Lanthanide and actinide series do not start with lanthanum and actinium explain?

Answer 1

Lets take the Lanthanides as an example. Lanthanum has its 5d sublevel filled before its 4f, even though the 4f is lower in energy and should, by the Aufbau Principle, be filled first. However, this is an exception because it happens to be the most stable configuration. It has one electron in the 5d sublevel and none in the 4f, and since it has no "f" electrons, it cannot be part of the f-bar's upper row (the Lanthanide series).

The Lanthanides themselves will each also have one electron in the 5d sublevel, and the first element will also have one in the 4f sublevel. This trend will continue for all 14, and thus the 4f sublevel is filled. Hafnium, the element immediately after the last Lanthanide, will resume filling the 5d sublevel as normal, with two electrons in it and a completely filled 4f sublevel. The anomaly does not really change much; after all, if the valence electron in Lanthanum were not in the 5d sublevel and instead in the 4f, it would start off the Lanthanide series, there would be 14, and it would simply end one element before it normally does. The numbers all add up, in any case, and this is the natural order of things.

For Actinides, replace the word Lanthanum with Actinium, Hafnium with Darmstadtium, 5d with 6d, and 4f with 5f. Everything works out perfectly; all the confusion is just caused by the stubborn little electron who doesn't want to be the first f-block element (and so he moves up into 5d, and stays there while the others take their appropiate places, which are moved one up thanks to his desire to maintain stability).

Answer 2

The lanthanide series starts with cerium because it was discovered before lanthanum. The actinide series starts with thorium because it has similar chemical properties to actinium. So, lanthanum and actinium do not come first in their respective series due to historical reasons related to the discovery of elements.