What is the last element in period 4 of the periodic table?

Krypton.

The last element in period 4 of the periodic table is Krypton, which is a noble gas with the atomic number 36. Krypton is characterized by a complete set of electrons in its outer shell, which makes it extremely stable and largely unreactive. This chemical inertness is a typical feature of noble gases.

Krypton was discovered in 1898 by Sir William Ramsay and Morris Travers in Britain during their study of the residue left after evaporating nearly all components of liquid air. The name “krypton” comes from the Greek word “kryptos,” meaning “hidden.” This name reflects the fact that the gas was well hidden in the air and not easy to find.

In terms of physical properties, krypton is a colorless, odorless, tasteless gas under normal conditions. It can emit a characteristic pale white to greenish glow in an electric field, which makes it useful in certain types of lighting. Krypton is used in fluorescent bulbs, where it helps to extend the life of the filament and improve the quality and consistency of light. Additionally, the gas is used in some types of photographic flashes for high-speed photography.

Krypton is also known for its role in the discovery of the krypton fluoride laser, which is notable for being a powerful source in the ultraviolet spectrum. The rarity and high cost of krypton limit its usage to applications where its specific properties are indispensable. In scientific research, the isotope krypton-86 was used to define the standard meter from 1960 to 1983, based on the wavelength of light emitted by this isotope.

Despite being relatively scarce in Earth’s atmosphere, making up only about 1 part per million by volume, krypton remains an element of significant scientific and practical interest, embodying the distinctive characteristics of the noble gases in the periodic table.