Lanthanide contraction

Japanese: ランタニド収縮 - ランタニドしゅうしゅく(英語表記)lanthanide contraction
Lanthanide contraction
The phenomenon in which the ionic or atomic radius of lanthanides (formerly called lanthanide elements) decreases with atomic number. A similar phenomenon is seen in actinides, and is called actinide contraction. In general, the size of an atom or ion is determined by the position of the outermost extranuclear electron. For example, the atomic radius generally increases with increasing atomic number within the same group in the periodic table, but decreases with increasing atomic number within the same period. This is because in atoms of elements in the same group, the number of electrons increases, and the outermost electrons move into outermost orbits. Also, in atoms of elements in the same period, as the atomic number increases, the additional electrons move into the same outermost orbit, so the charge of the nucleus increases by one unit and attracts more strongly.

Source: Heibonsha World Encyclopedia, 2nd Edition Information

Japanese:
ランタノイド(古くはランタニド元素と呼んだ)のイオン半径あるいは原子半径が,原子番号にしたがって減少する現象。同じような現象はアクチノイドでもみられ,これはアクチニド収縮と呼ばれる。一般に,原子あるいはイオンの大きさは最外側の核外電子の位置によって決まるが,たとえば原子半径では,一般的にいって周期表中の同じ族内では原子番号の増大とともに増加し,同じ周期内では原子番号の増大とともに減少している。これは同族内の元素の原子では,電子数が増加し,最外側電子はより外側の軌道に入るためであり,また同じ周期内の元素の原子では,原子番号の増大とともに,一つずつ電子が増加しても最外側の同じ軌道に入るため,原子核の電荷が1単位ずつ増加してそれだけ強くひきつけられるからである。

出典 株式会社平凡社世界大百科事典 第2版について 情報

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