Period 6 of the periodic table

Period 6 is the sixth row of the periodic table of elements. The elements in this period share the property that their outermost electrons fill the sixth shell of the electron configuration. The period begins with cesium, which is an alkali metal, and ends with radon, a naturally occurring radioactive gas.

Period 6 is the first period to contain 32 elements, making it longer than periods 4 and 5, each of which contains 18 elements. The reason for this extension is that this period fills the f-block orbitals (4f), giving rise to a group of elements known as the lanthanides. Because the full periodic table would become unwieldy, the lanthanides are typically displayed in a separate row below the main table.

Classification of elements

The period is organized by which electron orbitals are being filled. Cesium and barium occupy the s-block, one being an alkali metal and the other an alkaline earth metal. These are followed by lanthanum and the lanthanides (4f elements). Next are the transition metals of the 5d block: hafnium, tantalum, tungsten, rhenium, osmium, iridium, platinum, gold, and mercury. Finally comes the p-block: thallium, lead, bismuth, polonium, astatine, and radon.

General properties

The elements of period 6 have large atoms and high atomic masses compared to earlier periods. A significant phenomenon is lanthanide contraction: the filling of the 4f orbitals causes atomic radii to decrease more than expected, making the 5d transition metals similar in size to their 4d counterparts. Additionally, relativistic effects influence the properties of gold, mercury, and lead.

Radioactivity

Most elements in this period have stable isotopes, but radioactivity becomes significant toward the end of the period. Polonium, astatine, and radon are all radioactive, found only in small amounts occurring naturally from the decay of uranium and thorium. Bismuth was long considered stable but has since been found to have an extremely long half-life.