Which of the following has the biggest radius?

To decide which ion has the largest radius we look at periodic trends for cations.

1. Size down a group: As you move down a group in the periodic table, each element adds an extra electron shell. More shells mean a larger distance from the nucleus, so the ion’s radius increases.

2. Size across a period: Moving from left to right, the nuclear charge (number of protons) increases while the number of electron shells stays the same. The stronger attraction pulls electrons closer, making the radius smaller.

3. Effect of charge: Removing electrons to form a cation reduces the electron‑electron repulsion and increases the effective nuclear charge felt by the remaining electrons. A higher positive charge (e.g., Mg²⁺) therefore produces a smaller ion than a lower‑charged ion of similar size.

Applying these rules to the given ions:

• Cs⁺ is in group 1, period 6. It has the most electron shells (six) among the choices.
• Na⁺ is also in group 1 but only in period 3, so it has fewer shells.
• Li⁺ is in period 2, even fewer shells.
• Mg²⁺ is in period 3 but carries a +2 charge, which pulls its electron cloud tighter than the +1 charge of Na⁺, making it smaller than Na⁺.

Because Cs⁺ has the greatest number of electron shells and only a +1 charge, it experiences the least effective nuclear pull relative to its size, giving it the largest ionic radius of the four options.

Hence, the ion with the biggest radius is Cs⁺.