2.2 The Group number of the noble gases
In Figure 18, the Period numbers increase steadily from 1 to 7 down the columns. It obviously seems appropriate that the Group numbers should show a similar steady increase from I to VIII across the rows. However, this numbering scheme puts the noble gases in Group VIII. As Section 2.1 makes clear, almost none of these six elements then obeys generalisation (iii). For example, with this Group numbering, generalisation (iii) predicts the formula AO4 for the highest normal oxides of the noble gases, where A represents a noble gas atom. Only for xenon is such a compound known.
The situation is improved if one changes the Group number of the noble gases from VIII to zero. This is because there are no known oxides or binary fluorides of helium, neon or argon. In the case of the noble gases, generalisation (iii) then fails only at xenon when predicting oxide formulae, and at krypton, xenon and radon when predicting fluoride formulae. So in introducing chemical periodicity through generalisations (ii) and (iii), it makes sense to number the first 7 Groups from I to VII as in Figure 18, but to use zero for the noble gases (Group 0). This was also the Group numbering favoured by Mendeléev. In this course, however, we shall use the scheme of Figure 18 in which the noble gases are designated as Group VIII, and the Group numbers increase regularly across each row. The reasons for this change are given in Section 3.4.