Calcium hydride, denoted as CaH2, is correctly named without the prefix "di-" because it is an ionic compound. In ionic compounds, the use of prefixes like mono-, di-, or tri- is unnecessary due to the specific charges of ions, which clarify the ratio of cations to anions. Both "calcium hydride" and "calcium dihydride" are technically correct, but the former is the preferred nomenclature in accordance with IUPAC guidelines.
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Ygggdrasil said:Generally for ionic substances (usually substances formed between a metal and non-metal), you don't need to use the mono-, di-, tri- etc. prefixes to specify number. This is because most ions have a specific charge (or for transition metals, the charge is specified), so one can figure out the correct proportion of cations and anions.