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

Options:

1. K and Br
2. Na and Cl
3. K and Se
4. Rb and Br

Solution:

Across a period, atomic radius decreases from left to right because effective nuclear charge increases.

Period 4 begins with:

K → Ca → Sc → ... → Br → Kr

Therefore:

• Largest radius = Potassium (K)
• Smallest radius among the given options = Bromine (Br)

Answer:

K and Br (Option 1)