Question

In period 4 of the periodic table, the elements with highest and lowest atomic radii respectively are:

• A. K & Se
• B. K & Br
• C. Rb & Br
• D. Na & Cl

Hint:

Across a period, increasing nuclear charge dominates over shielding, causing atomic size to decrease.

Answer 1

The Correct Option is B

Concept: Atomic radius depends on:
Number of electron shells
Effective nuclear charge Across a period:
Atomic radius {decreases} from left to right
Due to increasing nuclear charge pulling electrons closer
Step 1: List elements of period 4 \[ \text{K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Br, Kr} \]
Step 2: Identify the largest atomic radius
Potassium (K) is the first element of period 4.
It has the lowest effective nuclear charge in the period. \[ \Rightarrow \text{K has the largest atomic radius} \]
Step 3: Identify the smallest atomic radius
Atomic radius decreases continuously across the period.
Noble gases are generally excluded due to van der Waals radii considerations. Thus, among the chemically relevant elements: \[ \Rightarrow \text{Br has the smallest atomic radius in period 4} \]
Step 4: Eliminate incorrect options
Option A: Se is not the smallest; Br is smaller.
Option C: Rb is not in period 4.
Option D: Na and Cl belong to period 3. Final Answer: \[ \boxed{\text{K and Br}} \]