calculate the formal charge of an atom in an organic molecule or ion. identify and recognize the bonding patterns for atoms of carbon, hydrogen, oxygen, nitrogen and the halogens that have a formal charge of zero.

You are watching: What is the formal charge on the central o atom?


Key Terms

Make certain that you can define, and use in context, the key term below.

valence electrons bonding and non-bonding electrons formal charge carbocations
Study Notes

It is more important that students learn to easily identify atoms that have formal charges of zero, than it is to actually calculate the formal charge of every atom in an organic compound. Students will benefit by memorizing the "normal" number of bonds and non-bonding electrons around atoms whose formal charge is equal to zero.


Determining the Formal Charge on an Atom

A formal charge compares the number of electrons around a "neutral atom" (an atom not in a molecule) versus the number of electrons around an atom in a molecule. Formal charge is assigned to an atom in a molecule by assuming that electrons in all naipublishers.comical bonds are shared equally between atoms, regardless of relative electronegativity. To calculate formal charges, we assign electrons in the molecule to individual atoms according to these rules:

*
*

Non-bonding electrons are assigned to the atom on which they are located. Bonding electrons are divided equally between the two bonded atoms, so one electron from each bond goes to each atom.

The formal charge of each atom in a molecule can be calculated using the following equation:

Formal Charge = (# of valence electrons in free atom) − (# of lone-pair electrons) − (1/2 # of bond pair electrons) Eqn. 2.3.1

To illustrate this method, let’s calculate the formal charge on the atoms in ammonia (NH3) whose Lewis structure is as follows:

*

Formal Charge of H = (1 valence e-) − (0 lone pair e-) − (1/2 x 2 bond pair e-) = 0

The sum of the formal charges of each atom must be equal to the overall charge of the molecule or ion. In this example, the nitrogen and each hydrogen has a formal charge of zero. When summed the overall charge is zero, which is consistent with the overall neutral charge of the NH3 molecule.

*

Typically, the structure with the most formal charges of zero on atoms is the more stable Lewis structure. In cases where there MUST be positive or negative formal charges on various atoms, the most stable structures generally have negative formal charges on the more electronegative atoms and positive formal charges on the less electronegative atoms. The next example further demonstrates how to calculate formal charges for polyatomic ions.



*

Write the formal charges on all atoms in BH4−.

Answer

.

*


Determining the Charge of Atoms in Organic Structures

The calculation method reviewed above for determining formal charges on atoms is an essential starting point for a novice organic naipublishers.comist, and works well when dealing with small structures. But this method becomes unreasonably time-consuming when dealing with larger structures. It would be exceptionally tedious to determine the formal charges on each atom in 2"-deoxycytidine (one of the four nucleoside building blocks that make up DNA) using equation 2.3.1. As you get more experience with organic structures, you will be able to quickly look at this type of complicated structure and determine charges on each atom.

*

2"-deoxycytidine

You need to develop the ability to quickly and efficiently draw large structures and determine formal charges. Fortunately, this only requires some practice with recognizing common bonding patterns.

Organic naipublishers.comistry only deals with a small part of the periodic table, so much so that it becomes convenient to be able to recognize the bonding forms of these atoms. The figure below contains the most important bonding forms. These will be discussed in detail below. An important idea to note is most atoms in a molecule are neutral. Pay close attention to the neutral forms of the elements below because that is how they will appear most of the time.

AtomPositiveNeutralNegative
C
*
*
*
N
*
*
*
*
Figure (PageIndex1): Structures of common organic atoms and ions.

Carbon

Carbon, the most important element for organic naipublishers.comists. In the structures of methane, methanol, ethane, ethene, and ethyne, there are four bonds to the carbon atom. And each carbon atom has a formal charge of zero. In other words, carbon is tetravalent, meaning that it commonly forms four bonds.

See more: How To Watch 50 Shades Of Darker Online Free In 2019, Watch Fifty Shades Darker Full Movie Online Free

*

Carbon usually makes four bonds

Carbon is tetravalent in most organic molecules, but there are exceptions. Later in this chapter and throughout this book are examples of organic ions called ‘carbocations’ and carbanions’, in which a carbon atom has a positive or negative formal charge, respectively. Carbocations occur when a carbon has only three bonds and no lone pairs of electrons. Carbocations have only 3 valence electrons and a formal charge of 1+. Carbanions occur when the carbon atom has three bonds plus one lone pair of electrons. Carbanions have 5 valence electrons and a formal charge of 1−.

*