Question

Which of the following statements is/are correct?
I. Atomic radius increases as we go from left to right in a period
II. Atomic size increases as we go down a group.

• A. Both I and Il
• B. Neither I nor II
• C. Only II
• D. Only I

Hint:

Remember: Across a period atomic size decreases, whereas down a group atomic size increases due to the addition of new shells.

Answer 1

The Correct Option is C

Concept: Atomic radius is one of the most important periodic properties. It refers to the distance between the nucleus and the outermost shell of an atom. The variation of atomic size across periods and down groups is governed mainly by two factors:
• Effective nuclear charge
• Number of electron shells As we move across a period, electrons are added to the same shell while the nuclear charge increases. This causes a stronger attraction between the nucleus and the electrons, resulting in a decrease in atomic size. As we move down a group, new electron shells are added. The increase in the number of shells outweighs the increase in nuclear charge, causing the atomic size to increase.

Step 1: Examine Statement I. Statement I says: \[ \text{Atomic radius increases from left to right in a period} \] This statement is incorrect. Across a period, the effective nuclear charge increases continuously and pulls the electrons closer to the nucleus. Therefore: \[ \text{Atomic radius decreases from left to right} \] Hence Statement I is false.

Step 2: Examine Statement II. Statement II says: \[ \text{Atomic size increases down a group} \] As we move downward in a group, a new electron shell is added at each step. For example: \[ Li < Na < K < Rb < Cs \] Thus atomic size increases down the group. Hence Statement II is true.

Step 3: Choose the correct option. Statement I is false. Statement II is true. Therefore only Statement II is correct. \[ \boxed{\text{Correct Option } = (3)} \]