Electronic Structure and Position in the Periodic Table

1An address for every element

A letter to a pupil in Rwanda is addressed by district, then sector, then cell. The periodic table gives every element a similar two-part address: a period (row) and a group (column). The remarkable thing is that the address is not arbitrary. If you know an element's electronic configuration, you can work out its address, and from its address you can predict how it behaves.

Figure 3.1: The first 20 elements in the modern periodic table, arranged in order of atomic number. The configuration is printed at the bottom of each box.

2Reading the address from the configuration

Look along any row of Figure 3.1 and down any column, and two rules appear:

$$\text{period number}=\text{number of occupied shells}\backslash text\{period\; number\}\; =\; \backslash text\{number\; of\; occupied\; shells\}$$

$$\text{group number}=\text{number of outer electrons (groups 1 and 2)},\text{or outer electrons}+10\text{ (groups 13 to 18)}\backslash text\{group\; number\}\; =\; \backslash text\{number\; of\; outer\; electrons\; (groups\; 1\; and\; 2)\},\; \backslash qquad\; \backslash text\{or\; outer\; electrons\}\; +\; 10\; \backslash text\{\; (groups\; 13\; to\; 18)\}$$

The "+10" is there because the ten groups of transition metals (3 to 12) sit between groups 2 and 13 from period 4 onwards. Many books, including older Rwandan ones, number the main groups I to VIII instead; then the group number simply equals the number of outer electrons. Both numbers are shown at the top of Figure 3.1.

One exception: helium has only 2 electrons, but its shell is full, so it sits with the other noble gases in group 18.

Figure 3.2: Finding magnesium's address from its Bohr diagram.

Table 3.1: The main groups of the first 20 elements