Why are lanthanides and actinides separated from the main periodic table?

[mickeyw]

I have a question about lanthanides and actinides... is there any reason that they broke both the lanthanides and actinides from the periodic table?

 

[Astronuc]

I have a question about lanthanides and actinides... is there any reason that they broke both the lanthanides and actinides from the periodic table?

That is not necessarily the case. This website - chemlab.pc.maricopa.edu/periodic/periodic.html - has alternative forms of the periodic table, however there seems to be some problem with it at the time this is posted. But the main idea is that there are alternate forms that do not 'break out' the lathanides and actinides. They are grouped however for reasons of similarity in some chemical properties. For example, the lanthanides tend to form 3+ valences, and so the oxides are generally M₂O₃. The atomic radii are fairly close for the lanthanides and actinides - http://www.webelements.com/webelements/properties/text/image-balls/atomic-radius-emp.html

The transitional metals fall nicely between alkali Earth's and poor metals and metalloids - see http://en.wikipedia.org/wiki/Periodic_table for an explanation.

See also http://www.webelements.com/.

One could put the lanthanides and actinides (periods 6 and 7, respectivley) between groups 2 and 13, but then the table would be very wide. See - http://en.wikipedia.org/wiki/Periodic_table_%28wide%29

 

[Tzar]

They broke them off just so that the periodic table would look nicer. If they we all together they would have to be in the middle of two other rows (as opposed to having two rows of their own) and thus the periodic table would be impracticably long and hard to print.

 

[mickeyw]

y thanx!

 

[madhusudan]

why the last electron of the element lanthanum enters the d orbital instead of entering the f orbital ?Same in actinum ? Why ?

 

[Astronuc]

http://www.webelements.com/webelements/elements/text/La/econ.html
There is a really cool flash (9) player for electron configs. One can browse by Z.

Ostensibly the 5d¹ level is at a lower energy state than 4f¹. Cerium (Z=58) then has 5d¹4f¹, but praseodymium (Z=59) has 4f³.

Then Gd (Z=64) has electronic config [Xe].4f⁷.5d¹.6s², and all the 4f electrons are unpaired. Gd has some interesting magnetic properties.