Ionic and Covalent Bonding

There are primarily two forms of bonding that an atom can participate in: Covalent and Ionic. Covalent bonding involves the sharing of electrons between two or more atoms. Ionic bonds form when two or more ions come together and are held together by charge differences.

So how do you know what kind of bond an atom will make? That is actually the easy part. Metals and Non-Metals when combined make ionic compounds. Non-Metals when combined with other Non-Metals make covalent compounds. So all you need to be able to do is figure out what elements are Metals and which are Non-Metals. For that information we can use the periodic table:

Tutorial - Covalent Bonding
© 2008 WGBH Educational Foundation. All Rights Reserved.

Covalent Bonds

As we mentioned before, the electrons in an atom are what is responsible for forming bonds. What we did not discuss previously is which electrons in the atom are involved in bonding. The bonding electrons are called the VALENCE electrons and they are the electrons that are found in the outermost shell of the atom. In the periodic table below, you can see diagrams of each element that shows how many valence electrons it possesses. Conveniently, the Group Number at the top of each column in the periodic table also gives the number of valence electrons. For example, Boron (represented as B in the periodic table) is in Group 3A and has 3 valence electrons; Carbon (represented as C) is in Group 4A and has 4 valence electrons.

Once you know how many valence electrons an atom has, you can start to build molecules. There are a couple of rules to follow however as you build:

1. The Octet Rule: The atoms that participate in covalent bonding share electrons in a way that enables them to acquire a stable electron configuration, or full valence shell. This means that they want to acquire the electronic configuration of the noble gas of their row. The Noble Gases are like the Michael Jordan's of the Chemistry world and every one of them wants to "be like Mike".
2. There are several exceptions to the octet rule however:
   • Hydrogen (H) only requires 2 electrons to have a full valence shell since it only needs to be like Helium (He).
   • Elements on the 3rd Period (3rd row) of the periodic table and below can actually have more than 8 electrons around them. They have extra space to allow for the extra electrons.

Now that you know the number of valence electrons and the rules you can start making molecules. For instance, looking at hydrogen we know that it is in Group I and thus has 1 valence electron, if it bound itself to another hydrogen they could share the two electrons between them and both be "happy". See below.

If that same hydrogen bonded to Chlorine, the hydrogen would get the two electrons it needs to be complete and the chlorine which has 7 valence electrons would get the one more to fulfil its octet. See above.

Now that you can form covalent compounds we need to go over how to name these compounds. Nomenclature is the fancy way of saying the rules for naming.

Covalent Compound Nomenclature

1. The first element is named first, using the elements name:

SF₆ Sulfur Hexafluoride

2. Second element is named using the suffix "-ide"

SF₆ Sulfur Hexafluoride (Fluorine becomes Fluoride)

3. Prefixes are used to denote the number of atoms

SF₆ Sulfur Hexafluoride (There are 6 Fluorines so Hexa is used as the prefix)

4. "Mono" is not used to name the first element

SF₆ Sulfur Hexafluoride (Note that there is only one Sulfur but no Mono prefix)

Note: when the addition of the Greek prefix places two vowels adjacent to one another, the "a" (or the "o") at the end of the Greek prefix is usually dropped; e.g., "nonaoxide" would be written as "nonoxide", and "monooxide" would be written as "monoxide". The "i" at the end of the prefixes "di-" and "tri-" are never dropped.

Ionic Bonding

Ionic bonds are formed by the combination of positive and negative ions; the combination of these ions form in numerical combinations that generate a neutral (zero charge) molecule.

So how do you know what kind of ion an element will form?

Again, our answers can be found using the periodic table:

Just as with the covalent compounds, each ion wishes to form an octet and be like the nearest noble gas. Sometimes it is easier for the element to gain electron(s) (anions) to produce the octet and sometimes it is easier for the element to lose electron(s) (cations). If you look at the periodic table above you will note that the Group 1A, 2A and 3A elements all form positive ions or Cations. This is because it is easier energetically for those elements to lose 1, 2, or 3 electrons than it would be for them to gain 5, 6 or 7 electrons. The gain or loss of an electron generally requires energy and once you exceed the gain or loss of 3 electrons the energy cost is simply too high for most atoms to accomplish. You should also notice that the elements on the right side of the periodic table (the non-metals) in Groups 5A, 6A and 7A all form negative ions or Anions for the same reason.

You can determine the charge that an element will form as an ion by looking at how far that element is from the nearest noble gas. For example, elements in Group 2A are 2 columns away from the nearest noble gas so losing 2 electrons will give them the noble gas number of electrons; Group 5A elements are 3 columns away from the nearest noble gas so addition of 3 electrons will work best for them and so on.

Tutorial - Ionic Bonding
© 2008 WGBH Educational Foundation. All Rights Reserved.

Forming Ionic Compounds

As was mentioned above, ions come together in compounds to form neutral (uncharged) molecules. This means that the positive and negative ions have to be balanced so that their charges all add up to zero:

In the examples to the right, the sodium is +1 and the chloride is -1 so adding them together to form a neutral molecule (positive charges + negative charges = zero) only requires 1 of each. NaCl

But in the case of calcium which forms a +2 ion and chlorine which forms a -1 ion, we need two chlorines to balance the charge of the one calcium. CaCl₂

Here are a few more general rules to follow when building and naming ionic molecules:

1. The number of ions in the compound is indicated as a subscript after the element's symbol: MgF₂ (Magnesium Fluoride), AlCl₃ (Aluminum Chloride), or Al₂O₃ (Aluminum Oxide)
2. The cation is generally listed first in the compound: MgF₂ (Magnesium Fluoride), AlCl₃ (Aluminum Chloride), or Al₂O₃ (Aluminum Oxide)
3. The name of the compound is simply the name of the positive element followed by the name of the negative element adding the –ide suffix: MgF₂ (Magnesium Fluoride), AlCl₃ (Aluminum Chloride), or Al₂O₃ (Aluminum Oxide)

Notice that in ionic nomenclature you do not use the Greek prefixes to indicate the number of atoms in the molecule. This is because as chemists we know the number since the charge the ions take on is predictable.

So to sum up the process for identifying, writing and naming compounds:

Up until now we have not discussed the metals beyond those in the Groups IA, 2A and 3A. The metals in the B Groups in the middle of the periodic table are also involved in ionic bonding. Their charges as an ion are less predictable however and they can actually have more than one charge as an ion:

Whenever you write an ionic compound that contains a transition metal ion, you have to indicate in the name which ion you are using by the inclusion of a Roman numeral in the name:

Fe²⁺ + Br^- → FeBr₂   Iron (II) Bromide

Fe³⁺ + Br^- → FeBr₃   Iron (III) Bromide

Polyatomic Ions

Another special case for creating and naming compounds derives from the existence of polyatomic ions. Polyatomic ions are ions that are made up of non-metals that when combined form a charged molecule. A table of the more common of these ions is shown below:

Common Polyatomic Ions

When a polyatomic ion is part of an ionic compound the rules for assembly are the same: the ions must combine to make a neutral molecule. But because the polyatomic ion must be treated like a single substance parenthesis are placed around it in the formula if more than one ion is required.

For instance, if you combined Magnesium Ion, Mg²⁺ and Phosphate Ion, PO₄^3-, to balance the charges you would need 3 magnesium ions and 2 phosphate ions: Mg₃(PO₄)₂ The parenthesis are placed around the polyatomic ion to indicate that the subscript creates a multiple of the entire ion not just a single atom. The parentheses are only used in cases where there is more than one polyatomic ion in the molecule. So for instance, MgSO₄ contains the sulfate ion (SO₄^2-) but since only one is required to balance the molecule, no parentheses are needed.

Let's Practice: