Metallic Bonding

The metals are the most numerous of the elements. About 80 of the 100 or so elements are metals. You know from your own experience something about how metallic atoms bond together. You know that metals have substance and are not easily torn apart. They are ductile and malleable. That means they can be drawn into shapes, like the wire for this paper clip, and their shape can be changed. They conduct heat and electricity. They can be mixed to form alloys. How is it that metallic bonding allows metals to do all these things?

The nature of metals and metallic atoms is that they have loosely held electrons that can be taken away fairly easily. Let's use this idea to create a model of metallic bonding to help us explain these properties. I will use potassium as an example. Its valence electron can be represented by a dot. When packed in a cluster it would look like the image at right. The valence electron is only loosely held and can move to the next atom fairly easily. Each atom has a valence electron nearby but who knows which one belongs to which atom? It doesn't matter as long as there is one nearby.

To emphasize that the valence electron is very loosely held, we can separate it from the rest of the atom and write it as "K⁺" and e^-" rather than "K⋅ ". Packed in a cluster they look like the image at right. These electrons are more or less free to move from one atom to another. Chemists often describe metals as consisting of metal ions floating in a sea of electrons

The mutual attraction between all these positive and negative charges bonds them all together. Atom to electron to atom to electron and so forth. We have an array of atoms bonded to one another, that is, a network. We can compare this network to the network of a diamond or silicon dioxide that we saw in the previous section.

In those (covalent) networks, each atom is bonded to a fixed set of other atoms, and the electrons are locked into place between those atoms. This makes the substances strong, but brittle, and it means that electricity cannot flow easily through them.

In metallic networks, the bonding is still fairly strong, but much more flexible, because each metal atom is only loosely tied to any single valence electron. This gives metals their characteristic ability to bend, as well as to be hammered into sheets or drawn into wire. And because the electrons are free to move around the network, it makes metals electrically and thermally conductive.

Alloys

The example above, of potassium metal, is a case of metallic bonding with just a single element. As you may know, it is often fairly easy to mix metal elements together into a mixture known as an alloy. In cases like these, atoms of one metal are substituted with atoms of another metal. Alternately, if the atoms of the second metal are much smaller they may be places in between the atoms of the first. Because of the flexibility of the bonding network, it is generally easy for these atoms to be added in almost any proportion.

Because alloys do not have fixed ratios of elements, they are considered mixtures, not compounds, and they are usually not represented with chemical formulas. They also do not have systematic names. You may already be familiar with the common names of many alloys, such as bronze (copper + tin), brass (copper + zinc), and pewter (tin + antimony). Steel is an alloy of iron and carbon (even though carbon is not a metal, it can participate in metallic bonding), often with nickel, chromium, or other elements added. Different types of steel are often identified by the percentage of carbon and other components in the mixture.

You will not be required to know the names or makeup of alloys for this class. However, you should be able to describe metallic bonding, define an alloy, and explain why alloys are classified as mixtures rather than compounds.

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