Answer:
Cesium
Explanation:
As you go down a Group in the Periodic Table from top to bottom, the number of energy levels or electron shells increases so the atomic radius of the elements increases.
In general, the atomic radius of elements decreases as you go across a Period from left to right.
This means, we would expect that the last element in group 1 to have the largest atomic radius. This is Francium. But the atom with the largest atomic radius is referred to as Cesium.
Why aren't francium atoms the biggest? The usual periodic trend for atomic size places larger atoms at the left of a row and towards the bottom of a column on the periodic table. It's no surprise that cesium is large. But shouldn't francium, in the next period with an even larger valence shell, be even larger?
The answer is "possibly, but we just don't know yet." Francium isn't easy to study. It's the least stable of the first 103 elements; the most stable Fr isotope has a half-life of just 22 minutes The distance between atoms in metallic francium has not yet been measured.
The element which has the largest known atomic radius is Cesium.
