Orbitals and Hybridization - MCAT Chemical and Physical Foundations of Biological Systems

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Electronegativity is an important concept in physical chemistry, and often used to help quantify the dipole moment of polar compounds. Polar compounds are different from those compounds that are purely nonpolar or purely ionic. An example can be seen by contrasting sodium chloride, NaCl, with an organic molecule, R-C-OH. The former is purely ionic, and the latter is polar covalent.

When comparing more than one polar covalent molecule, we use the dipole moment value to help us determine relative strength of polarity. Dipole moment, however, is dependent on the electronegativity of the atoms making up the bond. Electronegativity is a property inherent to the atom in question, whereas dipole moment is a property of the bond between them.

For example, oxygen has an electronegativity of 3.44, and hydrogen of 2.20. In other words, oxygen more strongly attracts electrons when in a bond with hydrogen. This leads to the O-H bond having a dipole moment.

When all the dipole moments of polar bonds in a molecule are summed, the molecular dipole moment results, as per the following equation.

Dipole moment = charge * separation distance

A single polar bond, such as the O-H bond described in the passage, is made up of two shared electrons. Before they join together in a bond, what subshell corresponds to the second quantum number (L) of the highest energy valence electrons of oxygen and hydrogen, respectively?

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Answer

The highest energy electrons are those that are in the orbitals most removed from "s," in the order of .

As you add electron shells (n = 1 to n =2 to n = 3 and so on), you add one more of these orbitals. The result is that hydrogen only has an "s" orbital because it has n = 1, but oxygen has "s" and "p" because it has n = 2.

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