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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?

Thanks.

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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.
    Commented Aug 15, 2018 at 15:13

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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-5.4.2.2 "Use of charge and oxidation numbers" neither mentions "Type I/II cations" nor treats the zinc case separately)

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