Period 6: cesium to radon, with the lanthanides

Four subshells: 6s, 4f, 5d and 6p

Cesium ([Xe] 6s¹) and barium ([Xe] 6s²) fill 6s. Lanthanum adds a single 5d electron, and then the lanthanides fill the seven 4f orbitals, which hold 14 electrons, reaching 4f¹⁴ at ytterbium. Hafnium through mercury complete the 5d subshell, and thallium through radon fill 6p, ending at [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁶.

The capacities 2 + 14 + 10 + 6 add up to 32 elements, against 18 in period 5. The 4f electrons are buried deep inside the atom, which makes the lanthanides so alike that chemists struggled for decades to separate them.

Trends across period 6

Cesium has the lowest first ionization energy of any element, 3.894 eV, and an electronegativity of just 0.79, while radon’s ionization energy is 10.745 eV. Gold has an unusually high electronegativity for a metal, 2.54, and can even form the negative ion Au⁻, as in cesium auride (CsAu).

Across the lanthanides the atoms shrink slightly, an effect called the lanthanide contraction, so the 5d metals that follow are compact and very heavy. Tungsten melts at 3695 K, the highest of any metal, and osmium, at 22.59 g/cm³, is the densest element. Mercury is the opposite extreme: it melts at 234.32 K and is a liquid at room temperature.

Where you meet period 6

Gold and platinum go into jewelry, electronics and catalysts because they do not corrode, and lead, though toxic, is still used in car batteries and radiation shielding. Neodymium makes the strongest permanent magnets, used in electric motors and wind turbines, while europium and terbium help produce the colors on screens and in energy-saving lamps.

Barium sulfate lets doctors see the gut on X-rays, and cesium atomic clocks define the length of a second. Promethium and the last three elements of the row, polonium, astatine and radon, have no stable isotopes, and radon gas seeping out of rocks can build up in basements.