7. Comparison of the original Mendeleef type of periodic table
(Figure 1.2} and the modern periodic table (p. (/)) shows that the
original group numbers are retained but Group I, for example, now
contains only the alkali metals, i.e. it corresponds to the top two
Group I elements of the Mendeleef table together with Group I A. At
the other end of the table, Group VII now contains only the halogens,
i.e. the original Group VIIB. The transition elements, in which the
inner d orbitals are being filled, are removed to the centre of the table
and the "rare earth' elements, in which the^/ orbitals are being filled,
are placed, for convenience, at the bottom of the table, eliminating
the necessity for further horizontal expansion of the whole table.
The original lettering of the transition metal groups, for example
VIB, VIIB and so on is still used, but is sometimes misleading and
clearly incomplete. However, we may usefully refer, for example, to