What Is Selenium?
With chemical symbol Se and atomic number 34, Selenium belongs to the nonmetal category of the periodic table.
A nonmetal with photovoltaic properties. Essential trace element in human nutrition.
As a nonmetal in Period 4 and Group 16 (Oxygen Group / Chalcogens), selenium's placement on the table reflects its electron configuration of [Ar] 3d¹⁰ 4s² 4p⁴, which governs its bonding behavior and the type of compounds it typically forms.
Position on the Periodic Table
Selenium is located in Period 4, the first row to include the transition metals.
Within that row, selenium sits in Group 16 (Oxygen Group / Chalcogens), alongside elements that share a similar number of valence electrons and, by extension, similar chemical behavior.
This position is not arbitrary — the periodic table was deliberately organized so that an element's row and column reveal its expected reactivity, bonding pattern, and physical state at a glance.
Physical & Chemical Properties
Selenium has an atomic mass of 78.971 u and a density of 4.81 g/cm³. Its melting point is 221°C and its boiling point is 685°C.
As a nonmetal, selenium conducts heat and electricity poorly compared to metals and tends to form covalent bonds by sharing electrons.
Electron Configuration Explained
Selenium's electron configuration is [Ar] 3d¹⁰ 4s² 4p⁴, describing how its electrons are distributed across shells and subshells around the nucleus.
Reading the configuration in order shows electrons filling shells of increasing energy, following the standard Aufbau principle that explains how atomic structure builds up across the periodic table.
The outermost (valence) electrons shown in this configuration are what ultimately determine selenium's chemical reactivity and the type of bonds it forms with other elements.
History & Discovery
Selenium was formally discovered in 1817, Berzelius, identified through the careful isolation and analytical techniques typical of that era's chemical research.
Many elements from this period were anticipated by Dmitri Mendeleev's periodic table, which successfully predicted gaps corresponding to undiscovered elements well before laboratory instruments existed to confirm them directly.
What Is Selenium Used For?
Selenium's specific properties make it useful in several applications, including:
- Solar cells
- Photocopiers
- Glass pigments
- Nutritional supplements
- Anti-dandruff shampoo
These uses reflect selenium's underlying classification as a nonmetal, since the same properties that define its category in chemistry also determine where it becomes practically useful.
Fun Facts About Selenium
- Named after Selene, Greek goddess of the moon.
- Selenium is element number 34 on the periodic table, in Period 4 and Group 16.
- Its standard atomic mass is 78.971 u, calculated from the natural abundance of its isotopes.
Frequently Asked Questions
What is Selenium's atomic number?
Selenium has atomic number 34.
What is Selenium's chemical symbol?
Selenium's symbol is Se.
What group and period is Selenium in?
Selenium is in Period 4, Group 16 (Oxygen Group / Chalcogens).
What is Selenium's atomic mass?
Selenium's standard atomic mass is 78.971 u.