Bonding Between Atoms

With the review material from the previous pages about classifications of atoms and tendencies to gain and lose electrons in mind, let's consider bonding between atoms. The very heart of bonding is the attraction between positive and negative charges, specifically the positive charge of the nucleus and the negative charge of the electrons. The varying tendencies of atoms to gain or lose electrons allows them to attract one another in various ways and form different kinds of bonds.

Determining Bond Types

Say I give you an arbitrary pair of elements: iron and iridium; strontium and sulfur; carbon and chlorine. By the time you've finished with this lesson and the next one, you'll be able to tell me what kind of bonding each pair would do. This determination is based on whether the elements involved are both metals, both non-metals, or a mixed pair.

Before we dive in, let's set the noble gases aside. They are not particularly good at either gaining or losing electrons, so the almost never form bonds, not even to themselves (i.e. oxygen forms molecules of O₂ that are connected by a bond, but argon just sits there as free atoms with no bonds).

So, if we ignore the inert gases, then we have two types of atoms: metallic atoms and nonmetallic atoms. There are three combinations in which these types of atoms can bond to one another. First, metal atoms can bond to other metal atoms in what we call, naturally, metallic bonding. Second, nonmetal atoms can bond to other nonmetal atoms in what should perhaps be called nonmetallic bonding, but instead we call it covalent bonding. Third, metal atoms can bond to nonmetal atoms in what we call ionic bonding.

Keep in mind throughout this lesson and in lesson 8 that you can (and should) use this simple idea to determine the type of bonding by looking at the types of atoms that are involved. Like many generalities this is an oversimplification (particularly with transition metals and metalloids), but it can be very useful one. Going back to our examples above: iron and iridium would form metallic bonds, strontium and sulfur would form ionic bonds, and carbon and chlorine would form covalent bonds.

At this point I want to stop and discuss the difference between bonding type (which we are learning about now) and substance type (elements vs. compounds, which we covered in Lesson 2). As you should remember, compounds are when multiple elements combine with a fixed ratio. Mixtures are when multiple substances combine with no fixed ratio.

Hopefully this diagram will make it clear how bonding type overlays onto compounds, elements, and mixtures.

Ionic bonding is simplest: it always produces compounds. A single element could never be ionic, as ionic bonding requires both a metal and a non-metal. And for reasons we will see soon, those elements always combine in a fixed proportion. Easy enough.

Covalent bonding occurs between non-metal atoms. If you have two different non-metal elements (as in SO₃) then they will form a compound. If the non-metal atoms are all the same (such as four phosphorus atoms in P₄) then that's just an element.

Metallic bonding occurs between metal atoms. If you only have a single element, then you end up with a big clump of atoms. It might contain a billion atoms, or a billion billion; the point is, the number is not fixed. So we just write the symbol (Au) even though we have a huge number of atoms all bonded together. If you have multiple elements, you get an alloy. Bronze is an example of an alloy consisting of 85-90% copper and 10-15% tin. Because the ratio is variable, we do not write a formula for bronze, and we classify it as a mixture.

Note the last entry on the table: Ne. Neon falls outside our three bonding boxes because it is an inert gas. It does not bond at all. A sample of neon consists entirely of free neon atoms, bopping through the universe unattached to anything else.

So far this probably all seems quite abstract. The central thing to take away from this section, though, is quite concrete. When looking at sets of elements, if the set is all metals then metallic bonding is present. If it's all non-metals, then covalent bonding is present. If it's metals and non-metals, then ionic bonding is present.

*We will see later that sometimes, if you have a metal and two non-metals, then both covalent and ionic bonding may be present. But that is a wrinkle for another section.