Chemistry
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The Actinoids are a group of 15 elements in Period 7 of the periodic table characterized by filling of their 5f electron subshell. Ac, Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No, and Lw are sometimes classed as actinoids. Exactly where they fit in the periodic table is debated. Some schemes assign Ac to Group 3 (under Sc, Y, and La) because its 6d1 7s2 valence electron configuration, and put the rest of the series between Group 3 and Group 4. Other schemes (including Wikipedia's) place Ac after Group 2 and place Lw, which has a 5f14 6d1 7s2 in Group 3. The list given above contains 15 elements because one scheme doesn't consider Ac a true actinoid and the other main scheme does not consider Lw a true actinoid. Since there are 14 5f orbitals available, it is necessary to pick one classification scheme or the other.

Actinoids are Period 7 homologs to Period 6's lanthanoids. Their physical and chemical properties are much more varied than are those of the lanthanoids. Some of this is due to the kinetic energy required for an electron exposed to the high nuclear charge of actinoids. Even outer electrons spend part of their time close to the nucleus, so are affected. KE is great enough so that effective mass of an electron exceeds its rest mass; causing s and p orbitals to shrink while d and f orbitals expand due to the more effective screening provided by the shrunken s and p electrons. In a more practical sense, properties of actinoids are difficult to measure with precision because their radiation damages crystal structure and introduces impurities into a sample.

The name of this group is an example of physicists and chemists separated by a common language. Physicists prefer "actinide" to "actinoid".

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