What NEET Asks
- Identify the fundamental property used in the Modern Periodic Law.
- Understand Moseley's experimental findings and its implications.
- Relate Moseley's Law to atomic number and X-ray frequency.
- Explain how Moseley's work resolved anomalies in Mendeleev's table.
Key Points
- Mendeleev's Periodic Law: Properties of elements are periodic functions of their atomic masses.
- Moseley's Experiment (1913): Studied the characteristic X-ray spectra of elements. He observed that the square root of the frequency (√ν) of the emitted X-rays was directly proportional to the atomic number (Z) of the element.
- Moseley's Law: √ν = a(Z-b), where 'a' and 'b' are constants (b ≈ 1 for K-series X-rays).
- Atomic Number (Z): Moseley concluded that atomic number, not atomic mass, is a more fundamental property of an element. It represents the number of protons in the nucleus.
- Modern Periodic Law: Properties of elements are periodic functions of their atomic numbers. This is the basis of the Long Form of the Periodic Table.
- Resolution of Anomalies: Arranging elements by atomic number successfully placed elements like Ar (Z=18) before K (Z=19) despite Ar having a higher atomic mass, resolving ambiguities in Mendeleev's table.
Must-Know Formula / Reaction
- Moseley's Law: √ν = a(Z-b)
- ν: frequency of the characteristic X-rays emitted.
- Z: atomic number of the element.
- a: proportionality constant (depends on the series of X-rays).
- b: screening constant (approximately 1 for K-series X-rays).
Common Mistakes
- Students often confuse the basis of Mendeleev's Periodic Law (atomic mass) with the Modern Periodic Law (atomic number).
- Don't forget that Moseley's work involved X-ray spectra, not visible light spectra.
- Misinterpreting the direct proportionality in Moseley's Law: it's √ν vs Z, not ν vs Z.
Rapid Revision
Moseley's work with X-rays proved atomic number (Z) is fundamental, leading to the Modern Periodic Law (properties periodic functions of Z). His law √ν = a(Z-b) resolved Mendeleev's anomalies, affirming Z as the correct ordering principle for the periodic table.