The Electron Configurations of Atoms

The electron configuration of an atom shows the number of electrons in each sublevel in each energy level of the ground-state atom. To determine the electron configuration of a particular atom, start at the nucleus and add electrons one by one until the number of electrons equals the number of protons in the nucleus. Each added electron is assigned to the lowest-energy sublevel available. The first sublevel filled will be the 1s sublevel, then the 2s sublevel, the 2p sublevel, the 3s, 3p, 4s, 3d, and so on. This order is difficult to remember and often hard to determine from energy-level diagrams such as Figure 5.8

A more convenient way to remember the order is to use Figure 5.9. The principal energy levels are listed in columns, starting at the left with the 1s level. To use this figure, read along the diagonal lines in the direction of the arrow. The order is summarized under the diagram.

FIGURE 5.9 The arrow shows a second way of remembering the order in which sublevels fill.

An atom of hydrogen (atomic number 1) has one proton and one electron. The single electron is assigned to the 1s sublevel, the lowest-energy sublevel in the lowest-energy level. Therefore, the electron configuration of hydrogen is written:

For helium (atomic number 2), which has two electrons, the electron configuration is:

He: 1s²

Two electrons completely fill the first energy level. Because the helium nucleus is different from the hydrogen nucleus, neither of the helium electrons will have exactly the same energy as the single hydrogen electron, even though all are in the 1s sublevel. The element lithium (atomic number 3) has three electrons. In order to write its electron configuration, we must first determine (from Figure 5.9) that the 2s sublevel is next higher in energy after the 1s sublevel. Therefore, the electron configuration of lithium is:

Li: 1s²2s¹

Boron (atomic number 5) has five electrons. Four electrons fill both the 1s and 2s orbitals. The fifth electron is added to a 2p orbital, the sublevel next higher in energy (Figure 5.9). The electron configuration of boron is:

B: 1s²2s²2p¹

Table 5.2 shows the electron configurations of the elements with atomic numbers 1 through 18. The electron configurations of elements with higher atomic number can be written by following the orbital-filling chart in Figure 5.9.

A. Box Diagrams of Electron Configuration
If an atom has a partially filled sublevel, it may be important to know how the electrons of that sublevel are distributed among the orbitals. Research has shown that unpaired electrons (a single electron in an orbital) are in a lower energy configuration than are paired electrons (two electrons in an orbital). The energy of the electrons in a sublevel would then be lower with half-filled orbitals than with some filled and some empty. We can show the distribution of electrons by using box diagrams, where each box represents an orbital and the arrows within the boxes represent the electrons in that orbital. The direction of the arrow represents the spin of the electron. (Recall from Section 5.3B that two electrons in an orbital spin in opposite directions on their axes.) Therefore, if an orbital contains two electrons, its box will contain two arrows, one pointing up and the other down.

Using a box diagram, we show the electron configuration of nitrogen as:

Notice that the 2p electrons are shown as

rather than

which would mean that, of the three p orbitals, one is filled, one is half-filled, and one is empty.