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

Step 1: Understanding the Concept:
Atomic radius decreases across a period from left to right due to increasing effective nuclear charge. It increases down a group as the number of energy shells increases.
Step 2: Detailed Explanation:
Period 4 starts with Potassium (K) and ends with Krypton (Kr).
- Highest Radius: Within a period, alkali metals (Group 1) have the largest atomic radius because they have the lowest effective nuclear charge in that row. Thus, K is the largest.
- Lowest Radius: As we move right, the radius decreases. Halogens (Group 17) have the smallest {covalent} radii in their periods. Although Noble gases (Group 18, Kr) come last, their radii are often measured as Van der Waals radii, which are larger than covalent radii. Therefore, among standard reactive elements, the halogen Br has the lowest radius.
Options (3) and (4) involve elements from other periods (Rb is period 5, Na/Cl are period 3).
Step 3: Final Answer:
The elements are K and Br.