FIGURE 2.1 Lewis structures for formaldehyde (left) and water (right) show the presence of four electrons (each represented by a dot) on the oxygen atoms of the two molecules.These electrons are not involved in the formation of any bond. The pairs of electrons involved in holding the molecule together are represented by lines.

Molecules with equal numbers of protons and electrons are not charged. But the distribution of electrons in three dimensions need not be uniform in the molecule with respect to the distribution of the nuclei. In this case, the distribution of negative charge in a molecule may be different from the distribution of positive charge, and the molecule has one or more bond dipole moments, the magnitude of which depends on these distributions. The more uneven the distributions of charge in space, the more polar the molecule is.a

In general, atoms with similar electronegativities share electrons equally. Carbon and hydrogen, as atoms, have similar electronegativities. Hence, the carbon-hydrogen bond is not generally associated with a large bond dipole moment, and molecules that contain carbon-hydrogen units are not particularly polar. In contrast,bonds joining carbon and hydrogen atoms to heteroatoms (atoms other than carbon or hydrogen)are often quite polar. For example, the oxygen-hydrogen bond, a bond between atoms with very different electronegativities, is generally associated with a large dipole moment. As a consequence,molecules that contain — OH groups are generally polar. Molecules that contain many unshared pairs of electrons are also more polar.

The nature and distribution of its charge are the dominant features in determining a molecular structure’s bulk physical properties. Knowing that a molecule is an ion is generally more important than almost any other piece of information for understanding the physical behavior of the molecule. If a molecule lacks a charge, then a statement about the magnitude of the dipole moment is most informative about its physical properties.

One of the more prominent features of polar molecules and ionic species is their ability to dissolve in polar solvents. Water, in turn, is one of the more polar solvents,because the distribution of electrons is quite different from the spatial arrangement of protons; the oxygen atom of H2O carries more negative charge, while the hydrogen atoms carry more positive charge (Figure 2.3).

As a consequence, water dissolves many salts and many molecules that have large dipole moments. Water does not dissolve molecules that lack a charge or a substantial dipole moment. Nonpolar molecules that contain many carbon-hydrogen and carbon-carbon units are called hydrocarbons and are well known as oils and fats.Their nonpolarity is the general reason that oil (petroleum oil and vegetable oil alike) and water do not mix.

2.1.3
Distribution of Charge Can Be Inferred from Molecular Structures

The polarities of molecules can be estimated from their molecular structures. The electronegativities of the constituent atoms are the key to making those estimates. The structure of the organic molecule is scanned, polar bonds between atoms with different electronegativities are identified, and the number of those polar bonds present in the molecule relative to the nonpolar bonds between carbon and hydrogen, or between

FIGURE 2.2 The sodium cation,the chloride anion, and the hydroxide anion, with their charges represented by the sign in a circle, have different numbers of protons and electrons.

a

The bond dipole moment is a measure for the polarity of a chemical bond between atoms within a molecule. For a complete molecule the total molecular dipole moment may be approximated as the vector sum of individual bond dipole moments.



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