Look up the electron configurations for the lanthanide series elements. What do you notice about their configurations? can you propose an explanation for what you observe?.

Lanthanides have the general electron configuration of the type (Xe)4f n 6s2. They're called 4f elements as they have incompletely filled 4f subshells.

Explanation:

radioactive decay occurs in unstable atomic nuclei – that is, ones that don't have enough binding energy to hold

the nucleus together due to an excess of either protons or neutrons.

they are all solids.