What it’s used to determine the number of each atom in an ionic formula

If the electronegativity difference is greater than 2.0, then the bond is ionic.

Explanation:

For ex:

Na has the electronegativity of 0.93

Cl has the electronegativity of 3.16

we subtract 3.16-0.93

We have 2.23 electronegativity difference between Na and Cl

because 2.23 > 2.0

therefore NaCl compound has an ionic bond.  

each chemical family is represented as a group or column on the periodic table. based on chemical properties, there are 6 types of chemical families. group 1 represents alkali metals, group 2 represents alkaline earth metals, groups 3 to 12 represent transition metals, groups 13 to 16 represent non-metals and other metals, group 17 represents halogens and group 18 represents noble gases.