Q.

An electron in hydrogen-like atom makes a transition from nth orbit and emits radiation  corresponding to Lyman series. If de-Broglie wavelength of electron in nth orbit is equal to the wavelength of radiation emitted, find the value of n. The atomic number of atom is 11.

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answer is 25.

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Detailed Solution

If A is de-Broglie wavelength, then for nth stationary orbit2πrn=nλwhere rn is radius of nth orbit.rn=ε0h2n2πmZe2∴ 2πε0h2n2πmZe2=nλ⇒1λ=mZe22ε0h2n    ........(i)For Lyman series of hydrogen like atom, wavelength is given as,1λ=Z2R112−1n2    ........(ii)From Eqs. ) and (i), Z2R1−1n2=mZe22ε0h2nRydberg constant, R=me48ε02ch3∴    Z2me48ε02ch31−1n2=mZe22ε0h2n or     1 1−1n2=4ε0chm2ZSubstituting given values, we get=4×8.85×10−12×3×108×6.62×10−34n×1.6×10−192×11=25n∴ n2−1=25n or  n2−25n−1=0n−25±(−25)2+4×1×12≈25±6252≈25 As negative n is not possible, n≈25Alternative solution  :     Energy of emitted photon = = hCλ= 13.6 z2(1-1n2)   where λ = wavelength of emitted photon                =de broglie wavelength in n th orbit = 3.3nzA0Solving n = 25
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