The short answer is that the charge of the iron atom in FeO is +2, because it needs to balance the -2 charge of the oxygen atom to create a neutral FeO molecule.
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Iron is an interesting metal because it can have different oxidation states (aka charges in a molecule). A good way to explore this is to look at the different iron oxide compounds that can exist. There is FeO (where Fe is +2 and O is -2), which is also called wüstite. There is Fe2O3 (where each Fe is +3 and each O is -2). It is also possible for iron to form a mixed oxide (which contains Fe in both the +2 and +3 states), such as Fe3O4. What are the oxidation states (charges) of each Fe atom in the Fe3O4 molecule? Remember, in a mixed oxide, the charges on Fe aren”t all the same!
Many atoms almost always take the same oxidation state. Iron just doesn”t. The reason for this, simply put, is that iron”s place in the periodic table puts it in a group of elements called transition metals.
Transition metals can take oxidation states ranging from +1 to +4, though most commonly they take +1 and +2. The valence electrons of transition metals (which are the electrons they “lose” when they bond with an atom like oxygen) are very close in energy, meaning that the atom feels only a small energy change–relative to the energy change of bonding–when it loses 1 versus 2 versus 3 electrons. This is what makes iron tricky, but it also makes it a versatile material that we encounter in many forms in nature.
When we say “charge” in an atom, we use a number that determines the electromagnetic interactions of the atom. In Iron (II) oxide, or Ferrous Oxide, or FeO, the Iron element (Fe) is bonded to the Oxygen, in the oxidation state of “2”. This means that the Iron has accepted 2 electrons from the Oxygen. Because of this electromagnetic interaction, the charge of the Iron (Fe) in FeO is 2+.
Remember that oxidation describes the loss of an electron by a molecule or atom. In FeO, you have one atom of Fe with 2 extra protons (+charge), and the Oxygen is an atom with two extra electrons (-charge). Both atoms will attract to each other because of their fundamental force of nature, the electromagnetic interaction.
Can you figure out what is happening in the next reaction known as rusting?:4Fe + 3O2 = 2 Fe2O3