The bonding properties of metals and non‐metals will be discussed in Chapter 2.
1.2.2 Electron energy levels
Electrons are arranged outside the nucleus of the atom in energy levels that are sometimes called principal quantum shells. You can imagine these as spherical layers extending out from the nucleus as in Figure 1.4. The energy levels are numbered from 1 to 7, with the level closest to the nucleus being level 1. These numbers are referred to as the principal quantum number of the electrons in that energy level. The principal quantum number has the symbol n. As the distance from the nucleus gets bigger, the volume of the layers increases. In addition, as the distance from the nucleus increases, the energy of the level increases along with the energy of the electrons in the level. The seven principal quantum levels relate to the seven rows across the periodic table.
Figure 1.4 The energy levels (EL) in an atom. The integers represent the principal quantum number, n.
1.2.3 Simple electronic configurations
The arrangement of electrons in the outer shells or principal quantum levels of the atom is called the electronic configuration. Electrons fill the lowest energy levels in an atom first. As n increases, the energy levels increase in both size and energy and can hold more electrons.
The maximum number of electrons that can be held in the first three energy levels is shown in Table 1.2. In this course, we will not be concerned with electrons in energy levels beyond n = 4.
Table 1.2 The maximum number of electrons in the first four energy levels.
| Energy level or shell | n | Maximum number of electrons |
|---|---|---|
| First | 1 | 2 |
| Second | 2 | 8 |
| Third | 3 | 18 |
| Fourth | 4 | 32 |
The first shell can contain only two electrons. The elements that only have electrons in the first shell are hydrogen and helium, with one and two electrons, respectively. Once the first shell is full, at helium, the next electron (in lithium) must be placed in the second shell. The second shell can hold up to eight electrons. Once a shell is full, an element is said to be stable. The noble gases (Group 8, also called Group 18) all have a full outer shell, so they are stable and are very unreactive. Table 1.3 shows the arrangement of the electrons in shells for the first 11 elements in the periodic table.
Table 1.3 Arrangement of electrons in the first 11 elements of the periodic table.
| Element | Number of electrons or Z | Number in 1st shell (n = 1) | Number in 2nd shell (n = 2) | Number in 3rd shell (n = 3) |
|---|---|---|---|---|
| H | 1 | 1 | 0 | 0 |
| He | 2 | 2 | 0 | 0 |
| Li | 3 | 2 | 1 | 0 |
| Be | 4 | 2 | 2 | 0 |
| B | 5 | 2 | 3 | 0 |
| C | 6 | 2 | 4 | 0 |
| N | 7 | 2 | 5 | 0 |
| O | 8 | 2 | 6 | 0 |
| F | 9 | 2 | 7 | 0 |
| Ne | 10 | 2 | 8 | 0 |
| Na | 11 | 2 | 8 | 1 |
You can see from the table that each consecutive element has one more electron, and these fill energy levels from n = 1 upwards. The first energy level is full at helium, and so the next electron (in lithium) enters the n = 2 shell. The second energy level is full at neon (Z = 10), and so the next electron (in sodium) enters the n = 3 energy level.
Electron configurations can be represented as in the diagram in Figure 1.5 for neon. This shows that the first and second shells are both full.
You will notice that there are four paired electrons and two single electrons in the second energy level of the O atom in Worked Example 1.4. Electrons in the same energy level tend to remain unpaired unless they have to pair up with another electron.
Figure 1.5 The structure of neon.