Elements from atomic number 90 to 103 are classified as _______.

Concept — an element belongs to a family according to the subshell that its differentiating electron enters. Beyond the s-, p- and d-blocks lies the f-block, also called the inner transition series: two families of fourteen elements each, one filling the 4f subshell and the other filling the 5f subshell. Each of these two f-block families is named after the element that immediately precedes it in the periodic table.

Application — place the span of atomic numbers 90 to 103 inside that scheme, step by step.

1. Atomic number 90 is thorium and atomic number 103 is lawrencium, so the span covers exactly fourteen consecutive elements — the size of one complete inner transition family.

2. This block of fourteen elements comes immediately after actinium, atomic number 89, and its differentiating electron enters the 5f subshell.

3. A fourteen-member 5f family named after the element preceding it is the second inner transition series, the one named for actinium — the actinoids.

Cross-check — the other families named in this question occupy different parts of the periodic table:

So the span of atomic numbers 90 to 103 is the actinoid series, and the elements in it are classified as actinoids.