n21​
  • PnP_nPn​ is negative and proportional to −1n2-\dfrac{1}{n^2}−n21​
  • EnE_nEn​ is negative and proportional to −1n2-\dfrac{1}{n^2}−n21​

  1. Consider transition to a higher level

A transition to a higher level means:

n→n′withn′>nn \to n' \quad \text{with} \quad n'>nn→n′withn′>n

Hence,

1n′2<1n2\frac{1}{n'^2}<\frac{1}{n^2}n′21​<n21​

Now check each quantity.


  1. Change in kinetic energy KKK

Since

Kn=13.6n2K_n=\frac{13.6}{n^2}Kn​=n213.6​

when nnn increases, KKK decreases.

So, kinetic energy decreases.


  1. Change in potential energy PPP

Since

Pn=−27.2n2P_n=-\frac{27.2}{n^2}Pn​=−n227.2​

for higher nnn, the value becomes less negative.

Example:

P1=−27.2 eV,P2=−6.8 eVP_1=-27.2\,\text{eV}, \qquad P_2=-6.8\,\text{eV}P1​=−27.2eV,P2​=−6.8eV

Clearly,

−6.8>−27.2-6.8 > -27.2−6.8>−27.2

So, potential energy increases.


  1. Change in total energy EEE

Since

En=−13.6n2E_n=-\frac{13.6}{n^2}En​=−n213.6​

for higher nnn, the total energy also becomes less negative.

Example:

E1=−13.6 eV,E2=−3.4 eVE_1=-13.6\,\text{eV}, \qquad E_2=-3.4\,\text{eV}E1​=−13.6eV,E2​=−3.4eV

Thus,

−3.4>−13.6-3.4 > -13.6−3.4>−13.6

So, total energy increases.


  1. Match with options
  • KKK decreases
  • PPP increases
  • EEE increases

Therefore, the correct option is:

B\boxed{\text{B}}B​

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