Ang arseniko o arsenik (Kastila: arsenico, Ingles: arsenic) ay isang uri ng elementong kimikal at lasongmetaliko. Mayroon itong atomikong bilang na 33, atomikong timbang na 74.992, balensiyang 3 o 5, at sagisag na As. Natutunaw ang abuhing arseniko sa temperaturang 817°C (habang nasa 28 na presyong atmosperiko, sagisag atm), sumisingaw (sublima) sa temperaturang 613°C, at may espesipikong grabidad na 5.73. Mayroon itong tatlong anyong alotropiko, na nasa anyong may kulay na dilaw, itim, at abuhing kristalina. Ginagamit ang arseniko bilang insektisidyo o pamatay ng pesteng mga kulisap, pamatay ng mapinsalang mga damo, at inihahalo rin sa mga aloy.[11]
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