Atom Energy Levels
Our present model of the atom is based on the concept of energy levels for electrons within an atom and on the mathematical interpretation of detailed atomic spectra. The requirements for our model are:
- Each electron in a particular atom has a unique energy that depends on the relationship between the negatively charged electron and both the positively charged nucleus and the other negatively charged electrons in the atom.
- The energy of an electron in an atom can increase or decrease, but only by specific amounts, or quanta.
A. Energy Levels
We picture an atom as a small nucleus surrounded by a much larger volume of space containing the electrons. This space is divided into regions called principal energy levels, numbered 1, 2, 3, 4, . . . . , outward from the nucleus.
Each principal energy level can contain up to 2n2 electrons, where n is the number of the level. Thus, the first level can contain up to 2 electrons, 2(12) = 2; the second up to 8 electrons, 2(22) = 8; the third up to 18, 2(32) = 18; and so on. Only seven energy levels are needed to contain all the electrons in an atom of any of those elements now known.
As stated earlier, the energy associated with an energy level increases as the distance from the nucleus increases. An electron in the seventh energy level has more energy associated with it than does one in the first energy level.
The lower the number of the principal energy level, the closer the negatively charged electron in it is to the positively charged nucleus and the more difficult it is to remove this electron from the atom.
B. Sublevels and Orbitals
When an electron is in a particular energy level, it is more likely to be found in some parts of that level than in others. These parts are called orbitals. Orbitals of equivalent energy are grouped in sublevels. Each orbital can contain a maximum of two electrons. When in a magnetic field, the two electrons in a particular orbital differ very slightly in energy because of a property called electron spin. The theory of electron spin states that the two electrons in a single orbital spin in opposite directions on their axes, causing an energy difference between them. (Like many models, this explanation is an oversimplification, but for the purpose of this course it is a useful description.)
Each principal energy level has one sublevel containing one orbital, an s orbital, that can contain a maximum of two electrons. Electrons in this orbital are called s electrons and have the lowest energy of any electrons in that principal energy level. The first principal energy level contains only an s sublevel; therefore, it can hold a maximum of two electrons.
Each principal energy level above the first contains one s orbital and three p orbitals. A set of three p orbitals, called the p sublevel, can hold a maximum of six electrons. Therefore, the second level can contain a maximum of eight electrons – that is, two in the s orbital and 6 in the three p orbitals.
Each principal energy level above the second contains, in addition to one s orbital and three p orbitals, a set of five d orbitals, called the d sublevel. The five d orbitals can hold up to 10 electrons. Thus, the third level holds a maximum of 18 electrons: 2 in the s orbital, 6 in the three p orbitals, and 10 in the five d orbitals.
The fourth and higher levels also have an f sublevel, containing seven f orbitals, which can hold a maximum of 14 electrons. Thus, the fourth level can hold up to 32 electrons: 2 in the s orbital, 6 in the three p orbitals, 10 in the five d orbitals, and 14 in the seven f orbitals. The sublevels of the first four principal energy levels and the maximum number of electrons that the sublevels can contain are summarized in Table 5.1.
To distinguish which s, p, d, or f sublevel we are talking about, we precede the letter by the number of the principal energy level. For example, the s sublevel of the second principal energy level is designated 2s; the s sublevel of the third principal energy level is designated 3s; and so on. The number of electrons occupying a particular sublevel is shown by a superscript after the letter of the sublevel. The notation
1. Orbital shapes and sizes
Each orbital has a unique shape and size. The shapes of s and p orbitals are shown in Figure 5.5. In these diagrams, the nucleus is at the origin of the axes. The s orbitals are spherically symmetrical about the nucleus and increase in size as distance from the nucleus increases. The 2s orbital is a larger sphere than the 1s orbital, the 3s orbital is larger than the 2s orbital, and so on (see Figure 5.6).
|FIGURE Perspective representations of the s and the three p orbitals of a single energy level. The clouds show the space within which the electron is most apt to be. The lower sketch shows how these orbitals overlap in the energy level.|
|FIGURE Cross-sectional view of the s orbitals of an atom showing their relative sizes and overlap.|
|FIGURE The shapes and orientations of the d orbitals.|
2. The energy of an electron versus its orbital
Within a given principal energy level, electrons in p orbitals are always more energetic than those in s orbitals, those in d orbitals are always more energetic than those in p orbitals, and electrons in f orbitals are always more energetic than those in d ortitals. For example, within the fourth principal energy level, we have:
There is also an interweaving of energy levels. Figure 5.8 shows, in order of increasing energy, all the orbitals of the first four energy levels. Notice that the energy of a 3d orbital is slightly higher than that of a 4s orbital, and that of a 4d orbital is a little higher than that of a 5s orbital. Note especially the overlap of orbitals in the higher principal energy levels.
|FIGURE The principal energy levels of an atom and the sublevels and orbitals each contains. The arrows show the order in which the sublevels fill.|