All practice pages/Interatomic theory/Elements
Element 36, a noble gas
A hidden noble gas that once defined the length of a meter, and that does, reluctantly, form a few compounds.
Period 4, group 18. The other noble gases are tinted, and every cell opens its own article.
Krypton has 36 electrons, one for every proton, so the atom carries no charge. Below are the same 36 electrons written two ways.
1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶
Short form: [Ar] 3d¹⁰ 4s² 4p⁶, where [Ar] stands for the 18 electrons argon already has.
That list runs by shell. In the order the subshells fill it reads [Ar] 4s² 3d¹⁰ 4p⁶, 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.
Krypton sits in period 4 and has 4 shells. The period number counts the shells.
The outer shell holds 8 electrons. Those are the ones krypton 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 18 means 8.
A full valence shell, so krypton is very unreactive. Under extreme conditions it does form a few compounds with fluorine, such as krypton difluoride.
About 1 part per million of air, collected when air is liquefied and separated.
William Ramsay and Morris Travers found it in May 1898 in the last traces left after liquid air evaporated, weeks before they found neon and xenon. They named it from the Greek kryptos, hidden.
Insulating gas in energy-efficient windows, some fluorescent and high-performance lamps, camera flashes, and krypton fluoride lasers for research. From 1960 to 1983 the meter was defined by the wavelength of orange-red light given off by krypton-86.