Which of the following arrangements is in decreasing order of atomic radius?

Analyze the question: We need the order of atomic radius from largest to smallest for Li, Be, B and C. These are all Period 2 elements.

Concept: Across a period, from left to right, the nuclear charge (number of protons) increases. The electrons are added to the same shell, so the shielding stays about the same. The nucleus therefore pulls the outer electrons in more strongly, and atomic radius decreases across a period.

Order in Period 2: Li (Z=3), Be (Z=4), B (Z=5), C (Z=6)

Atomic radius decreases as Z increases:

Li (152 pm) > Be (112 pm) > B (85 pm) > C (77 pm)

Checking the options:

• A. Li>Be>C> is incomplete, with nothing after C, and it leaves out B.
• B. Li>B>Be>C puts B before Be. This is wrong, because Be is larger than B.
• C. Li>Be>B>C matches the correct trend.
• D. C>B>Be>Li is the reverse order, so it is wrong.

Answer: C

Tip: Remember "across a period, atoms shrink; down a group, atoms grow." For Period 2, the order is Li > Be > B > C > N > O > F, and the same trend holds for the elements to the right as well.