Many objects are made of iron. The iron can react with oxygen, forming rust, which is porous. This allows more oxygen to get to the iron underneath, which further rusts the object until the object virtually falls apart. For the purposes of this scenario, assume that iron has 3 valence electrons. 4Fe(s) + 3O2(g) → 2Fe2O3(s). Using your knowledge of chemistry, explain the characteristics of rust and how you would determine the bonding present.

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For purposes of this question, we are instructed to assume that iron has three valence electrons and that the chemical equation for the formation of rust is 4 Fe(s) + 3 O2(g) --> 2 Fe2O3(s). This is a simplification of the nature of naturally-occurring rust, which includes iron in both the +2 and +3 oxidation states as well as hydrides of the mixed oxides of iron.

Iron, like many other metals, readily loses electrons in the presence of an oxidizing agent, forming a positive ion. In this case we assume iron has 3 valence electrons and loses them all in this reaction, forming iron(III) or Fe3+. Oxygen, a well-known oxidizing agent, accepts two electrons per atom, forming O2-. This gives it a complete valence shell of eight electrons, as neutral oxygen has six. The presence of moisture is an important factor in rust formation, assisting in the transfer of electrons, particularly if electrolytes are also present. Familiar electrolytes that often contribute to rust formation include road salt and the dissolved salts in seawater.

The porosity of rust, which allows corrosion to continue and sets it apart from the nonporous, protective oxides formed by many other metals, is most likely related to the mixed nature of real-world rust. Other properties, however, can be explained using the given formula of Fe2O3 and a knowledge of basic chemistry.

Because rust formation is known to be a redox (oxidation-reduction) reaction, and because it involves a metal and a nonmetal, we infer that the compound formed is ionic. A check of the electronegativities of iron (1.83) and oxygen (3.44), with a difference of 1.61, confirms that the bonding is ionic or mostly ionic (electronegativity values from third reference link).

Ionic compounds are expected to be brittle, which rust certainly is, and to have high melting points. One seller of "red iron oxide" with the formula Fe2O3 gives the melting point as 1475-1565 degrees C, which is indeed high (first reference link).

Ionic compounds in which the ions are relatively small (low radii) and relatively high in charge typically have low solubilities in water. This is explained by the strong electrostatic attraction between oppositely charged ions that have high charges and can be packed close together by virtue of their small size. Oxygen is one of the smallest nonmetals, and its 2- ion has an ionic radius of 140 pm making it the second smallest of all anions, after F-. Iron(III) or Fe3+ has an ionic radius of 65 pm, smaller than many metal ions (ionic radii from second reference link).

As Fe2O3 meets the criteria of consisting of small ions with relatively high charges, we would predict a low solubility in water. Wikipedia describes it as insoluble in water, although soluble in acids and sugar solutions.

We would also predict that iron(III) oxide, being ionic, is electrically insulating. This matches real-world observations that corrosion must be cleaned off of electrical contacts in order for them to work.

Using the formula Fe2O3 to represent rust, along with explanations from basic chemistry, we are thus able to correctly predict many of the properties of rust.

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