Polar covalent compounds are formed between atoms of somewhat different electron affinities. The shared electrons in such molecules are closer to the more non-metallic atom and are sometimes drawn that way in diagrams. Atom Groups are commonly encountered ionic combinations of atoms that tend to stay together as a unit through many chemical reactions. Occasionally you will find a positively charged group such as NH. 9.14. But a few such as Copper, Iron, Tin, Lead, and Mercury have an alternate state.
Many sulfates are insoluble.
Obviously, a fundamental rule is that the same number of each type of atom must appear on each side of the equation. Making sure this is right is called balancing the equation.