Which of the following best describes the general trend of ionic radius across a period and down a group in the periodic table?

• A. Ionic radius increases across a period and decreases down a group.
• B. Ionic radius decreases across a period and increases down a group.
• C. Ionic radius increases both across a period and down a group.
• D. Ionic radius decreases both across a period and down a group.

Answer: (B)

Verified step by step guidance

1. Understand that the ionic radius refers to the size of an ion, which can be either a cation (positive ion) or an anion (negative ion). The size depends on the number of electrons and the effective nuclear charge experienced by those electrons.
2. Across a period (left to right on the periodic table), the number of protons in the nucleus increases while electrons are added to the same principal energy level. This causes the effective nuclear charge to increase, pulling electrons closer to the nucleus and decreasing the ionic radius.
3. Down a group (top to bottom on the periodic table), new electron shells are added, increasing the distance between the nucleus and the outermost electrons. This results in an increase in ionic radius despite the increase in nuclear charge because the shielding effect reduces the pull on outer electrons.
4. Therefore, the general trend is that ionic radius decreases across a period due to increasing effective nuclear charge and increases down a group due to the addition of electron shells and increased shielding.
5. Compare the given options with this understanding to identify the correct description of the ionic radius trend.