Chapter Review
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Chapter 31: Atomic Physics Chapter Review |
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Chapter Review
31-6 Multi-Electron Atoms and the Periodic Table
Unlike with hydrogen, the energies of the states of multi-electron atoms depend on both n and 
. Electrons that have the same value of n are said to be in the same shell. The case of n = 1 is called the K shell, n = 2 is called the L shell, and so on, in alphabetical order. Electrons with the same value of are in the same subshell. The case = 0 is called the s subshell, = 1 is the p subshell, = 2 is the d subshell, and = 3 is the f subshell. After the f subshell the names continue in alphabetical order.
Atoms naturally exist in their ground states unless some interactions are present to excite them. Finding the ground state structure of multi-electron atoms requires what is known as the Pauli exclusion principle. This principle states that
only one electron can occupy a given state of an atom.
This means that no two electrons can have the same four quantum numbers. Because of this principle, the ground state of an atom is obtained by filling up the allowed energy states, starting with the lowest (n = 1), until all of the electrons are accounted for (see example 31.4 below).
The electronic configuration of an atom is the specification of how many electrons exist in certain subshells. A particular notation for this specification lists the value of n, followed by the name of the subshell with the number of electrons in that subshell as a superscript. For example, the ground state of helium consists of two electrons each with n = 1, = 0, and m = 0, one electron with ms = 1/2, and the other with ms = -1/2. The electronic configuration of this atom is 1s2. The electronic configuration of atoms leads to a better understanding of the periodic table of the elements. This table is a grouping of the chemical elements according to their properties. It is now known that elements with similar chemical properties correspond to elements with similar outer electron configuration.
Example 31.4 The Ground State of Boron: The boron atom has an atomic number of 5; write out the electronic configuration of its ground state.
Solution
A neutral boron atom will have the same number of electrons as protons. Since its nucleus has an atomic number of 5, it also has 5 electrons surrounding the nucleus. For the n = 1 shell, can only be zero corresponding to the s-subshell. For the electrons in this 1s subshell, one can have ms = -1/2 and the other can have ms = +1/2 which places two electrons in this subshell; so the configuration starts with 1s2.
For the n = 2 shell, can only be either 0 or 1. There will be another two electrons in the s-shell ( = 0) for the same reason as above; so we write 2s2 for the configuration of these electrons. This leaves one electron unaccounted for. This last electron goes into the p-subshell ( = 1). Therefore, the electronic configuration of the ground state of boron is
1s22s2p1.
Notice that the principle quantum number n = 2 is only indicated once for both the s and p subshells. (However, you should be aware that some authors would write this as 1s22s22p1).
Practice Quiz
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31-7 Atomic Radiation
The spectra of multi-electron atoms are more complicated than that of hydrogen. Three types of atomic radiation that are of practical use are discussed in this section. X-rays are high energy photons that are often produced in tubes, called X-ray tubes, in which electrons are accelerated to high speeds and collided into a target. Most of the X-rays, called bremsstrahlung, are generated by the rapid deceleration of the electrons. Some of the X-rays are produced because of transitions, of orbital electrons, in the target atoms which occur because the collision between the incident electrons and the target removes some of the lower energy electrons in target atoms. When the states vacated by these electrons are filled, X-ray photons are given off.
The light given off by a laser is also due to interactions at the atomic level. With laser light, excited atoms are stimulated to emit radiation by incident light. The emitted photons then stimulate other excited atoms to emit photons, and so on. All of these photons have the same energy, phase, and move in the same direction. This process is why they are called lasers which is an acronym for Light Amplification by the Stimulated Emission of Radiation.
Fluorescence occurs when atoms are illuminated by light and the energy of that light excites the atoms into a higher state. Once the illumination ceases, the electrons in the atoms spontaneously fall back to lower energy levels giving off (fluorescent) light of lower frequency than the light used to illuminate the atoms. A related phenomena is phosphorescence in which the material continues to glow long after the original illumination ceases.
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