How can I tell what chemical formulas are ionic compounds??

Question

I have a list of formulas such as MgO, Na*subcript 2*O, H2O, etc., but I have no idea how to figure out what formulas are ionic compounds.

Answer 1

Ionic compounds are always a metal and a non-metal bonded together. But covalent compounds are 2 non-metals bonded together.
MgO is a metal and a non-metal so that would be ionic.
H20 is a non-metal and a non-metal so that would be covalent.
Na20 is a metal and a non-metals so that would be ionic.
I hope I've helped.

Answer 2

Not always easy to do so.

In general, if you have something from the far left of the periodic table paired up with something from the far right of the table, they are, more often than not, ionic. But then there's Calcium Fluoride which SHOULD be ionic - but isn't. Has to do with atomic sizes.

If there is a lot of carbon, nitrogen, oxygen involved, you are looking at organics, which are usually covalent.

If you have H2SO4, that's sufuric acid - where the SO4 part involves covalent bonds between sulfur and oxygen. Taken as SO4(-2), the hydrogen attaches to the sulfate moeity via ionic bonds. I.e. you can have two different types of bonds in the same molecule. Sulfur and oxygen form covalent bonds, and the tip-off is how close they are to each other in the periodic table.

But is there a hard and fast rule to recognize ionic bonds? Not a perfect one, or at least I dont' recall one.

Answer 3

Oh snap! And along came Percent Ionic Character....which is just another refinement of Chemical Bonding....You can find periodic tables with this info tabulated on it. But how to determine it from Chem Formula....that skill will grow with your knowledge and experience in chemistry. Usually to "determine," which I interpret as "calculate," you need info regarding bond distances. But you can get good at rough guessing it after a short time. Salts are always Ionic, but sometimes a salt has a component with covalent bonds, like Sodium phosphate, which is Ionic in terms of its "bond" between the Metal and the Phosphate itself but has covalent bonds between Phosphorous and Oxygen atoms in the Phosphate. You just have to get used to the chemical species.