Comparing Bonds and Compounds

Lesson Objectives

The student will:

  • identify the most likely type of bonding (ionic, covalent, or polar covalent) for each compound given binary formulas and an electronegativity chart.
  • compare the properties of ionic and covalent compounds

Vocabulary

  • dipole: the separation of charges within a covalently bonded molecule (so that one end of the bond has a partial negative charge, and the other end has a partial positive charge.)
  • polar covalent bonds:covalent bonds where the electrons are still shared by the two atoms, but they are not shared equally

Introduction

Recall from the lesson on "Chemical Periodicity" that the ability for an atom in a molecule to attract shared electrons is called electronegativity. The electronegativity of atoms has been quantified in several ways. One method that is widely accepted is that of Linus Pauling. The Pauling electronegativity values are shown below.

When two atoms combine, the difference between their electronegativities is an indication of the type of bond that will form. If the difference between the electronegativities of the two atoms is small, neither atom can take the shared electrons completely away from the other atom, and the resulting bond will be covalent. If the difference between the electronegativities is large, the more electronegative atom will take the bonding electrons completely away from the other atom (electron transfer will occur), and the bond will be ionic. There is no exact number for the difference in electronegativity that will produce an ionic bond, but many chemists suggestas the approximate difference that will produce an ionic bond.

If we compare a few electronegativities, we can see thatis a reasonable suggestion. The difference between the electronegativities of hydrogen and oxygen is, and we know the bonds in the water molecule are covalent. In magnesium chloride, the electronegativity difference is, and we know this molecule contains ionic bonds.

The Partial Ionic Character of Covalent Bonds

So far, we have discussed two extreme types of bonds. One case is when two identical atoms bond. They have exactly the same electronegativities, thus the two bonded atoms pull exactly equally on the shared electrons. The other case is when the bonded atoms have a very large difference in their electronegativities. In this case, the more electronegative atom will take the electrons completely away from the other atom, forming an ionic bond.

What about the molecules whose electronegativities are not the same but the difference is not as large as? For these molecules, the electrons are still shared by the two atoms, but they are not shared equally. The shared electrons are pulled closer to the more electronegative atom. This results in an uneven distribution of electrons over the molecule and causes slight charges on opposite ends of the molecule. Such molecules are said to have a dipole or are called polar molecules (see figure below). These charges are not fullandcharges, but rather they are fractions of charges. For small fractions of charges, we use the symbolsand.

Atoms whose electronegativities are not the same but are not different enough to cause complete electron transfer will formpolar covalent bonds.With the introduction of the polar covalent bond, we now know of three different types of bonds.

In the figure below, molecule A represents a perfectly covalent bond that would form between identical atoms, or between atoms with electronegativity differences less than 0.5. Molecule B is a polar covalent bond formed between atoms whose whose electronegativity difference is greater than 0.5 but less than, while molecule C is an ionic bond formed between atoms whose electronegativity difference is greater than.

Properties of Covalent Compounds

Carbon will share electrons with hydrogen, or other atoms, to get an octet.

Covalent compounds have very different properties from ionic compounds. Ionic compounds have high melting points, causing them to be solid at room temperature. Covalent compounds have low melting points and many are liquids or gases at room temperature. Whereas most ionic compounds are capable of dissolving in water, many covalent compounds do not. Unlike ionic compounds, covalent compounds that are dissolved in water are not good conductors of electricity.

When elements combine to form covalent compounds, the properties of the compound are different from the properties of the elements from which the compound is formed. (Recall that the same was true for ionic compounds.) Consider, for example, sugar, formed from the elements carbon, hydrogen, and oxygen: C12H22O11. Forms of carbon you are probably familiar with include coal and graphite (pencil lead). Oxygen is a gas necessary for your survival, and hydrogen is a very flammable gas. You wouldn't want the sugar you put on your cereal to taste like coal or be as flammable as hydrogen and oxygen gases. When combined, a new compound is made with its own unique properties, different from the elements that formed the compound.

What happens when these elements combine in different ratios, forming compounds such as isopropyl alcohol (commonly called rubbing alcohol),, or acetone (the main ingredient in most finger nail polish removers), C3H6O? Does rubbing alcohol have the same properties as finger nail polish remover or sugar? No! When elements combine in different ratios, different compounds are formed which have their own unique properties. Each compound will typically have its own melting point, boiling point, and density. Frequently, they will have a unique smell or taste. They will also have unique chemical properties and react differently than other compounds.

Lesson Summary

  • A difference of approximately 1.7 or more in the electronegativities of bonded atoms will produce an ionic bond.
  • Bonds between atoms whose electronegativity difference is between 0.5and 1.7 will be polar covalent.
  • Bonds between atoms whose electronegativity difference is less than 0.5 will be nonpolar covalent.
  • Molecules with an uneven distribution of electrons over the molecule have slight charges on opposite ends of the molecule, and are said to have a dipole.
  • Ionic compounds form ionic crystal lattices rather than molecules. They have very high melting and boiling points, and tend to be brittle solids. They are generally soluble in water, and the resulting solutions will conduct electricity.
  • Ionic compounds have chemical properties that are unrelated to the chemical properties of the elements from which they were formed.
  • Covalent compounds are formed by non-metals sharing electrons. They tend to have low melting and boiling points. Although some are soluble in water, they do not conduct electricity when dissolved.

Further Reading / Supplemental Links

To see a video and clips discussing the types of bonds, go to Go to the k-12 library. Search for covalent bond, metallic bond, or ionic bond. (username to pioneer: pioneer; password: time)

This video is a ChemStudy film called "Shapes and Polarities of Molecules: