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The Lewis model is based on the octet rule and concerns simple molecules without transition elements, for instance:
The Lewis model concerns also simple multiatomic ions, for example:
Green:
The outer electrons of the neutral atoms
Red:
The supplementary electrons which are responsible for the charge of the ion.
The atoms form as many covalent bonds as they have outer unpaired electrons
The oxygen atom O has to unpaired electrons able to form two covalent bonds in the water molecule, whereas the hydroxide ion has only a single unpaired electron left to form a covalent bond!