Editing Period 4 element (section)

==p-block elements==

===Gallium===
[[File:Gallium crystals.jpg|frameless|left|upright=0.3]]
[[Gallium]] (Ga) is an element in [[boron group|group&nbsp;13]], under [[aluminium]]. Gallium is noteworthy because it has a melting point at about 303 [[kelvin]]s, right around room temperature. For example, it will be solid on a typical spring day, but will be liquid on a hot summer day. Gallium is an important component in the alloy [[galinstan]], along with tin. Gallium can also be found in semiconductors.

===Germanium===
[[File:Polycrystalline-germanium.jpg|frameless|left|upright=0.3]]
[[Germanium]] (Ge) is an element in [[carbon group|group&nbsp;14]]. Germanium, like [[silicon]] above it, is an important [[semiconductor]] and is commonly used in diodes and transistors, often in combination with arsenic. Germanium is fairly rare on Earth, leading to its comparatively late discovery. Germanium, in compounds, can sometimes irritate the eyes, skin, or lungs.

===Arsenic===
[[File:Arsen 1a.jpg|frameless|left|upright=0.3]]
[[Arsenic]] (As) is an element in [[pnictogen|group&nbsp;15]], the pnictogens. 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.

===Selenium===
[[File:SeBlackRed.jpg|frameless|left|upright=0.3]]
[[Selenium]] (Se) is an element in [[chalcogen|group&nbsp;16]], the chalcogens. Selenium is the first nonmetal in period&nbsp;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. Selenium is red in monomolar structure but metallic gray in its crystalline structure.

===Bromine===
[[File:Bromine 25ml.jpg|frameless|left|upright=0.3]]
[[Bromine]] (Br) is an element in [[halogen|group&nbsp;17 (halogen)]]. It does not exist in elemental 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.

===Krypton===
[[File:Krypton discharge tube.jpg|frameless|left|upright=0.3]]
[[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.