Let's analyze the periodic table to determine which element will form the ion with the largest ionic radius. The ionic radius of an element is significantly influenced by its position on the periodic table, specifically its group and period.
1. Understanding the Periodic Table Trends:
- Within the same group (vertical column), the ionic radius increases as we go down the column. This is because each subsequent element down a group has an additional electron shell, making the ion larger.
- Within the same period (horizontal row), the ionic radius decreases from left to right because the increasing positive charge of the nucleus pulls the electron cloud closer.
2. Grouping the Elements:
- Group 1 (1A): H, Na, K, Rb, Cs
- As we go down this group, the elements are:
- H (Hydrogen)
- Na (Sodium)
- K (Potassium)
- Rb (Rubidium)
- Cs (Cesium)
- Cesium (Cs) is at the bottom of this group.
- Group 2 (2A): Be, Mg, Ca, Sr, Ba
- As we go down this group, the elements are:
- Be (Beryllium)
- Mg (Magnesium)
- Ca (Calcium)
- Sr (Strontium)
- Ba (Barium)
- Barium (Ba) is at the bottom of this group.
3. Comparing Ionic Radii Across Groups:
- Between the groups we compared:
- From Group 1A: Cesium (Cs)
- From Group 2A: Barium (Ba)
Since both Cs and Ba are the heaviest (lowest) in their respective groups, they would have the largest ionic radii among their groups. However, comparing these two elements directly:
- Cesium (Cs) usually forms a +1 ion (Cs⁺).
- Barium (Ba) usually forms a +2 ion (Ba²⁺).
Generally, a +1 ion will be larger than a +2 ion from the same period because it has less nuclear charge pulling the same electron cloud (assuming similar electron configurations). So between Cs and Ba, Cesium (Cs) is more likely to have the largest ionic radius.
Therefore, Cesium (Cs) forms the ion with the largest ionic radius. This conclusion leads us to the atomic number of Cesium:
### Conclusion:
- Cesium (Cs) with an atomic number of 55 forms the ion with the largest ionic radius.
