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TABLE 8.3

Electron configuration for the first 20 elements
Atomic Number Element Electron Configuration
1 Hydrogen 1s 1
2 Helium 1s 2
2
3 Lithium 1s 2s 1
2
4 Beryllium 1s 2s 2
2
2
Spin up Spin down 5 Boron 1s 2s 2p 1
2
2
6 Carbon 1s 2s 2p 2
FIGURE 8.15 Experimental evidence supports the concept
2
2
7 Nitrogen 1s 2s 2p 3
that electrons can be considered to spin one way or the other as 2 2 4
they move about an orbital under an external magnetic field. 8 Oxygen 1s 2s 2p
2
2
9 Fluorine 1s 2s 2p 5
2
2
10 Neon 1s 2s 2p 6
6
2
2
11 Sodium 1s 2s 2p 3s 1

magnetic field would have a different energy than one
6
2
2
12 Magnesium 1s 2s 2p 3s 2
spinning the other way (say, counterclockwise). Th e spin
2
2
2
6
13 Aluminum 1s 2s 2p 3s 3p 1
quantum number describes these two spin orientations
2
6
2
2
14 Silicon 1s 2s 2p 3s 3p 2
(Figure 8.15).
2
2
2
6
15 Phosphorus 1s 2s 2p 3s 3p 3
Electron spin is an important property of electrons that helps 16 Sulfur 1s 2s 2p 3s 3p 4
2
2
2
6
determine the electronic structure of an atom. As it turns out, two 2 2 6 2 5
17 Chlorine 1s 2s 2p 3s 3p
electrons spinning in opposite directions, called an electron pair,
2
2
6
2
18 Argon 1s 2s 2p 3s 3p 6
can occupy the same orbital. This was summarized in 1924 by
2
6
2
6
2
19 Potassium 1s 2s 2p 3s 3p 4s 1
Wolfgang Pauli, an Austrian physicist. His summary, now known
2
2
6
2
6
20 Calcium 1s 2s 2p 3s 3p 4s 2
as the Pauli exclusion principle, states that no two electrons in an
atom can have the same four quantum numbers. This provides the
key for understanding the electron structure of atoms.
Table 8.3 gives the electron configurations for the first
8.4 ELECTRON CONFIGURATION
20 elements. The configurations of the p energy sublevel have
The arrangement of electrons in orbitals is called the electron con- been condensed in this table. There are three possible orienta-
6
figuration. Before you can describe the electron arrangement, you tions of the p orbital, each with two electrons. This is shown as p ,
need to know how many electrons are present in an atom. An which designates the number of electrons in all of the three pos-
atom is electrically neutral, so the number of protons (positive sible p orientations. Note that the sum of the electrons in all the
charge) must equal the number of electrons (negative charge). orbitals equals the atomic number. Note also that as you pro-
The atomic number therefore identifies the number of electrons ceed from a lower atomic number to a higher one, the higher
as well as the number of protons. Now that you have a means of element has the same configuration as the element before it with
finding the number of electrons, consider the various energy lev- the addition of one more electron. In general, it is then possible
els to see how the electron configuration is determined. to begin with the simplest atom, hydrogen, and add one electron
According to the Pauli exclusion principle, no two electrons at a time to the order of energy sublevels and obtain the electron
in an atom can have all four quantum numbers the same. As configuration for all the elements. The exclusion principle
it works out, this means there can only be a maximum of two limits the number of electrons in any orbital, and allowances
e lectrons in any given orbital. There are four things to consider: will need to be made for the more complex behavior of atoms
(1) the main energy level, (2) the energy sublevel, (3) the num- with many electrons.
ber of orbital orientations, and (4) the electron spin. Recall that The energies of the orbital are different for each element,
the lowest energy level is n = 1, and successive numbers identify and there are several factors that influence their energies.
progressively higher-energy levels. Recall also that the energy The first orbitals are filled in a straightforward 1s, 2s, 2p, 3s,
sublevels, in order of increasing energy, are s, p, d, and f. This then 3p order. Then the order becomes contrary to what you
electron configuration is written in shorthand, with 1s stand- might expect. One useful way of figuring out the order in
ing for the lowest energy sublevel of the first energy level. A which orbitals are filled is illustrated in Figure 8.16. Each row
superscript gives the number of electrons present in a sublevel. of this matrix represents a principal energy level with possible
Thus, the electron configuration for a helium atom, which has energy sublevels increasing from left to right. The order of
2
two electrons, is written as 1s . This combination of symbols has filling is indicated by the diagonal arrows. There are excep-
the following meaning: The symbols mean an atom with two tions to the order of filling shown by the matrix, but it works
electrons in the s sublevel of the first main energy level. for most of the elements.
8-13 CHAPTER 8 Atoms and Periodic Properties 215