Hi, I am a high school chemistry student (by no means a chemist or professional). I am wondering how polyatomic ions obtain extra electrons/how they are formed. I understand that polyatomic ions are covalently bonded molecules, but I am curious on how they obtain the extra electrons needed to form anions and am struggling to find the answer online. Thank so much!
The electrons to make anions must come from electron donor atoms/molecules. Those are called cations and include elements such as sodium, potassium, calcium and many other elements on the left side of the periodic table or many other molecules. Polyatomic simply means it has more than one atom.
As John said, the electrons for the anion come from an atom that forms a cation by "giving up" the electron. You can't have an anion without a cation. So while your chemistry book might show a table of anions in their charged form alone without a cation this would never happen in "real life". There would always be a cation with it - and John gives good examples. The polyatomic anion is covalently bonded among atoms in that group but that doesn't mean it could not add an additional electron or two from another atom. (I assume you have done Lewis dot structures. You can show how the "extra" electron(s) are shared among the covalent bonds in the polyatomic group.)
Most polyatomic anions are molecules with acidic group that lose a proton. They are closed shell species with an even number of electrons. This is the case with DNA for example. In solution, there will always be counter ions, but you can produced polyanions in the gas phase by electrospray. The type of anion formed by adding electrons will be an open shell anion and will be much more reactive.