What Is Rust?What is rust? Rust is the result of the reaction between a metal and a non-metal. To have the clearest view possible of this process we will take the example of metallic Iron reacting with Oxygen to form the brownish, flaky rust we are all used to seeing on our cars and drain pipes. The Metal Lattice We have previously seen how metals come to form lattices that consist of alternating layers of positively charged ions and free electrons. This layered structure of metal is presented below:
Oxygen Gas 
Oxygen gas, O2, consists of two atoms of Oxygen covalently bonded together. This is a fairly stable state for Oxygen but given the opportunity each Oxygen atom will absorb two electrons from elsewhere and have a full outer shell of 8 electrons without having to share. This is the most stable state for any Oxygen atom. What is Rust? It's The Reaction Between the Two We know that the metal Iron has loose electrons. We also know that each of the atoms in the oxygen molecule would prefer to have 8 electrons without having to share. Let's look at the process in a series of visual steps. First the oxygen gas molecules, O2, approach the metal:
Then the Oxygen molecule breaks apart and each oxygen atom soaks up two of the metal's free electrons, forming the stable oxide ion:
The metal lattice now has less electrons than it needs and so some of the positively charged Fe ions will be shed from the lattice.
The loose +3 Fe ions and the stable O2- ions combine to form the neutral compound Fe2O3, as follows. Note that this is an ionic compound (a metal and a non-metal together) and that all ionic compounds have an overall charge of ZERO:
And so a single particle of the rust Fe2O3 is formed. Since this does not form a perfect seal over the surface of the metal, further oxygen can get to the surface of the metal, and the rusting continues. Rusting: The Transfer Of Electrons So finally, what is rust? It is the product of the transfer of electrons from the metal (loss of electrons is called oxidation) to the oxygen molecules (gain of electrons is called reduction). Batteries take advantage of this process of natural decay of metals. However, instead of allowing the metals to decay immediately, they make the electrons travel to the other end of the battery first, and so the current can be made to pass through a circuit. Return from What Is Rust to Solar Batteries or return to the Green Planet home page for more Solar Power Facts.
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