How do i determine which compound is more ionic?

Ionic bonds are an electrostatic attraction between oppositly charged ions. Ionic bonds are formed between a non-metal (positively charged cation) (group I,II, III on periodic table) and a metal (negatively charged anion) (group V, VI,VII) . Ionic bonds are formed in a molten state or in solution. 

We can determine which of two ionic compounds is more negative by checking the data booklet for electronegativty. In the first step we calculate the difference in electronegativity(x) (the electronegativity refers to the tendency of an atom to attract electrons) of a compound e.g NaCl. (average electronegativities for elements can be found in the Ib data booklet)

Δx=|x₁-x₂|

Δx(NaCl)=|x_N-x_Cl|= |0.9-3.2|=2.3

in step two we calculate the average electronegativity of the compound. 

∑x=(x₁+x₂)/2

∑x(NaCl)=(x_N+x_Cl)/2= (0.9+3.2)/2= 2.05

we can then plot that on the Arkel Kekelaar Triangle of Bonding and comapare it to other compounds calculating their difference in electronegativity and average electronegativity the same way as shown above. the highest and most center one will be the most ionic compound. 

Arkel Kekelaar Triangle of Bonding:

http://www.meta-synthesis.com/webbook/37_ak/purdue.gif

It is important to rememebr that bonding is a spectrum. Compounds are not either ionic or covalent but most are something inbetween. The procedure above shows how to distinguish between two ionic compounds. But electronegativity differences can also be used to determine what time of bonding occurs in a compound. electronegative differences ubelow 1.8 are classified as covalent bonding whereas anything above 1.8 is classified as an ionic bond.