Question

Which statement(s) is(are) false for the periodic classification of elements?

• A. The properties of the elements are the periodic functions of their atomic numbers
• B. Non-metallic elements are lesser in number than the metallic elements
• C. The first ionization energies of the elements along a period do not vary in a regular manner with increase in atomic number
• D. For transition elements, the d-electrons are filled monotonically with increase in atomic number
• E. Both (C) and (D)

Hint:

Whenever you see the word "monotonically" or "always" in chemistry, look for exceptions like Chromium or Copper in the d-block.

Answer 1

The Correct Option is D

Concept: This question tests the understanding of Modern Periodic Law and the specific anomalies in electronic configurations of transition elements.

Step 1: Evaluate Statement (D).
"Monotonically" means the number of $d$-electrons increases by exactly one for every increase in atomic number. However, transition elements have exceptions due to the extra stability of half-filled and fully-filled subshells. For example:
• Chromium (Cr, Z=24): Expected $[Ar]3d^4 4s^2$, Actual $[Ar]3d^5 4s^1$.
• Copper (Cu, Z=29): Expected $[Ar]3d^9 4s^2$, Actual $[Ar]3d^{10} 4s^1$. Because of these jumps/exceptions, the filling is not strictly monotonic.

Step 2: Verify other statements.
(A) is True (Modern Periodic Law). (B) is True (Metals make up about 80% of the table). (C) is True (Variations exist, e.g., N vs O, Be vs B).