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Hydrogen monochloride

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.

Exceptions to the Octet Rule

Oxygen, fluorine, nitrogen, and carbon always obey the octet rule. However, it is often said in jest that the only rule in science is, “There is an exception to every rule.” Indeed, the octet rule applies to most atoms in molecules, but there are exceptions. Some elements, such as Boron, tend to have fewer than 8 valence electrons. Likewise, other elements can have more than 8 valence electrons. Only the elements located in or below period 3 (row 3) of the periodic table tend to exceed an octet.

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Experiment Molecular Modeling and Lewis Structures

Consider the Lewis structure for iodine tetrachloride (ICl4 -). The sum of the valence electrons is

36 (7+(4)7+1=36). Drawing the molecule and placing lone pairs around the Cl atoms results in a representation of only 32 electrons.

Where do the remaining 4 electrons go? Since iodine is located below period 3 of the periodic table, the remaining lone pairs may be placed around the central iodine atom. (You may learn later in your course that this occurs because elements like iodine can exceed the octet by using their empty valence d orbitals.) In the case of ICl4

-, iodide breaks the octet rule, while the Cl atoms obey the octet rule.

As shown above, the total number of valence electrons (36) are represented in the Lewis structure. The placement of the Cl atoms and the lone pairs are adjusted so that the central iodide atom is evenly surrounded.

Valence shell electron pair repulsion (VSEPR)

While Lewis structure models describe atoms in the two-dimensional sense, the valence shell electron pair repulsion (VSEPR) model describes the three-dimensional arrangement of the molecule.

The VSEPR model arranges atoms in a manner that minimizes electron pair repulsion, maintaining the most stable form of the molecule. Consider the two-dimensional Lewis structure for methane (CH4) shown in Figure 6. The three-dimensional VSEPR model has bond angles of 109.5 degrees, placing the hydrogen atoms as far from one another as possible

Figure 6. Lewis structure and VSEPR model of methane (CH4)

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Experiment Molecular Modeling and Lewis Structures