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Why are uranium, neptunium, and plutonium among the densest metals?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — actinoid element Modern version of the periodic table of the elements (printable).

Uranium, neptunium, and plutonium are among the densest metals because of their unique electronic and structural characteristics. A primary explanation for the high density and the variety of crystal forms observed in these metals is the mixing of 5f and 6d orbitals within their atoms.1

In these lighter actinoid elements, some electrons enter unfilled 6d orbitals rather than the expected 5f orbitals. This interaction leads to the formation of multiple hybrid electronic states that possess nearly the same energy, which contributes to the existence of several unusual metallic structures. For example, neptunium exhibits a density of 20.48 grams per cubic centimetre when crystallized into its orthorhombic form at 25 °C (77 °F), making it one of the densest metals known.1

This behavior contrasts with heavier actinoids, such as americium and beyond, where the electron energy levels are sufficiently separated to prevent this orbital mixing.1