Editing Period 4 element (section)

===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.