by 4f electrons; the 5p and 5s electrons (which are the outermost electrons in the 3+ ions) are drawn towards the nucleus, thus resulting in a smaller ionic radius.

In single-electron atoms, the average separation of an electron from the nucleus is determined by the subshell it belongs to, and decreases with increasing charge on the nucleus; this, in turn, leads to a decrease in atomic radius. In multi-electron atoms, the decrease in radius brought