The lanthanides and actinides are set apart even further from the transition metals. In most versions of the periodic table, lanthanum (57) is followed by hafnium (72) in the transition metals section of the chart. Similarly, actinium (89) is followed byrutherfordium(104). The "missing" metals—lanthanides and actinides respectively—are listed at the bottom of the chart. There are reasons for this, as well as for the names of these groups.
After the 6sorbital fills with the representative elemen barium(56), lanthanum does what a transition metal does—it begins filling the 5dorbital. But after lanthanum, something strange happens:cerium(58) quits filling 5d,and moves to fill the 4forbital. The filling of that orbital continues throughout the entirelanthenide series, all the way tolutetium(71). Thus lanthanides can be defined as those metals that fill the 4forbital; however, because lanthanum exhibits similar properties, it is usually included with the lanthanides.
A similar pattern occurs for the actinides. The 7sorbital fills withradium(88), after which actinium (89) begins filling the 6dorbital. Next comesthorium, first of the actinides, which begins the filling of the 5forbital. This is completed with element 103,lawrencium. Actinides can thus be defined as those metals that fill the 5forbital; but again, because actinium exhibits similar properties, it is usually included with the actinidesThank You