Arsenic vs Selenium

Side-by-side comparison of Arsenic (As, element 33) and Selenium (Se, element 34) by atomic structure and physical properties.

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PropertyAs Arsenic Se Selenium
Atomic number3334
SymbolAsSe
NameArsenicSelenium
Categorymetalloidnonmetal
Standard Atomic Weight74.921595 u78.971 u
Period44
Group1516
Blockpp
Phase at STPSolidSolid
Electron configuration[Ar]3d104s24p3[Ar]3d104s24p4
Oxidation states[-3, -2, -1, 0, 1, 2, 3, 4, 5][-2, -1, 0, 1, 2, 3, 4, 5, 6]
Electronegativity (Pauling)2.182.55
Electron affinity0.81 eV2.0206 eV
Density5776 kg/m³4809 kg/m³
Melting point816.85 °C220.5 °C
Boiling point613.85 °C684.85 °C
Discovery year12501817

Key Differences

PropertyAs ArsenicSe Selenium
Standard Atomic Weight74.921595 u ↓ lower78.971 u ↑ higher
Density5776 kg/m³ ↑ higher4809 kg/m³ ↓ lower
Melting point816.85 °C ↑ higher220.5 °C ↓ lower
Boiling point613.85 °C ↓ lower684.85 °C ↑ higher
Electronegativity (Pauling)2.18 ↓ lower2.55 ↑ higher
Electron affinity0.81 eV ↓ lower2.0206 eV ↑ higher
Group15 16
Categorymetalloid nonmetal
Electron configuration[Ar]3d104s24p3 [Ar]3d104s24p4
Discovery year1250 1817