<P> Arsenic (As) is an element in period 4, between germanium and selenium . Arsenic, as mentioned above, is often used in semiconductors in alloys with germanium . Arsenic, in pure form and some alloys, is incredibly poisonous to all multicellular life, and as such is a common component in pesticides . Arsenic was also used in some pigments before its toxicity was discovered . </P> <P> Selenium (Se) is an element in period 4, between arsenic and bromine . Selenium is the first nonmetal in period 4, with properties similar to sulfur . Selenium is quite rare in pure form in nature, mostly being found in minerals such as pyrite, and even then it is quite rare . Selenium is necessary for humans in trace amounts, but is toxic in larger quantities . </P> <P> Bromine (Br) is an element in period 4, between selenium and krypton . Bromine is a halogen, never existing in pure form in nature . Bromine is barely liquid at room temperature, boiling at about 330 kelvins . Bromine is also quite toxic and corrosive, but bromide ions, which are relatively inert, can be found in halite, or table salt . Bromine is often used as a fire retardant because many compounds can be made to release free bromine atoms . </P> <P> Krypton (Kr) is a noble gas, placed under argon and over xenon . Being a noble gas, krypton rarely interacts with itself or other elements; although compounds have been detected, they are all unstable and decay rapidly, and as such, krypton is often used in fluorescent lights . Krypton, like most noble gases, is also used in lighting because of its many spectral lines and the aforementioned reasons . </P>

What is the name of the 4th halogen