Question

In period 4 of the periodic table which elements have the lowest and highest atomic radii respectively?

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

Hint:

While comparing atomic radii:

Radius decreases across a period.
Radius increases down a group.
Noble gases are usually excluded from atomic radius comparisons.

Answer 1

The Correct Option is A

Concept: Atomic radius in a period generally:

Decreases from left to right due to increasing nuclear charge.
Electrons are added to the same shell while effective nuclear charge increases.
Noble gases are usually excluded when comparing atomic radii due to van der Waals radius.
Step 1: Identify 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: Determine the element with the highest atomic radius. In a period, the alkali metal (first element) has the largest atomic radius. \[ \Rightarrow \text{Highest atomic radius} = \text{K} \]
Step 3: Determine the element with the lowest atomic radius. Atomic radius decreases across the period. Excluding noble gases, the smallest atomic radius in period 4 is of the halogen. \[ \Rightarrow \text{Lowest atomic radius} = \text{Br} \]
Conclusion: \[ \boxed{\text{Lowest atomic radius} = \text{Br}, \quad \text{Highest atomic radius} = \text{K}} \] Hence, the correct answer is (1) K and Br.