Question

In $4^{\text{th}}$ period of periodic table the elements with the largest and smallest size respectively is :

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

Hint:

Size Trend: Alkali Metal > Alkaline Earth > ... > Halogen.

Answer 1

The Correct Option is A

To determine the elements with the largest and smallest size in the 4^th period of the periodic table, we must understand periodic trends, particularly atomic size or atomic radius.

1. Understanding Atomic Size: Atomic size generally decreases across a period from left to right due to an increase in effective nuclear charge. This means as we move from left to right, each added electron is pulled closer to the nucleus, making the atom smaller.
2. Identifying Elements in the 4^th Period: The elements in the 4^th period are K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Br, and Kr.
3. Largest Element: Potassium (K): As the first element in the 4^th period, potassium has the largest atomic size. It has the least effective nuclear charge affecting valence electrons, resulting in a larger radius.
4. Smallest Element: Bromine (Br): Moving across the period, bromine has a significantly higher effective nuclear charge compared to potassium, leading to a smaller atomic size. The helium nucleus attracts the valence electrons more strongly, reducing the atomic radius.

Thus, in the 4^th period, the element with the largest atomic size is potassium (K), and the element with the smallest atomic size is bromine (Br).

Conclusion: Based on the periodic trend concept, the correct answer is K and Br.