All practice pages/Interatomic theory/Elements
Element 43, a transition metal
The first element made by people rather than found in nature, and today a workhorse of medical imaging.
Period 5, group 7. The other transition metals are tinted, and every cell opens its own article.
Technetium has 43 electrons, one for every proton, so the atom carries no charge. Below are the same 43 electrons written two ways.
1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d⁵ 5s²
Short form: [Kr] 4d⁵ 5s², where [Kr] stands for the 36 electrons krypton already has.
That list runs by shell. In the order the subshells fill it reads [Kr] 5s² 4d⁵, 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 3 here is 3s² 3p⁶ 3d¹⁰, and 2 + 6 + 10 is 18.
Technetium sits in period 5 and has 5 shells. The period number counts the shells.
The outer shell holds 2 electrons. Those are the ones technetium can lose, gain or share, so they are the ones that decide what it bonds with.
Every technetium atom is radioactive; it is the lightest element with no stable isotope. Chemically it resembles rhenium, the element below it.
Almost none exists on Earth; traces form in uranium ores when uranium atoms split. In 1952 the astronomer Paul Merrill found its spectral lines in red giant stars, proof that stars make heavy elements as they age, since any technetium present when those stars formed would have decayed long ago.
Mendeleev had left a gap for element 43, and several claimed discoveries turned out to be wrong. In 1937 Carlo Perrier and Emilio Segrè found it in a strip of molybdenum that had been bombarded in the Berkeley cyclotron. They named it from the Greek technetos, artificial.
Technetium-99m is injected for tens of millions of diagnostic scans a year, of bones, heart, brain and other organs. It gives off gamma rays a camera can see, and it has a half-life of only six hours, so it fades from the body quickly.
Radioactive. The tiny doses used in medicine are considered safe, but larger amounts are handled behind shielding.