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Tribromine octoxide

Experiment Molecular Modeling and Lewis Structures

Consider the outcome if the Lewis structure was drawn with a single bond, and the octet rule was fulfilled. The total number of electrons would be 14. In step 1, we determined that the molecule has 10 valence electrons, so we know that the Lewis structure below is incorrect, even though the octet rule is fulfilled.

Consider the outcome if the Lewis structure was drawn with the correct number of total valence electrons and a single bond. The Lewis structures below are both incorrect because, although 10 electrons are present in both structures, the octet rule is not fulfilled.

Consider the outcome if the Lewis structure was drawn with a double bond and the octet rule was fulfilled. The Lewis structure below is incorrect because, yet again, the number of valence electrons is incorrect. The total number of electrons in the structure below is 12, and it was determined that the molecule has 10 valence electrons.

The correct Lewis structure has a triple bond; carbon has one lone pair, and nitrogen has one lone pair. The octet rule is fulfilled, and the valence electrons total 10, as calculated in step 1.

However, the Lewis structure is not complete. The charge of the molecule is denoted by placing the structure in brackets and writing the charge in the upper right-hand corner.

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Experiment Molecular Modeling and Lewis Structures

Resonance Structures

Resonance occurs when more than one valid Lewis structure exists. In this scenario, the various molecular structures may be referred to resonance structures.

Consider the Lewis structure for the polyatomic ion CO3 -2. Following the steps for drawing Lewis

structures, we find that CO3 -2 has a total of 24 electrons (4+(3)6+2=24). Through trial-and-error, it

is determined that one double bond, two single bonds, and 8 lone pairs exist.

A Lewis structure for CO3 -2 can be drawn as follows:

In the diagram above, the oxygen atoms are evenly distributed around the central carbon atom. However, notice that there is more than one option for the placement of the double bond. The double bond could be placed between the C atom and any one of the O atoms. Thus, the Lewis structure may be represented in three drawings.

The CO3 -2 molecule has resonance, which occurs when more than one valid Lewis structure

exists. In reality, the electron structure of CO3 -2 is a combination of all three resonance structures.

Resonance is represented by double-headed arrows as follows. Notice that the -2 charge is denoted for each resonance structure.