JEE Main 2025 Jan 23 Shift 2, Chemistry Q19: Bohr's Atomic Model

JEE Main2025Jan 23, Shift 2Chemistry

Given below are two statements about X-ray spectra of elements :

Statement (I) : A plot of $ν$ ( $ν =$ frequency of X-rays emitted) vs atomic mass is a straight line.

Statement (II) : A plot of $ν$ ( $ν =$ frequency of X-rays emitted) vs atomic number is a straight line.

In the light of the above statements choose the correct answer from the options given below :

A
Statement I is true but Statement II is false
B
Both Statement I and Statement II are true
C
Both Statement I and Statement II are false
D
Statement I is false but Statement II is true

Solution

Moseley's law for characteristic X-rays states $ν = a (Z - b)$ , where $Z$ is the atomic number, not the atomic mass.

Statement I plots $ν$ against atomic mass. Because atomic masses do not increase strictly linearly with $Z$ (isotope mixes and packing irregularities) the plot is not a true straight line, it is at best a stepwise increasing curve. Statement I is false.

Statement II plots $ν$ (not $ν$ ) against $Z$ . Since $ν \propto (Z - b)^{2}$ , the plot is a parabola, not a straight line. Statement II is false.

$∴$ Both statements are false.

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JEE Main 2025 Jan 23 Shift 2, Chemistry Q19: Bohr's Atomic Model

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