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The Structure of the Atom for AP Chemistry (page 2)

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By — McGraw-Hill Professional
Updated on Feb 8, 2011

Electron Shells, Subshells, and Orbitals

According to the latest atomic model, the electrons in an atom are located in various energy levels or shells that are located at different distances from the nucleus. The lower the number of the shell, the closer to the nucleus the electrons are found. Within the shells, the electrons are grouped in subshells of slightly different energies. The number associated with the shell is equal to the number of subshells found at that energy level. For example, energy level 2 (shell 2) has two subshells. The subshells are denoted by the symbols s, p, d, f, etc. and correspond to differently shaped volumes of space in which the probability of finding the electrons is high. The electrons in a particular subshell may be distributed among volumes of space of equal energies called orbitals. There is one orbital for an s subshell, three for a p, five for a d, seven for an f, etc. Only two electrons may occupy an orbital. Table 5.2 summarizes the shells, subshells, and orbitals in an atom. The chapter on Spectroscopy, Light, and Electrons, Chapter 10 has a discussion of the origin of this system.

Energy-Level Diagrams

The information above can be shown in graph form as an energy-level diagram, as shown in Figure 5.1:

Be sure to fill the lowest energy levels first (Aufbau principle) when using the diagram above. In filling orbitals having equal energy, electrons are added to the orbitals to half fill them all before any pairing occurs (Hund's rule). Sometimes it is difficult to remember the relative energy position of the orbitals. Notice that the 4s fills before the 3d. Figure 5.2 may help you remember the pattern in filling. Study the pattern and be able to reproduce it during the exam.

Following these rules, the energy-level diagram for silicon (Z = 14) can be written as shown in Figure 5.3

Although this filling pattern conveys a lot of information, it is bulky. A shorthand method for giving the same information has been developed—the electronic configuration.

Electronic Configurations

The electronic configuration is a condensed way of representing the pattern of electrons in an atom. Using the Aufbau build-up pattern that was used in writing the energy-level diagram, consecutively write the number of the shell (energy level), the type of orbital (s, p, d, etc.), and then the number of electrons in that orbital shown as a superscript. For example, 1s22s1 would indicate that there are two electrons in the s-orbital in energy level (shell) l, and one electron in the s-orbital in energy level 2. Looking at the energy-level diagram for silicon above, the electronic configuration would be written as:

    silicon : 1s22s22p63s2 3p2

The sum of all the superscripts should be equal to the number of electrons in the atom (the atomic number, Z). Electronic configurations can also be written for cations and anions.

Practice problems for these concepts can be found at:

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