Lanthanide contraction

Lanthanide contraction is the gradual decrease in atomic and ionic radii across the lanthanide series as nuclear charge increases. It results chiefly from the weak shielding of nuclear charge by 4f electrons.

Effective nuclear charge: The net positive charge experienced by an electron after shielding by other electrons is taken into account. Across the series, rising effective nuclear charge pulls the electron cloud closer to the nucleus.
Zirconium: A transition metal with atomic number 40, widely used in alloys and nuclear technology. Zirconium and hafnium have nearly identical radii and chemistry because of the contraction.
Actinide contraction: The decrease in atomic and ionic radii across the actinide series as nuclear charge increases. It is a related contraction associated with filling 5f orbitals rather than 4f orbitals.
Lanthanide series: The sequence of elements from lanthanum to lutetium in which the 4f subshell is progressively occupied. The contraction is measured across this specific sequence.
Electron shielding: The reduction in nuclear attraction experienced by an electron because other electrons repel it. 4f electrons shield added nuclear charge poorly, allowing the contraction to accumulate.
Hafnium: A transition metal with atomic number 72, chemically similar to zirconium and used in control rods and high-temperature alloys. Its size resembles zirconium despite its much greater atomic number.
Atomic radius trend: The broad pattern of changes in atomic size across periods and down groups in the periodic table. Unlike the contraction within a series, this trend combines several directions and electron-shell effects.
Aufbau principle: The rule of thumb that electrons occupy available atomic orbitals in order of increasing energy. It helps explain why 4f orbitals fill across the lanthanide series.
4f orbital: An atomic orbital in the f subshell with principal quantum number four. The lanthanide series fills 4f orbitals, whose electrons provide weak shielding.
Lanthanide separation: The chemical processing used to isolate individual lanthanide elements from mixtures. Their steadily changing ionic radii create subtle differences exploited in separation processes.

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