Question

What is the difference between `"NI"` and `"Ni"`? How many elements are in each?

Answer 1

Well, for one, `"NI"` doesn't exist (on the other hand, `"NI"_3` has been witnessed; `"PF"_3` exists, so by implication of elements on the same column of the periodic table being similar in valence electronic behavior, `"NF"_3` exists, and thus, so might `"NI"_3`).

Despite both of their (`"N"`, `"I"`) natural oxidation states being negative, the least you could have is charge magnitudes matching, which would require more than one `"I"` atom bonded to `"N"`. If it did exist in this unrealistic exercise, it would be different in that it has two elements.

`"Ni"` is the nickel atom, and clearly, there is only one element in one atom, because every atom derives from a single element (a neutral atom IS the element).