Formation of Compound
We get chemical formula of molecule by writing valence of 1st atom in front of 2nd atom and valence of 2nd atom in front of 1statom.
Example: valence of carbon is 4 and oxygen is 2, write 4 in front of O and 2 in front of C in subscripts as shown below.
as we see, both valences are multiple of two so divide it by two to get CO2 formula.
Na++ CO3-2 —> Na2CO3
Al+3 + SO4-2 —> Al2(SO4)3
Mg+2+ N-3 —> Mg3N2