What is wrong with the following formulas

a.) Al2S

b.) PNa3

c.) Mg2S2

a) Al=3+ and S=2- so it should be Al2S3

b) You cant have a formula ending in Na - should begin with Na.

c) Mg and S are both 2+/- so they cancel out =MgS

a.) Should be Al2S3

b.) Should be Na3P (so what is given is not necessarily wrong)

c.) MgS

a.) Al2S3
b.) Na3P
c.) MgS