Q.The statement that is not correct for periodic classification of elements is:

The key idea is Effective Nuclear Charge — the pull felt by valence electrons, which governs periodic trends.

Reasoning:

1. Statement (A) is the Modern Periodic Law — correct.
2. Statement (B) is true: most elements are metals. …

1. Statement (A): "The properties of elements are periodic functions of their atomic numbers."

   This is the modern periodic law, established by Moseley. It replaced Mendeleev’s law based on atomic mass. So (A) is correct.

2. Statement (B): "Non-metallic elements are less in number than metallic elements."

   Look at the periodic table: metals occupy most of the left, centre, and lower regions (about 80–90 elements), while non-metals are only about 20 (mainly in the upper right). So (B) is correct.

3. Statement (C): "For transition elements, the 3d-orbitals are filled with electrons after 3p-orbitals and before 4s-orbitals."

   This is the trap. The Aufbau principle says that 4s fills before 3d (because 4s has lower energy than 3d for neutral atoms). For example, potassium (Z=19) has configuration $[Ar]4s^1$, not $[Ar]3d^1$. So 3d is filled after 4s, not before. The statement says "before 4s-orbitals" — that is false. …