Question

Which of the following is not the correct order of atomic radius of the elements given?

• A. \(Br < Ge < Ga < Ca\)
• B. \(Cr < V < Ti < Sc\)
• C. \(F < Cl < K < Cs\)
• D. \(O < P < K < Ge\)

Hint:

Remember that transition metal radii decrease across a period due to poor shielding of d-electrons, while main group element radii follow periodic trends dominated by shell number and effective nuclear charge.

Answer 1

The Correct Option is D

Step 1: Recall the periodic trends.
Atomic radius grows as we go down a group (new shells are added) and shrinks as we go left to right across a period (the nucleus pulls harder). We test each option against these rules to find the wrong one.
Step 2: Check option 1.
$Br < Ge < Ga < Ca$ lists 4th-period elements from right to left. Radius increases as we move left, so this ordering is correct.
Step 3: Check option 2.
$Cr < V < Ti < Sc$ are 3d transition metals. From Sc to Cr the nuclear charge rises and radius drops, so listed smallest to largest this is correct.
Step 4: Check option 3.
$F < Cl < K < Cs$: fluorine and chlorine are small Group 17 atoms while potassium and caesium are large Group 1 atoms, and radius grows down each group, so this is correct.
Step 5: Check option 4.
$O < P < K < Ge$: typical radii are about O $60$ pm, P $110$ pm, K $227$ pm, Ge $122$ pm. Here K ($227$) wrongly sits before Ge ($122$), so the order is broken.
Step 6: Identify the incorrect order.
Option 4 is the one that is not correct.
\[ \boxed{O < P < K < Ge} \]