Selenium
Selenium (Se, atomic number 34) is a Period 4 nonmetal (p-block, 78.971 u). Incorporated cotranslationally as the 21st proteinogenic amino acid, selenocysteine (via recoded UGA codons), at the catalytic center of glutathione peroxidase and thyroid ho
Selenium is a chemical element on the periodic table with the atomic symbol Se, atomic number 34, and standard atomic weight 78.971 u. Classified as a Nonmetal within Group 16 and Period 4 (p-block), selenium possesses a ground-state electron configuration of [Ar] 3d10 4s2 4p4 distributed across 4 concentric Bohr electron shells (2 · 8 · 18 · 6) with 6 outer-shell electrons. Incorporated cotranslationally as the 21st proteinogenic amino acid, selenocysteine (via recoded UGA codons), at the catalytic center of glutathione peroxidase and thyroid hormone deiodinases.
⚛️ Interactive Periodic Table Lab: Explore Selenium (Se, Z = 34) in the CRV Science Dynamic Periodic Table & Bohr Orbital Lab to visualize its animated 2 · 8 · 18 · 6 electron shells, simulate live phase transitions from 0 K to 6,000 K, and compare periodic heatmaps across all 118 IUPAC elements.
Atomic Structure and Periodic Trends: How Do Quantum Orbitals Define Selenium?
Electron Shell Architecture and Ground-State Configuration
A neutral atom of selenium contains 34 protons in its atomic nucleus balanced by 34 electrons occupying quantized atomic orbitals. In the Bohr-Sommerfeld and quantum-mechanical orbital models, these electrons populate 4 principal energy shells (K (n=1): 2e−, L (n=2): 8e−, M (n=3): 18e−, N (n=4): 6e−), terminating in the directional dumbbell-shaped p-orbital (azimuthal quantum number l = 1) with ground-state configuration [Ar] 3d10 4s2 4p4. Removing the most loosely bound valence electron from an isolated gaseous selenium atom requires a first ionization energy of 941 kJ/mol, while its tendency to attract shared bonding electrons is quantified by a Pauling electronegativity of 2.55.
Periodic Table Position and Neighboring Elements
Within the periodic table, Selenium occupies Period 4, Group 16 of the p-block (Reactive Nonmetals). Along Period 4, it is flanked by Arsenic (As, Z = 33) and Bromine (Br, Z = 35). Vertically within its periodic family, Selenium shares valence orbital symmetry and homologous chemical trends with Oxygen (O), Sulfur (S). Learners can interactively compare Selenium's orbital radii, ionization energy, and Pauling electronegativity against all neighboring elements in the Dynamic Periodic Table Lab.
Physical and Thermodynamic Properties: How Does Selenium Behave Across Temperature Regimes?
Standard Phase, Melting Point, and Boiling Point Dynamics
At standard ambient temperature and pressure (298.15 K / 25 °C, 1 atm), elemental selenium exists as a crystalline solid. As thermal energy increases, its crystal or molecular lattice undergoes solid-to-liquid fusion at a melting point of 494 K (+220.85 °C / +429.5 °F) and transitions to the vapor phase at a normal boiling point of 958 K (+684.85 °C / +1264.7 °F), spanning a liquid thermodynamic range of 464 K. In the CRV Science Dynamic Periodic Table, dragging the live temperature slider between absolute zero (0 K) and solar-photosphere temperatures (6,000 K) dynamically tracks Selenium's state transitions in real time alongside enthalpy and entropy phase equilibria.
Quantitative Atomic and Thermodynamic Parameter Matrix
| Parameter | Quantitative Value / Formula | Physical / Quantum Basis | Comparative Context in Group 16 / Period 4 |
|---|---|---|---|
| Atomic Number & Symbol | Z = 34 (Se) | 34 nuclear protons; nuclear charge +34e | Located in Period 4, Group 16 (p-block) |
| Standard Atomic Weight | 78.971 u (g/mol) | Abundance-weighted isotopic nucleon mass | ~45 neutrons in dominant/reference mass number |
| Electron Configuration | [Ar] 3d10 4s2 4p4 | Shells: 2 · 8 · 18 · 6 (6 outer-shell e−) | Reactive Nonmetals valence orbital architecture |
| Electronegativity & Ionization | χ = 2.55; IE1 = 941 kJ/mol | Pauling bond polarity scale & first valence ionization enthalpy | Governs covalent vs. ionic bond character |
| Melting & Boiling Points | MP: 494 K; BP: 958 K | Cohesive lattice energy and intermolecular forces | STP (298 K) phase: SOLID (Liquid range: 464 K) |
| Principal Oxidation States | +6, +4, -2 | Valence electron donation, sharing, or acceptance | Discovered / First Isolated: 1817 |
Chemical Reactivity, Bonding, and Oxidation States: How Does Selenium Form Compounds?
Characteristic Oxidation States and Coordination Chemistry
As a reactive p-block (or Period 1 s-block) nonmetal with high electronegativity and ionization energy, it readily shares or acquires valence electrons to build strong localized σ and π covalent bonds across molecular gases, polyatomic oxyanions, and biological macromolecules. In chemical compounds, selenium exhibits characteristic oxidation states of +6, +4, -2. Depending on the electronegativity difference (Δχ) of its bonding partners—such as Oxygen (χ = 3.44), Fluorine (χ = 3.98), Chlorine (χ = 3.16), Nitrogen (χ = 3.04), Carbon (χ = 2.55), or Hydrogen (χ = 2.20)—selenium forms redox-specific oxides, halides, hydrides, or coordination complexes governed by Gibbs free energy (ΔG = ΔH − TΔS).
Nucleosynthesis, Discovery History, and Elemental Origins
First identified or isolated in 1817, Selenium carries a distinct cosmological and geochemical signature. With atomic number Z = 34 beyond the iron-56 binding-energy peak, selenium is synthesized astrophysically via neutron-capture nucleosynthesis—combining slow neutron capture (the s-process in asymptotic giant branch stars) and rapid neutron capture (the r-process in neutron-star mergers and core-collapse supernovae).
Biological Role and Scientific Applications: Why Is Selenium Important in Nature and Technology?
Biochemical Classification: Essential Trace Cofactor
In comparative biochemistry and systems physiology, Selenium is classified as Essential Trace Cofactor. Catalytic transition metal or trace nonmetal required in metalloenzyme active sites (redox catalysis, O2 transport, hormone synthesis). Across terrestrial and marine ecosystems, its chemical speciation interacts with reactive oxygen species, enzymes, electron transport chain, oxidative phosphorylation, reactive oxygen species.
Key Mechanistic Functions and Technological Uses
Incorporated cotranslationally as the 21st proteinogenic amino acid, selenocysteine (via recoded UGA codons), at the catalytic center of glutathione peroxidase and thyroid hormone deiodinases. Researchers and students can further investigate Selenium's functional role alongside all 118 elements by launching the Interactive Periodic Table & Element Detective Lab on CRV Science Learn.
Frequently Asked Scientific Questions
What is the electron configuration and valence shell structure of Selenium (Se)?
Selenium (atomic number Z = 34) has the ground-state electron configuration [Ar] 3d10 4s2 4p4, distributing its 34 electrons across 4 shells as 2 · 8 · 18 · 6. Its outermost shell contains 6 electrons, placing Selenium in Group 16, Period 4 (p-block) as a member of the Reactive Nonmetals. You can watch these 34 electrons orbit the nucleus in real time in the Dynamic Periodic Table Bohr Lab.
What phase of matter is Selenium at room temperature, and when does it melt or boil?
At standard room temperature (298 K / 25 °C) and 1 atm pressure, Selenium is a crystalline solid. It melts at 494 K (+220.85 °C / +429.5 °F) and boils at 958 K (+684.85 °C / +1264.7 °F). Using the 0–6,000 K temperature slider in the Interactive Periodic Table, you can observe Selenium's exact solid, liquid, and gas transitions relative to all other elements.
How does Selenium function in biochemistry, materials science, and technology?
Incorporated cotranslationally as the 21st proteinogenic amino acid, selenocysteine (via recoded UGA codons), at the catalytic center of glutathione peroxidase and thyroid hormone deiodinases. In biological systems, it is categorized under Essential Trace Cofactor, with principal oxidation states of +6, +4, -2 and a first ionization energy of 941 kJ/mol.