that Lothar Meyer proposed a similar arrangement about the same time. In 1871
Mendeleev rearranged some elements and proposed a table of eight columns,
obtained by splitting each of the long periods across into a period of seven
elements, an eighth group containing the three central elements (such as Fe, Co,
Ni), and a second period of seven elements. The first and second periods of seven
across were later distinguished by use of the letters A and B attached to the group
symbols, which were Roman numerals. This nomenclature of periods (IA, IIA,
etc.) has been revised in the present Periodic Table, even in the extended form of
assigning Arabic numbers from 1–18 as shown in Table 4.
TIP
Periods are the horizontal rows 1–7. Groups are the vertical columns 1–18.Table 4. Periodic Table Properties