Question

which sequence of elements is arranged in order of decreasing atomic radius?
al, si, p
n, c, b
cl, br, i
li, na, k

Explanation:

Step1: Recall atomic - radius trends

Atomic radius decreases across a period (from left - to - right) and increases down a group (from top - to - bottom) in the periodic table.

Step2: Analyze option A

Al (aluminum), Si (silicon), and P (phosphorus) are in the same period (Period 3). As we move from Al to Si to P (left - to - right), the atomic radius decreases because the effective nuclear charge increases, pulling the electrons closer to the nucleus.

Step3: Analyze option B

N (nitrogen), C (carbon), and B (boron) are in the same period (Period 2). But the correct order of decreasing atomic radius should be B > C > N, not N > C > B.

Step4: Analyze option C

Cl (chlorine), Br (bromine), and I (iodine) are in the same group (Group 17). As we move down the group from Cl to Br to I, the atomic radius increases due to the addition of new electron shells.

Step5: Analyze option D

Li (lithium), Na (sodium), and K (potassium) are in the same group (Group 1). As we move down the group from Li to Na to K, the atomic radius increases due to the addition of new electron shells.

Answer:

A. Al, Si, P