what is actinide contraction

Actinide contraction is the gradual decrease in atomic and ionic radii of the actinide elements as the atomic number increases from actinium (Ac) to lawrencium (Lr). This phenomenon is caused by the poor shielding effect of the 5f electrons, which leads to a greater effective nuclear charge experienced by the outer electrons, pulling them closer to the nucleus and thus reducing the size of the atom or ion. The contraction in the actinide series is more pronounced than in the lanthanide series because 5f electrons shield the nucleus less effectively than 4f electrons. This contraction results in a steady decrease in atomic size across the actinide series, making the ionic sizes more similar and causing difficulties in separating the elements chemically. It also influences the chemical properties of the elements that follow the actinides, such as transition metals, by making their atomic sizes more uniform than expected. Additionally, about 40-50% of the actinide contraction can be attributed to relativistic effects. In summary, actinide contraction refers to the steady decrease in atomic and ionic radii across the actinide series due to the increasing nuclear charge poorly shielded by 5f electrons.