Why aren 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 [5]. The distance between atoms in metallic francium has not yet been measured.
