Binary ionic and binary molecular compounds have what two things that are similar in their composition

Best Ionic binary compounds -- contain a metal cation and an anion made of a nonmetal or a polyatomic anion. Consider calcium phosphate:
Ca = 2+ (Group IIA)
PO4 = 3- (phosphate)
The lowest common multiple of 2+ and 3- is 6. You must have 3 Ca2+ to make +6; you must have 2 PO43- to make -6. The sum of the positive and negative charges must be equal in a compound because they are electrically neutral.
Ca3(PO4)2 -- calcium phosphate

Binary molecular compounds -- contain two or more nonmetals bonded covalently. There are no ions present because the atoms share electrons. Prefixes are used to indicate the number of atoms of each kind present. Examples are:
SO2 -- sulfur dioxide
N2O4 -- dinitrogen tetraoxide
PCl3 -- phosphorous trichloride
CO -- carbon monoxide

The general formula for an acid is HX, where X is an anion. Binary acids contain H and a nonmetal and are named with the prefix "hydro" and the suffix "ic". Examples are:
HCl -- hydrochloric acid
H2S -- hydrosulfuric acid
HF -- hydrofluoric acid
Ternary acids contain H and a polyatomic anion. Examples are:
H2SO4 -- sulfuric acid
HNO3 -- nitric acid
HNO2 -- nitrous acid
HClO3 -- chloric acid
HClO2 -- chlorous acid
HClO -- hypochlorous acid
The names of the anions are changed as follows:
-ate anions form -ic acids
-ite anions form -ous acids
per...ate anions form per...ic acids
hypo...ite anions form hypo...ous acids.