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34 Se Selenium 78.9718

Selenium

POLYATOMIC NONMETAL
Bohr Model: Electrons arranged in energy shells around the nucleus.

Selenium is a chemical element with symbol Se and atomic number 34. It is a nonmetal with properties that are intermediate between those of its periodic table column-adjacent chalcogen elements sulfur and tellurium. It rarely occurs in its elemental state in nature, or as pure ore compounds.

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Quick Facts

Atomic Number 34
Period 4
Group 16
Phase Solid
Appearance black, red, and gray (not pictured) allotropes
About Polyatomic Nonmetals

Polyatomic nonmetals are elements that form bonds with distinct molecular structures. They are generally poor conductors of heat and electricity.

History

Discovered By Jöns Jakob Berzelius
Named By Unknown

Physical Properties

Atomic Mass 78.9718
Density 4.81 g/L
Molar Heat 25.363 J/(mol·K)
Phase Transition (Melting & Boiling Points)

Selenium melts at 494 K and boils at 958 K.

Atomic Properties

Electron Config [Ar] 3d10 4s2 4p4
Electronegativity 2.55
Electron Affinity 194.9587 kJ/mol
Ionization Energy 941 kJ/mol
Orbital Filling Diagram
This diagram visualizes electron configuration according to the Aufbau principle and Hund's rule. Boxes represent orbitals (s, p, d, f), while arrows indicate electrons with spin up or down. Electrons fill lower energy levels first and occupy orbitals singly before pairing.

Photograph

Image of Selenium Source: Wikipedia

External Resources

Wikipedia PubChem Royal Society of Chemistry Periodic Videos (YouTube)

Study Guide

Atomic Number

The number of protons in the nucleus, which defines the element.

Atomic Mass

The average mass of an atom, including protons and neutrons.

Electronegativity

A measure of how strongly an atom attracts electrons in a bond.

Ionization Energy

The energy required to remove an electron from an atom.

Electron Config

The arrangement of electrons in the atom's energy shells.