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    Se

    Selenium

    Nonmetal

    Atomic Number: 34 • Atomic Mass: 78.9710 u • Period: 4 • Group: 16

    Electron Configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁴
    Phase: Solid
    Electronegativity: 2.55
    Arsenic (As)Bromine (Br)
    OverviewPropertiesAtomic StructureOccurrenceApplicationsHistory

    Physical & Chemical Properties

    Physical Properties

    • •Exists in several allotropic forms, including red amorphous powder, black vitreous form, and metallic gray form
    • •Gray form (most stable) has metallic properties and conducts electricity better in light than in darkness
    • •Photovoltaic and photoconductive properties
    • •Burns with blue flame when ignited
    • •Melts at 494 K (221°C)
    Melting Point

    494 K (220.85 °C, 429.53 °F)

    Boiling Point

    958 K (684.85 °C, 1264.73 °F)

    Density

    4.809 g/cm³

    Electronegativity

    2.55 (Pauling scale)

    Chemical Properties

    • •Main oxidation states are -2, +4, and +6
    • •Chemically resembles sulfur
    • •Forms hydrogen selenide (H₂Se), a highly toxic gas
    • •Reacts with metals to form selenides
    • •Powerful oxidizing agent in the +6 state

    Electron Configuration

    1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁴

    Electron configuration describes how electrons are distributed in atomic orbitals, showing how they are arranged in energy levels and subshells.