Period 5: rubidium to xenon

Filling 5s, 4d and 5p

Rubidium ([Kr] 5s¹) and strontium ([Kr] 5s²) begin the fifth shell. Yttrium through cadmium then fill the 4d subshell, and indium through xenon fill 5p, ending at [Kr] 4d¹⁰ 5s² 5p⁶.

Because 4d and 5s are so close in energy, several of these metals have surprising configurations. Niobium, molybdenum, ruthenium, rhodium and silver all keep a single 5s electron, and palladium, [Kr] 4d¹⁰, has no 5s electrons at all.

Trends across period 5

Electronegativity climbs from 0.82 for rubidium to 2.66 for iodine, and first ionization energy from 4.177 eV for rubidium to 12.13 eV for xenon. Rubidium gives up its outer electron so easily that it can catch fire on contact with air.

In the middle of the row, molybdenum melts at 2896 K, the highest value in period 5, and ruthenium is the densest at 12.45 g/cm³. Toward the right the numbers fall again: tin melts at 505.08 K, and iodine, a shiny dark solid that gives off violet vapor, at 386.85 K.

Where you meet period 5

Silver conducts electricity better than any other metal and goes into jewelry, mirrors and electronics. Tin coats steel food cans and is the main metal in most solder, and indium tin oxide is the transparent conductor on touchscreens. Iodine is added to table salt because the thyroid gland needs it, strontium salts color fireworks red, and palladium helps clean car exhaust.

Technetium (43) has no stable isotopes at all; it was first made in a laboratory in 1937, and a short-lived form of it is used in medical scans. Xenon lights some car headlights and powers the ion engines of spacecraft, and in 1962 it became the first noble gas shown to form a compound.