While brushing up on my Chemistry basics, I came across a topic about naming ionic compounds. It is a question regarding Roman numerals. It says "Roman numerals should not be included in the name when the metal can only form one cation."

The cation of $\ce{Zn^{2+}}$ has +2 charge, and $\ce{NO3-}$ anion has -1 charge.

which results in having $\ce{Zn(NO3)2}$

If Roman numerals are used for transitional metals and given that zinc is part of the transitional metals, why is this ionic compound named as zinc nitrate instead of zinc(II) nitrate?


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    $\begingroup$ Zinc, mercury, and cadmium are considered post transitions metals in some circles. It's a blurred line for the group. $\endgroup$ – A.K. Aug 15 '18 at 15:13

Zinc and cadmium are generally considered to be "almost invariably" in the oxidation state +2 (cfr. the introduction to this paper about one of the few exceptions), therefore the indication of the charge is unnecessary.

Zinc is usually listed as a "Type-I cation" (see Monoatomic ion), i.e. a cation that only appears in one oxidation state, or its case is emphasised as an exception among the transition metals:

Several exceptions apply to the Roman numeral assignment: Aluminum, Zinc, and Silver. Although they belong to the transition metal category, these metals do not have Roman numerals written after their names because these metals only exist in one ion.

-- Chemistry LibreTexts

(For the sake of completeness, the IUPAC Red Book section IR- "Use of charge and oxidation numbers" neither mentions "Type I/II cations" nor treats the zinc case separately)


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