How do you write the name and formula of a binary compound?

A binary compound is made up of two elements. In ionic compounds , one element is a metal and the other element is a nonmetal. In covalent, or molecular, compounds both elements are nonmetals.

Ionic Compounds
When naming ionic compounds, the name of the metal goes first, followed by the nonmetal. The metal forms a positive ion, and the nonmetal forms a negative ion. The name of the nonmetal changes so that the last one or two syllables are dropped, and the suffix -ide is added. Some metals, primarily, but not limited to, the transition metals can form more than one positively charged ion, such as #"Fe"^(2+)"# and #"Fe"^(3+)"#, the ion is named for the charge using Roman numerals, such as iron (II) and iron (III).

Binary ionic compounds are named by writing the metal ion first, followed by the nonmetal. If the metal has only one ion, then the Roman numeral is not necessary. Also, Greek prefixes are not used when naming ionic compounds.

Examples of Ionic compounds

#"NaCl":# sodium chloride
#"K"_2"O":# potassium oxide
#"MgCl"_2:# magnesium chloride
#"Al"_2"O"_3":# aluminum oxide
#"FeCl"_3":# iron (III) chloride

Binary Molecular Compounds
When naming molecular, or covalent, compounds, if an element in the formula has a subscript, the name of that element includes a Greek prefix that means the subscript number. The name of the second element is changed in the same way as ionic compounds. Greek prefixes are used on both elements, if required. If the first element in the compound has no subscript, no prefix is used.

Examples of Molecular Compounds

#"NF"_3:"# nitrogen trifluoride
#"NO":# nitrogen monoxide
#"N"_2"O":# dinitrogen monoxide (laughing gas)
#"N"_2"O"_4":# dinitrogen tetroxide
#"PCl"_5":# phosphorus pentachloride
#"S"_2"F"_10":# disulfur decafluoride
#"H"_2"O":# dihydrogen monoxide (water)
#"NH"_3":# nitrogen trihydride (ammonia)