Formal Charges
Click to watch video
Formal Charges
An atom will have a negative charge if it contains an extra negative electron or two over what it normally has. An atom will have a positive charge if it contains one or two fewer negative electrons than it normally has. Chemists have agreed on a set of rules, or an accounting of electrons, to communicate how many electrons an atom has. This accounting is the atom’s formal charge. Understanding formal charges will be useful to you later in this course. It is one way we can identify the reactive sites on molecules. It is crucial to understand formal charges in order to draw good resonance structures. Resonance structures will help you predict how a reaction will proceed.
5. Consider the following oxygen atom/ion,
This oxygen atom/ion (above) contains _____ negative electrons around it. Oxygen’s placement in the periodic table tells us that it normally contains ____ negative electrons around it. Since this oxygen atom contains one more negative electron compared to normal, it has a -1 formal charge.
Now, consider this oxygen atom/ion,
This oxygen atom/ion contains _____ negative electrons around it. Normally, oxygen contains ____ negative electrons around it. Since this oxygen atom contains one fewer negative electron compared to normal, it has a +1 formal charge. That’s pretty simple, right
6. What are the formal charges of the following species? Two of the answers have been filled in for you as a self-check.
Usually, you will not be assigning formal charges to nonbonded atoms like the previous exercise. Instead, you will need to analyze atoms in molecules. When you analyze atoms in molecules, the trick is to remember which electrons are available to the atom. Every nonbonded electron is available to the atom. For every two electrons available in a bond, usually represented by a line (―), one-half of those electrons are available to be counted on the atom. Let’s look at the nitrogen atom in ammonia, NH3.
Bonding of ammonia
Normal nitrogen has five valence electrons. The nitrogen atom in the amide anion has six valence electrons. Since it has one extra negative electron compared to what is normal, the nitrogen atom in amide has a -1 formal charge. Since this is a non-zero formal charge, it must be written on the nitrogen atom. If the formal charge is +1 or -1, typically the “1” is omitted and only the charge is written.
Let’s analyze the nitrogen atom in the ammonium cation, NH4+.
Understanding formal charges in the ammonium cation
Normal nitrogen has five valence electrons. The nitrogen atom in ammonium has four valence electrons. Since it has one fewer negative electron compared to what is normal, the nitrogen atom in ammonium has a +1 formal charge. Since this is a non-zero formal charge, it must be written on the nitrogen atom.
7. Identify the formal charges on the non-hydrogen atoms in the following structures. Write the formal charge on the atoms. If the formal charge is zero, do not write it.
If you don’t like to waste your time, it is important to be able to recognize when atoms are normal, with a zero formal charge, or not. Because if we have a molecule we are inspecting and it has dozens of atoms in it, we don’t want to think about and analyze each of those atoms. This inspection of every atom would be a great waste of time. It is imperative that we be able to recognize normal bonding patterns. If we can quickly identify “normal” atoms, we can stop and analyze only the abnormal atoms. Take a moment to review the normal bonding patterns of atoms.
8. In the following molecules, try to quickly recognize the “normal” atoms with a zero formal charge, and then move on to the next atom. For the atoms that look abnormal, with a non-zero formal charge, stop and calculate its formal charge and write it on the atom.
Answers
5.
7, 6
5, 6
6.
7.
8.
