Question

What are the rules for prefix in a compound?

Answer 1

Prefixes are generally added at the beginning of a name, to denote some quality of the compound. They can represent the number of substituent groups, or the number of bonds present, etc. While naming binary molecular compounds we use Greek prefixes.

Complete answer:
We use chemical nomenclature to differentiate between different compounds and to have no ambiguity regarding the name or the compound the name refers. Every chemical substance has a unique name. IUPAC has established the most widely accepted nomenclature for naming compounds. Prefixes serve many purposes in naming substances. But Greek prefixes are used widely for denoting the number of atoms of a particular element in a binary (two-element) compound. The need for these prefixes is that, same set of elements can form two different compounds and it is important to differentiate them. For example, $H$ and $O$ can form both ${H_2}O$ and ${H_2}{O_2}$. If we try to name these compounds without using any prefix, then we will name both of them as hydrogen-oxide which is pretty confusing and destroys the whole purpose of using nomenclature.
Now we will look at the rules for naming molecular compounds.
(1) Write the more electropositive element first, followed by the other element.
(2) Remove the ending of the second element and add ‘ide’ instead.
(3) Use the Greek prefixes before each element to denote the number of atoms present.
(4) The prefix mono- is not used for the first element. It is dropped.
(5) If two vowels sounding similar come in a row after adding the prefix, then the one at the end of the prefix is dropped.

Note:
Note that molecules of elements that contain two or more atoms of a same element are given names by using these prefixes as well. For example, ${O_2}$ is named dioxygen and ${O_3}$ is named trioxygen, though they are commonly known as oxygen and ozone.