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

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Given: The question asks for the elements with the highest and lowest atomic radii in period $4$ .

Find: The correct pair of elements.

Concept: Atomic radius depends on the number of electron shells and effective nuclear charge. Across a period, atomic radius decreases from left to right because increasing nuclear charge pulls the electrons closer.

Step 1: List the 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 $4$ , so it has the largest atomic radius in that 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}}

Therefore, the correct option is B.

Given: Atomic radius trend across period $4$ .

Find: The correct option quickly.

Use the periodic trend directly: atomic radius decreases from left to right across a period. Therefore, the leftmost element in period $4$ has the largest radius and the right-side reactive non-metal before the noble gas has the smallest chemically relevant radius.

So, K is the largest and Br is the smallest. This immediately gives K & Br, which corresponds to option B.

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