Period 7: francium to oganesson

Filling 7s, 5f, 6d and 7p

Francium ([Rn] 7s¹) and radium ([Rn] 7s²) fill 7s. Actinium and thorium place their next electrons in 6d, then the actinides fill the 5f subshell, reaching 5f¹⁴ at nobelium. Rutherfordium through copernicium complete 6d, and nihonium through oganesson fill 7p, so oganesson ends at [Rn] 5f¹⁴ 6d¹⁰ 7s² 7p⁶.

The 5f electrons are less deeply buried than the 4f electrons of the lanthanides, so the early actinides use them in bonding more readily. Uranium, neptunium and plutonium each show oxidation states from +3 to +6, while most lanthanides stick to +3.

Trends across period 7

Francium has an electronegativity of 0.79 and a first ionization energy of 3.9 eV, almost the same as cesium above it. The early actinides are dense metals: uranium is 19.1 g/cm³, and neptunium, at 20.45 g/cm³, is the densest member of the row with a listed value.

After that, measured numbers run out. From fermium (100) onward this site lists no density, and from rutherfordium (104) onward no melting point, because only tiny amounts, sometimes just a few atoms, have ever been made. Theory suggests the heaviest members break familiar trends; oganesson, for example, may be a solid at room temperature even though it sits in the noble gas column.

Where you meet period 7

Uranium fuels nuclear power plants, and the steady heat of plutonium-238 powers space probes and Mars rovers. Americium-241 is the tiny radiation source inside many household smoke detectors, and radium, discovered in 1898, once made watch dials glow until its dangers became clear.

Many of the heavier elements honor scientists and places, such as curium, einsteinium, berkelium and californium. Tennessine (117), made in 2010, was the last to be discovered, and in 2016 the names of the final four elements were approved, completing all seven rows.