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Anions are atoms or radicals which are a group of atoms, that have gained electrons. Since they now have more electrons than protons, anions have a negative charge. For example Chloride ions Cl- , Bromide Br- , Iodide I-. These are monovalent anions as it has a combining capacity with only one ion of Hydrogen. Similarly there are bivalent anions, etc.
Anions are termed so because they get attracted towards the Anode(the positive electrode).