By Robert M. Hazen, Ph.D., George Mason University
There are two different types of chemical bondings: metallic and covalent. In metallic bonding, atoms adopt the strategy of sharing electrons. This behavior is distinctive from ionic bonds wherein one atom takes an electron, and the other one gives it away. Meanwhile, covalent bonding involves sharing of electrons, like metallic bonding, but in this case only locally with a few atoms.
How Metallic Bonding Works
Think about a large collection of sodium atoms—that’s element 11— and every one of those atoms wants to get rid of an electron. So when a bunch of sodium atoms comes together, that’s exactly what they do—they all give up electrons. And they form a sea of a negative charge, of free-floating electrons.
And in that sea, there are positive centers. The sodium atoms themselves dispersed around that negative sea. The negative sea holds the positive charges together, and that’s how metallic bonding works.
Metallic bonding in nature is extremely common. If you look at the periodic table, almost all the elements that are on the left side of the diagram are metals. Those are all elements that want to give up electrons. And, indeed, it turns out that about three-quarters of all the elements in the periodic table are metals.
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Formation of Alloy with Metallic Bonding
Elements can occur as metallic, but they can also combine with each other, and virtually any combination of two metallic elements forms an alloy, which also displays the properties of metals. There are many famous kinds of metallic alloys. For example, brass is a very beautiful alloy of copper and zinc and is used for many beautiful artifacts. It has been known for hundreds of years, and many antiques of brass are found, as well as brass musical instruments.
We also have bronze, which is an ancient alloy of copper and tin. Then there’s pewter. Pewter was used throughout the colonial period in America, and in Europe, it was commonly used to form cups and plates and other sorts of everyday items. It’s formed from the metals tin and antimony.
We have many modern alloys. Steels, for example, are alloys of iron and carbon with lots of other elements thrown in. Sometimes as many as 20 other elements: titanium and vanadium, manganese. And these other elements provide steels with very special properties. Specialty steels are a huge industry in this country.
Learn more about semiconductors and modern microelectronics.
Distinctive Properties of Metallic Bonding
Now, metallic bonds have a number of distinctive properties. Let’s look at strength, transparency, and electrical conductivity. The bonds between positive atoms in that sea of negative charge aren’t particularly directional, so you can actually take metals and bend them. In some cases, as in gold, you can actually hammer them out into paper-thin sheets.
In the process of gilding, chunks of gold are hammered paper-thin over and over again, and then these paper-thin sheets of gold are applied to surfaces. For example, the beautiful golden domes of many of the state capitals in North America.
Now, in terms of optical properties, that mobile sea of electrons scatters light, so metals show a metallic luster. Indeed, if you can take a piece of glass that’s very smooth and coat the backside of the glass with a metal, you get a perfect reflecting surface called a mirror.
And finally, metals make wonderful electrical conductors because of that sea of electrons. The electrons are free to move, and so electricity can pass through metals quite easily.
Learn more about properties of materials.
How Covalent Bonding Works
Think about what must happen when two hydrogen atoms come together. Hydrogen is element 1, and so it has one electron. So each hydrogen atom wants to see two electrons, and what happens in covalent bonding is exactly that: Two hydrogen atoms come together, and they share their two electrons collectively.
This forms an H2 molecule, and H2 is a gas that can occur in our atmosphere. Now, this link between the two hydrogen bonds, this shared electron bond, is called a covalent bond. This bond contrasts with a hypothetical hydrogen-hydrogen bond that could be ionic.
Imagine, for example, two hydrogen atoms coming together, and one takes the electron from the other one. So you have an H2 molecule, but with a positive charge and a negative charge in two ions.
That’s not what happens. Imagine also that you could have a hydrogen metal in which every hydrogen gives up its electron to a sea to have a negative sea with positive protons, the nucleus of the hydrogen floating around. And that doesn’t happen either. You get a covalent bond.
Properties of Covalent Bonding
The most versatile of all the covalent bonded elements is carbon. Carbon is element six, and its strategy is to gain four more electrons. It often achieves this state by linking to four other atoms. The simplest of these ways of doing this is to find four hydrogens. And so, you have a carbon surrounded by four hydrogens. CH4 is methane or natural gas.
Now, as with ionic bonding and metallic bonding, the properties of materials with covalent bonds reflect the behavior of their electrons. First of all, you’re forming small molecules, and small molecules typically form liquids or gases because they’re light, they’re moving around, and they fly off into space or form very liquid type arrangements.
Also, in terms of electrical conductivity, since all the electrons are shared locally and not through the entire material, covalently bonded materials are often very good electrical insulators. Indeed, we use covalently bonded materials for electrical insulators in our daily lives because ceramic materials—the ionic bonded material—are brittle. They break, and you want a wire to be flexible, and so you use covalently bonded material to surround your metal wire.
Common Questions about Metallic Bonding and Covalent Bonding
Metallic bonding is different from covalent bonding in that in metal bonding all atoms give off their extra electrons and form a sea of electrons, whereas, in covalent bonding, atoms share their electrons locally.
Different metals can also be combined with metallic bonding, resulting in the formation of alloys. An alloy has the properties of both metals.
One of the most important properties of metallic bonding is the electrical conductivity that occurs due to the electron sea. The electron sea also scatters light, and therefore the surface of such material is shiny.
