In atomic theory, what was Henry Moseley’s key contribution to organizing the periodic table?

• A. He proposed that electrons orbit the nucleus in fixed energy levels to explain atomic line spectra.
• B. He showed that elements should be arranged by increasing atomic number (based on nuclear charge) rather than by atomic mass.
• C. He discovered the neutron and used it to explain isotopes and atomic mass differences.
• D. He developed the first periodic table organized by recurring chemical properties and atomic mass.

Answer: (B)