Selenium (symbol Se, atomic number 34) is a nonmetal essential in trace amounts, used in photocells and glassmaking. It is classified as a reactive nonmetal and sits in group 16, period 4 of the periodic table.
Overview
Selenium is the chemical element with atomic number 34 and the symbol Se. With a standard atomic weight of 78.971, it is a reactive nonmetal. In the periodic table it occupies group 16 and period 4, and under standard conditions it is normally a solid. Selenium is a nonmetal essential in trace amounts, used in photocells and glassmaking.
Atomic structure
An atom of Selenium has 34 protons in its nucleus and, when neutral, 34 electrons arranged in the configuration [Ar] 3d10 4s2 4p4. Its standard atomic weight of 78.971 reflects the natural mix of its stable isotopes.
Discovery and place in the table
Selenium was discovered in 1817 by Jöns Jacob Berzelius. It follows Arsenic (As) and precedes Bromine (Br) in order of atomic number.
| Atomic number | 34 |
|---|---|
| Symbol | Se |
| Atomic mass | 78.971 |
| Category | Reactive nonmetal |
| Group | 16 |
| Period | 4 |
| Electron configuration | [Ar] 3d10 4s2 4p4 |
| Phase at STP | Solid |
| Discovered | 1817 (Jöns Jacob Berzelius) |