Mark Gaydos, age 13, of St. Louis, Missouri, for his question:
Why do some metals rust more than others?
A full answer to this question leads on and on through the field of chemistry. It calls for an understanding of atomic theory and the math to cope with it, plus patience and that joyful curiosity that keeps a person probing until he masters what he yearns to know. There is not room here for all these fascinating details. But Andy includes some suggestions to help you reach the complete answer for yourself.
The rusting of metals is corrosion and corrosion is a chemical activity involving atoms and energy. It is a transference of electrons between different atoms and the energy is electrochemical. The quota of energy available depends on the structure of the atoms or molecules in each metal. This inside factor governs both the rate and degree of corrosion in different metals. However, a specific set of outside conditions sets the corrosive activity in motion. For example, moist air triggers iron atoms to combine with oxygen and form molecules of rusty iron oxide.
Metals are metals because of the way their electrons hold the atoms together in set patterns. Each solid metal has its particular lattice crystal plus a cloud of foot loose electrons traveling at fantastic speeds throughout the structure. These teeming electrons form brief attachments from atom to atom, creating adhesive links and bonds. They also create a built in cloud of electrochemical energy part of which may be available for corrosion and other chemical activities.
The structure of atoms, on which the available energy depends, takes us to the Periodic Table to find the basic number of electrons alloted to the atom of each metal. This neat chart also shows the metals arranged in related groups, or families. Notice that sodium and potassium belong in the very, very active family of alkali metals. Sodium is so violently energetic that it ignites in water. Notice that zinc, gold and mercury belong in a family of lazy transition metals. They are slow to corrode and a lot of outside prodding is needed to make them participate in other chemical activities.
These behavior patterns depend somewhat on the size of the atoms but more on the structure of their electron shells. These features are detailed in good chemistry books and easier to understand when you read how several different experts explain the same facts. You will find your clues as you probe. For example, notice that most alkali and transition meta' have an outer shell of one elel~ on. Hence, the outer shell is not a factor. But most of the active alkali metals have a stable next to last shell of eight electrons. In the lazy transition metals, the next to last shell has 18 electrons, making it less stable. This is a valid clue. It is one factor used to grade the elements in a chart called the activity series. This short cut shows which metals corrode to what degree under certain conditions. But for a full understanding of why some metals are lazy and others have energy to squander on rusty corrosion, you need to know how each of the different atoms is put together.
Of the 92 elements in nature, about 70 are metals. But as you probe, you will come across metallic alloys. Then the plot thickens and the rules change. For example, the main ingredient in stainless steel is iron the champion ruster. Without a doubt, you will understand why a smidgeon of nickel added to the recipe inhibits the corro¬sive energy in the iron.