CHEMISTRY question???

Question

Which formula represents an ionic compound?

(1) H2 (3) CH3OH

(2) CH4 (4) NH4Cl

why isn't it choice 3?

Answer 1

Number 3 is methanol, which doesn't ionize; the OH group is firmly attached to the central carbon.  But ammonium chloride does ionize.

Answer 2

Compounds 1 through 3 are covalent compounds, nonmetals bound to nonmetals, while  compound 4 is an ionic compound.  It is certainly not choice 3.   For an easy rule, when you think ionic, think metal or polyatomic cation bound to nonmetal or polyatomic anion.  When you think covalent, think of nonmetal bound to nonmetal--Of course it helps if you have memorized the table of common polyatomic ions in your text.  

Compound four contains the polyatomic cation NH4+ and the anion Cl-.  Since it has these opposite charges on the cation and anion (inherently) the compound is ionic--Held together by the electrostatic force present as a result of the opposite charges of the two aforementioned species.  

It's not quite so black and white though.  For your purposes, probably a general chemistry question, it is perfectly fine.  There is crossover though, ie. ionic compounds exhibiting covalent character, vice-versa, etc.  But the rule I listed above will generally help you identify the difference