The actinoids (or actinides) are a series of ranging from atomic numbers 89 to 103. Named after their first member, actinium , these elements form the lower row of the f-block. Atomic Range: Actinium ( ) to Lawrencium (
Every single element in this series is radioactive . Natural vs. Synthetic: Only thorium ( ) and uranium (
As the atomic number increases, the atomic and ionic radii steadily decrease due to poor shielding by 5f electrons.
Their chemical behavior is defined by electrons progressively filling the 5f orbital.
