The Structure of Metal Metal Basics

The Crystals that Make the Metal

Let's take a look at a paper clip. A typical paper clip is made up of 1,000,000,000,000,000,000,000 atoms of iron. These atoms are tightly packed and in a crystalline structure, a regular arrangement of atoms that repeats itself many times.

This is the crystalline structure for iron atoms. The atoms of other metals, such as aluminum and zinc, have different arrangements.

The structure of iron atoms isn't continuous throughout the entire paper clip. When a metal cools and is transitioning from liquid to solid, its atoms come together to form tiny grains, or crystals. Even though the crystalline structure does not continue from crystal to crystal, the crystals are bound to one another.

In this diagram, each square represents an individual atom.

The Bonds that Connect the Crystals

The bond that holds the crystals of a metal together is actually the same that holds the atoms within each crystal together: the metallic bond.

There are three main types of bonds that bind atoms together: the ionic bond, the covalent bond, and the metallic bond. With the ionic bond, positively charged atoms are attracted to negatively charged atoms. Salt, or NaCl, is an example of a crystal held together by ionic bonds. With the covalent bond, atoms share their outermost electrons. The sharing of electrons between the carbon atoms of a diamond is an example.

With the metallic bond, atoms are also held together by their outermost electrons, but instead of being shared by a couple of atoms, these electrons act like a cloud that is able to move through the structure of the crystal. This cloud, which is negatively charged (electrons have a negative charge), pulls on the atoms, which are positively charged (each atom is missing one or more of its electrons). The attraction between the atoms extends beyond the crystal, pulling in on adjacent crystals.

The electron cloud moves freely when a voltage is applied. This movement of electrons is an electric current. The electron cloud can also transfer thermal energy, making it a good conductor of heat. Also, the electron cloud absorbs then radiates most of the light that falls on it, which gives metal its lustrous appearance.

The Flaws that Break the Bonds

When a metal crystal forms, the atoms try to assemble themselves into a regular pattern. But sometimes there isn't an atom available to fill in a space, and sometimes a growing layer is halted by other growing layers. There are many imperfections within each crystal, and these flaws produce weak points in the bonds between atoms. It is at these points, called slip planes, that layers of atoms are prone to move relative to adjacent layers if an outside force is applied.

Adding other elements to a metal can counteract the effects of the imperfections and make the metal harder and stronger. Carbon, for example, is added to iron to make steel, and tin is added to copper to make bronze.

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