METALLIC BOND

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 this (also in example 31 in your workbook). 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 this. 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. Network 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. The network in this paper clip (which of course is not made out of potassium), has a vast number of atoms that are all bonded to one another. This paper clip has on the order of 1022 atoms. The network of metallic bonding holds that entire chunk of metal together. Each metallic bond gives strength and the network extends that strength over the entire chunk of metal. How can this particular model of metallic bonding be used to explain the properties of metals (such as electrical conductivity, malleability, and thermal conductivity)? The key is in the loosely held electrons spread around and between all the metal atoms, or metal ions. These electrons can move easily from one place to another, allowing for good electrical conductivity. To a limited extent, the atoms can also move from one place to another and still remain in contact with and bonded to the other atoms and electrons around them. I will use these beads to represent the atoms. If these are shifted in position, the atoms still remain pretty much in contact with one another. Although the external shape of the metal is changing, the internal pattern is pretty much the same. Thus the shape of metals can be changed. The atomic vibration that we measure as heat can be easily passed from one atom to the next making metals good conductors of heat as well as electricity. Elements and Alloys If all of the atoms in a piece of metal are the same, we have a pure element. But what about metals like brass and steel that are not pure elements? What happens if we add different elements, different kinds of atoms, heat them until they melt and mix them? When we do that, we get a mixture of atoms that we call an alloy. The ratio of atoms of one kind to another is not fixed. The composition of the mixture will vary depending on how many of each kind of atom (or how much of each kind of element) happens to be available. Naming Alloys? There is not yet a systematic way of naming alloys like those you have learned for naming covalent and ionic compounds. In part, this is because alloys are not compounds, their composition is variable and their names need to reflect that. Generally, we use common names like brass, bronze, or steel or technical names like alnico I, alnico II, chromium steel or high carbon steel.