Determine Bond Polarity

Atoms bond together to form molecules by sharing electrons and this sharing of electrons can sometimes be equal (or nearly equal). Other times, one atom has more of the electrons on average. When one atom has a disproportional amount of the negative charge (the electrons) it means that the other atom will have a positive charge. This makes the bond a polar bond, meaning that it has a positive and negative pole. You can identify polar bonds by looking at the kinds of atoms bonded together, and the electronegativity of those atoms. You can then classify the bond as polar or nonpolar.

Steps

Looking at Bonded Atoms

1. Identify any metals. Metals are typically shiny and malleable. They often have loosely bound electrons. That means that they have weaker electronegativity than many nonmetals. This allows metals to “give” some of their electrons to nonmetals, resulting in a dipole.
   • A dipole is when a bond has a positive and negative charge at either end. The presence of a dipole indicates a polar bond.
2. Note any non metals. Non metals are usually hard and brittle, and lack luster (shine). They often have a greater electronegativity than metals. This means that they can “take” the electrons from the metal atoms to which they are bound. This will create a dipole in the bond.
3. Consider valence electrons for each atom. The Study the Chemical Theory of Valence Bonds of an atom are the ones that reside in its outer shell. For the most part, atoms follow the octet rule, meaning that eight valence electrons will create the most stable configuration. Atoms with nearly eight are likely to “take” other electrons, while atoms with only one or two valence electrons are likely to “give” up their outer electrons.
   • For example, sodium (Na) has one valence electron, and chlorine (Cl) has seven. When they bond, they form the salt sodium chloride (NaCl) because the sodium gives up its one valence electron and the chlorine accepts it. This is a polar bond.

Analyzing Electronegativity

1. Account for the electron affinity of each atom. The electron affinity of an atom is the measure of how likely that atom is to “take” electrons from another atom. Electron affinity grows as you go from left to right across the Study the Chemical and Physical Properties of Atoms in the Periodic Table, and up from bottom to top. In other words, small, non metallic atoms tend to have the highest electron affinity.
   • Electron affinity is one component of the atom’s electronegativity.
   • Some atoms with high electron affinity are fluorine, chlorine, oxygen, and nitrogen.
   • A few low affinity atoms are sodium, calcium, and hydrogen.
2. Consider the ionization energy for each atom. Ionization energy is the amount of energy needed to remove an electron from any particular atom. The ionization energy rises as you go from left to right across the periodic table, and from bottom to top. This means that small, non metallic atoms are the hardest atoms from which to remove electrons. Large, metallic atoms are the easiest.
   • Ionization energy is the other component of the atom’s electronegativity.
3. Utilize the periodic table trends. By looking at the periodic table, you can tell a lot about how polar a bond will be between any two atoms. Atoms at the top right of the table will tend to ionize and hold a negative charge. Atoms on the far left of the table tend to form positive ions. Atoms in the middle of the table form less polar bonds.

Classifying Bond Types

1. Consider all nonpolar bonds covalent. By definition, a non polar bond must be covalent. That means that electrons are shared evenly from one atom to the next. A truly nonpolar bond has a negativity difference of zero between the two atoms.
   • For example, hydrogen gas (H₂) forms a nonpolar bond between the two hydrogen atoms because they have the exact same electronegativity.
2. Recognize polar covalent bonds. Polar covalent bonds are formed when you have two atoms with similar (but not identical) electronegativities. This generally happens between two nonmetals, and has a weak dipole. These bonds have an electronegativity difference that is greater than zero, but less than two.
   • For example, a carbon hydrogen bond is weakly polar, making it a polar covalent bond.
3. Categorize ionic bonds. Ionic bonds usually form between metals and non metals. When each atom ionizes to an anion or cation, there is a strong dipole. Ionic bonds have atoms with an electronegativity difference greater than two.
   • Bonds between calcium and chlorine are ionic.

Solving Practice Problems

1. Determine the polarity of table salt. Table salt bears the chemical formula NaCl, as it is comprised of one sodium and one chlorine atom. To determine the polarity of table salt, you can find that the electronegativity of sodium is 0.9 and that of chlorine is 3.0. You will find the difference between the two electronegativities to be 2.1, which means that table salt is held together by an ionic bond (and thus is polar).
   • You could also make a reasonable assumption that the bond is polar simply by noticing where each atom lies on the periodic table.
2. Find the difference in electronegativity for carbon and hydrogen. Start by looking at a periodic table that lists electronegativities. You will find that of hydrogen to be 2.1 and that of carbon to be 2.5. The difference between the two is 0.4, meaning that a carbon-hydrogen bond is (slightly) polar.
3. Give an example of a truly nonpolar bond between two different atoms. To accomplish this, you must look at a periodic table that lists electronegativities. Locate two atoms that have identical electronegativities. These two atoms will form a covalent bond.
   • For example, hydrogen and tellurium will form a covalent bond.

Sources and Citations

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