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Instructions
Refresh yourself as to the structure of the periodic table. Specifically, recall that each row extends from an atom with a single electron in its outer shell in the leftmost column to a completely filled outer shell of eight electrons in the rightmost column.
Note that a nearly full outer electron shell can add an electron because the extra electron will be subjected to the same attractive force as the existing outer electrons. The outer electron in an atom with just one or two outer shell electrons is generally very loosely held because the inner electrons shield the nucleus.
Identify likely negative ions by examining the periodic table. Specifically, the atoms in the second and third columns from the right are likely candidates because they have room for one or two more electrons in their outer shells.
Put together the position on the periodic table with the knowledge that each electron carries a negative unit charge to determine the charge on a negative ion. So, fluorine and iodine, for example, which are one column from the rightmost in the periodic table, have room for one extra electron. Fluoride and iodide ions have a charge of -1. Oxygen and sulfur, one column to the left, will form oxide and sulfide ions with a charge of -2.