Why are #"actinides"# and #"lanthanides"# not placed into Groups?

Answer 1

#"f-element metals"# do not follow the Group numbers......

......they are a class of metals all by themselves, and they have a peculiar chemistry, which is marked by extreme Lewis-acidity. They commonly form #M^(3+)# ions. Because the Group 3 metals, scandium, and yttrium, also commonly form #M^(3+)# ions, their chemistry is often lumped together with that of the lanthanides.

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Answer 2

Henry Antrim

Because actinides and lanthanides have different electronic configurations and properties from elements in the main group, they are not classified in any particular group; instead, in order to preserve the overall structure of the periodic table, they are frequently displayed separately below the main table.

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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.