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

We need to identify the elements with the highest and lowest atomic radii in Period 4 of the periodic table.

Key Concept: Trend of Atomic Radius Across a Period

As we move from left to right across a period, the atomic radius generally decreases. This is because:

- The nuclear charge (number of protons) increases with each successive element.

- Electrons are added to the same principal energy level (same shell), so the shielding effect does not increase significantly.

- The increased effective nuclear charge pulls the electron cloud closer to the nucleus, reducing the atomic radius.

Period 4 elements: K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Br, Kr

Highest atomic radius:

Potassium (K) is the first element in Period 4 (Group 1, alkali metal). It has the lowest effective nuclear charge among Period 4 elements and its outermost electron is in the 4s orbital with minimal nuclear attraction. K has the largest atomic radius in Period 4 (approximately 227 pm).

Lowest atomic radius:

Noble gases (Kr) have special considerations - their atomic radii are measured differently (van der Waals radii vs covalent radii). When considering covalent atomic radii, Bromine (Br) is the element with the smallest atomic radius in Period 4, as it is the rightmost element before the noble gas Kr and has the highest effective nuclear charge among the elements that form covalent bonds. Br has a covalent radius of approximately 114 pm.

Note: Options 3 (Na and Cl) and 4 (Rb and Br) include elements not in Period 4. Na and Cl are in Period 3, and Rb is in Period 5. These can be eliminated immediately.

The correct answer is Option 1: K and Br.