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The shortest wavelength of He atom in Balmer series is x, then longest wavelength in the Paschene series of Li+2 is :
  • a)
    36x/5
  • b)
    16x/7
  • c)
    9x/5
  • d)
    5x/9
Correct answer is option 'B'. Can you explain this answer?
Verified Answer
The shortest wavelength of He atom in Balmer series is x, then longest...
The shortest wavelength of He+ ion spectrum in Balmer series is the transition from n = infinity ----> n = 2.

Hence, 1/λ1 = 1/x = RZ^2(1/2^2 − 1/∞^2)= R 2^2(1/4) = R 
or R = 1/x

Then, the longest wavelength in the Paschen series of line of Li2+ ion will be from n = 4 -----> n = 3

1/λ2 = RZ^2(1/3^2 − 1/4^2) = R 3^2(1/9 − 1/16) = R 9(7/144)=R 7/16 
or Since, R = 1/x
1/λ2 = 1/x x 7/16
Or λ2 =16x/7
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Most Upvoted Answer
The shortest wavelength of He atom in Balmer series is x, then longest...
Shortest wavelength of He atom in Balmer series

The Balmer series of hydrogen-like atoms consists of spectral lines that result from electron transitions from energy levels n ≥ 3 to n = 2. For He atom, the Balmer series corresponds to n = 3 to n = 2 transitions.

Let λ be the shortest wavelength in the Balmer series of He atom. Then, we have:

1/λ = R(1/2^2 - 1/3^2)

where R is the Rydberg constant for He atom.

Simplifying the above equation, we get:

λ = 8R/5

Longest wavelength in the Paschen series of Li 2

The Paschen series of hydrogen-like atoms consists of spectral lines that result from electron transitions from energy levels n ≥ 4 to n = 3. For Li 2, the Paschen series corresponds to n = 4 to n = 3 transitions.

Let λ' be the longest wavelength in the Paschen series of Li 2. Then, we have:

1/λ' = R'(1/3^2 - 1/4^2)

where R' is the Rydberg constant for Li 2.

Simplifying the above equation, we get:

λ' = 16R'/7

Relation between Rydberg constants for He and Li 2

The Rydberg constant for hydrogen-like atoms is given by:

R∝m/e^2

where m is the reduced mass of the atom and e is the charge on the electron.

For He atom, we have:

m = 2m_e

where m_e is the mass of electron.

For Li 2, we have:

m = m_e/2

Substituting these values in the above equation, we get:

R/R' = 4

Using this relation, we can write:

λ' = 16R'/7 = 16R/(4*7) = 4λ/7

Substituting the value of λ from the first part, we get:

λ' = 16x/7

Therefore, the correct option is B.
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