How do you find formulas for covalent compounds?

Nonmetals are not big on the give-and-take department but they can share electrons with other nonmetals to satisfy the octet rule. Why? It has something to do with their placement in the periodic table (they are too close to each other, hence, the difference in their electronegativities are between 0 and 1.6).

Based on the periodic table, there are seventeen elements that are classified as nonmetals.

Hence, in a substance say, lead sulfate #PbSO_4# you have the following chemical bonds:

1.) ionic bonding between the metal #Pb^"2+"# ion and nonmetal #SO_4^"2-"# ion
2.) covalent bonding between the #S# atom and the four #O# atoms in the #SO_4^"2-"# ion

Another thing that you can use is the electronegativity values to predict what kind of chemical bond would the elements undergo to.

If the difference in electronegativities between two bonding elements is less than 0.5, then you have a nonpolar covalent bond.

If the difference in electronegativities between two bonding elements is between 0.5 and 1.6, then you have a polar covalent bond.

If the difference in electronegativities between two bonding elements is greater than 2, then you have an ionic bond.

Examples: H= 2.1 ; Cl= 3.0; Na= 0.9; C= 2.5

#CH_4# = 2.5 - 2.1 = 0.4 #color (red) ("nonpolar covalent")#
#HCl# = 3.0 - 2.1 = 0.9 #color (red) ("polar covalent")#
#NaCl# = 3.0 - 0.9 = 2.1 #color (red) ("ionic bond")#