Two (-2) above the O3 means there are 2 extra electrons available for bonding.
Polyatomic ions are groups of atoms joined together by covalent bonds, like molecules, except that they have a charge. The only difference between the process for drawing Lewis diagrams for molecules and for ions is in determining the number of available electrons. Count the valence as before then add an electron for each unit of negative charge or subtract one for each unit of positive charge. For example, the sulfite ion has the formula SO32-. The number of valence electrons is 6 + 3*6 + 2 = 26.
<span>The atom with the highest electronegativity in the sulfite ion is the sulfur, so it belongs in the center of the diagram with oxygen atoms bonded to it. Fill in enough bonds to hold the ion together then add non-bonding electrons until all atoms obey the octet rule or you have used all that are available.
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