Chemistry · Periodic Classification Of Properties · NEET
It proved that atomic number (Z) is a more fundamental property of an element than its atomic mass. Moseley used this to fix the periodic table. When elements are arranged by atomic number instead of atomic mass, the small problems in Mendeleev's table disappear. For NEET, remember one line: Moseley showed the true basis of classification is atomic number, and this led to the Modern Periodic Law.
Each element gives off its own special X-rays when hit by fast electrons. These are called characteristic X-rays, and their frequency changes in a clean, step-by-step way from element to element. Ordinary visible light does not change this cleanly across all elements. X-rays come from deep inner electrons close to the nucleus, so their frequency depends directly on the number of protons (the atomic number). That is why X-rays were the perfect tool.
He plotted the square root of the X-ray frequency (written √ν, where ν is the frequency) on one axis, and the atomic number (Z) on the other axis. The result was a straight line. When he instead tried √ν against atomic mass, the points did NOT fall on a neat straight line. The clean straight line only appeared with atomic number, which is why atomic number won.
A straight line means the two things are directly and simply related, with no exceptions. Since √ν and atomic number gave a straight line, it showed atomic number is the real, orderly property behind the periodic table. Atomic mass gave a scattered, less orderly plot, so it is not the true basis. In science, a clean straight line is strong proof of a simple rule.
Atomic number (Z) is the number of protons in the nucleus, and it is a whole number that never has odd exceptions. Atomic mass is the total mass of protons and neutrons, and it can go slightly out of order between some element pairs. Moseley's straight line matched atomic number, so atomic number is fundamental. This is why the next step, Modern Periodic Law, is based on atomic number.
Mendeleev arranged elements by atomic mass, which caused a few wrong-order pairs (like Ar and K). After Moseley, the law was changed to arrange elements by atomic number instead. This updated version is called the Modern Periodic Law. So Moseley did not throw away Mendeleev's table; he corrected the property it was built on.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Henry Moseley was an English physicist. He did this X-ray experiment in 1913, showing that atomic number is the true basis of the periodic table.
Moseley found that √ν = a(Z − b), where ν is the X-ray frequency, Z is the atomic number, and a and b are constants. This equation is a straight line, which is exactly what he saw.
NEET usually tests the conclusion of Moseley's work, that atomic number is more fundamental than atomic mass, and that it led to the Modern Periodic Law. Knowing the √ν vs Z straight line and its meaning is enough.
Atomic number is the number of protons and decides an element's identity and chemistry. Moseley's straight line proved it is orderly, while atomic mass can go out of order between some pairs. See the next concept for a full comparison.
When fast electrons hit an element, that element emits X-rays of fixed frequencies unique to it. This set of frequencies is its characteristic X-ray spectrum, and Moseley studied it for many elements.
