All practice pages/Interatomic theory/Elements
Element 81, a post-transition metal
A soft metal named for a green spectral line, and a tasteless, odorless poison once called the poisoner's poison.
Period 6, group 13. The other post-transition metals are tinted, and every cell opens its own article.
Thallium has 81 electrons, one for every proton, so the atom carries no charge. Below are the same 81 electrons written two ways.
1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s² 6p¹
Short form: [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹, where [Xe] stands for the 54 electrons xenon already has.
That list runs by shell. In the order the subshells fill it reads [Xe] 6s² 4f¹⁴ 5d¹⁰ 6p¹, because a subshell fills by its energy, not by its shell number.
The simple form counts electrons shell by shell. The orbital form says which subshell each one sits in: s holds 2, p holds 6, d holds 10 and f holds 14. Shell 4 here is 4s² 4p⁶ 4d¹⁰ 4f¹⁴, and 2 + 6 + 10 + 14 is 32.
Thallium sits in period 6 and has 6 shells. The period number counts the shells.
The outer shell holds 3 electrons. Those are the ones thallium can lose, gain or share, so they are the ones that decide what it bonds with. For a main-group element the group number gives that count: group 13 means 3.
A metal that gives up its valence electrons to form positive ions, though less readily than the metals to its left.
Rare, found in trace amounts in sulfide ores of zinc, lead and copper and recovered from the dust of smelting them.
William Crookes found it in 1861 as a bright green line in the spectrum of residue from a sulfuric acid plant, and Claude-Auguste Lamy isolated the metal in 1862. Crookes named it from the Greek thallos, green shoot.
Infrared detectors and optics, special low-melting glass, and electronics. Thallium-201 was long used in heart scans. Thallium sulfate was once a rat poison and insecticide, but it is now banned in many countries.
Extremely toxic, and it can be absorbed through the skin. Its salts have no taste or smell, which made them a favorite of poisoners. Prussian blue is the antidote.