The elements at the bottom of the periodic table are placed there because the process of building up the atoms is different than those in the rest of the table above. The atoms in the first two columns are alike because they are constructed by filling the next available s-sublevel with electrons. Those in the last 6 columns are constrructed by filling the next available p-sublevel with electrons. The transition elements in the upper part are formed by filling up a d-sublevel. Those in the bottom are constructed by filling an f-sublevel.
Not every shell has all sublevels. The first shell has only an s sublevel, the second an s and a p, the third has s, p, and d, the fourth, has s, p,d and an f sub levels. The fifth and sixth shells haves,p,d,f,and theoretically a further sublevel that would be called g, except that such atoms would be so complex that they don't exist in nature. L ikewise the seventh shell only has s and p sublevels.
When filling shells with electrons the order in which levels are filled jumps around. They are filled in the following order: 1s,2s,2p,3s and 3p. Seems like an easy pattern so far, but then things get more complicated. Before a 3d level is filled the 4s is filled first (K, Ca) then the 3d (Sc through Zn) then the 4p (Ga though Kr). Then it is 5s, 4d, and 5p gives us the next row. The 4f level doesn't get filled until the sixth s is filled (Cs and Ba) and 5d has one electron (La). Then the 4f level is filled (Ce through Lu) before the next 5d electron (Ha). Since all the elements 58 through 71 share a Lanthium-like configuration they are called lanthanides. Likewise in period 7 Fr and Ra are formed by filling the 7s sublevel, adding one electron to the 6d level to produce actinium, the filling the 5f level Th through Lr, before adding a second 6 d electron. This second set are called the actinides because they share an actinium-like structure.
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