word-forming element in chemistry, used to coin element names, from Latin adjectival suffix -ium (neuter of -ius), which formed metal names in Latin (ferrum "iron," aurum "gold," etc.).

Why is ium suffix?

The -ium suffix is a Latin suffix which forms abstract nouns, thus it is used to form chemical elements' name from its naming origin, such as minerals (calcium from calx) or person names (gadolinium).

Is ium diminutive suffix?

(Greek: a suffix; scientific names; names of metallic elements; a part, lining, or enveloping tissue, region; little; representing a diminutive force)