Modern periodic tables are based on the atomic number of elements. The fundamental significance of atomic number for arranging elements was demonstrated by which scientist through studies of X ray spectra?

Introduction / Context:
This general knowledge question relates to the development of the modern periodic table. Early periodic tables arranged elements by atomic weight, but some inconsistencies remained. The concept of atomic number as the fundamental organising principle was established through experimental evidence involving X ray spectra. You are asked to identify which scientist work proved the significance of atomic number in the periodic classification of elements.

Given Data / Assumptions:

• The modern periodic law states that the properties of elements are periodic functions of their atomic numbers.
• The question mentions that this significance was proved by an experiment involving X rays.
• The options refer in summary form to work by Moseley, Bragg, Millikan and Roentgen.
• Standard historical facts from atomic and modern physics are assumed.

Concept / Approach:
Henry Moseley conducted experiments in which he measured the frequencies of characteristic X rays emitted by different elements. He found that the square root of the frequency of these X rays varied in a systematic way with the atomic number of the element. This relationship, known as Moseley law, showed that atomic number, not atomic weight, is the fundamental quantity determining the position of an element in the periodic table. Bragg worked on X ray diffraction and crystal structure, Millikan performed the oil drop experiment to measure the charge of the electron and Roentgen discovered X rays themselves. None of these other contributions specifically established the importance of atomic number for periodic classification.

Step-by-Step Solution:
Step 1: Recall that the modern periodic table arranges elements in order of increasing atomic number rather than atomic weight. Step 2: Henry Moseley measured frequencies of X rays emitted when elements were bombarded with high energy electrons. Step 3: He plotted the square root of the frequency against the atomic number and found a straight line relationship, now called Moseley law. Step 4: This proved that atomic number is a fundamental property of an element, directly related to its nuclear charge. Step 5: Bragg contributions relate to X ray diffraction in crystals, which help determine crystal structures but do not define atomic number. Step 6: Millikan oil drop experiment measured the charge of the electron, an important constant but not directly connected to the periodic table arrangement. Step 7: Roentgen discovery of X rays was crucial but did not by itself reveal the link between X ray frequency and atomic number.

Verification / Alternative check:
In summaries of the development of periodic classification, you will typically see a sequence from Mendeleev periodic table based on atomic weights to Moseley work that refined the table by using atomic numbers. For example, problems with the placement of elements like argon and potassium were resolved when arranged by atomic number rather than weight. The direct experimental link came from Moseley measurements of characteristic X rays, which allowed atomic numbers of elements to be determined unambiguously. The other scientists mentioned made important contributions to physics but are not credited with establishing atomic number as the basis of the periodic table.

Why Other Options Are Wrong:
Bragg work on X ray diffraction is important for crystallography and for determining interatomic distances in crystals, but it does not address periodic classification based on atomic number. Millikan oil drop experiment measured the elementary charge and gave evidence for quantisation of electric charge, but it is unrelated to the order of elements in the periodic table. Roentgen discovery of X rays opened a new field of research but did not itself link atomic number to X ray frequencies. Thus these options do not match the specific contribution described in the question.

Common Pitfalls:
Learners sometimes confuse any work involving X rays with Moseley law, leading them to think that Bragg or Roentgen might be the answer. Others may remember Millikan as a key figure in atomic physics but not distinguish his specific contribution. To avoid this confusion, pair each name with a key phrase: Moseley X ray spectra and atomic number, Bragg X ray diffraction and crystal structure, Millikan oil drop and electron charge, Roentgen discovery of X rays. This association makes it easier to select the correct scientist when questions mention the periodic table and atomic number.

Final Answer:
The significance of atomic number for the modern periodic table was demonstrated by Moseley work on X ray spectra.