First, it's ionic because it contains cobalt, a metal. The other part is SO₄, which you should recognize as sulfate, a polyatomic oxyanion with a -2 charge. If sulfate's charge is -2 and there are three of them, that makes a total negative charge of -6. The total positive charge on the cobalt atoms must be +6 so they balance out and the molecule is not left with a charge. Since there are two cobalts, each one must have a +3 charge.

Cobalt (III) sulfate

This contains a semimetal and a nonmetal. Name it as if it were covalent (binary molecular). The common name for this compound is arsenic trioxide, but I'd like you to use the more formal systematic name.

Diarsenic trioxide

This compound contains four different nonmetals, but it's not a covalent (molecular) compound. You should recognize the two groups, NH₄ and PO₂. When written like this as separate ions, they aren't really correct, because they have charges. The NH₄ ⁺ is ammonium ion, and it has a +1 charge. (Memorize this. You'll see it often.) The other one is PO₂, which is similar to PO₄ ^3-, phosphate ion, which has a -3 charge. This one has two fewer oxygens than phosphate, so it's called hypophosphite ion. Now that we know the names of the ions, we can name the compound.

ammonium hypophosphite

Note: Hypophosphite ion has the same charge as phosphate: -3. This is reassuring, because ammonium has a +1 charge and there are three of them. This total positive charge of +3 balances perfectly with the -3 charge of the hypophosphite ion. You don't need to think about this while you're naming the compound.

Two nonmetals: covalent (binary molecular). Use the prefixes.

dinitrogen pentoxide

Note: You can probably imagine, without much of a stretch, that the compound called diarsenic pentoxide also exists.

The hydrogen indicates this is an acid. It only has one other element, selenium. Therefore, it's a binary or simple acid. The selenium is actually a selenide, Se²⁺, ion. How did I know that? Because the H⁺ ion needs to balance with it. No net charge! Any acid of a simple ion (like selenide) has the prefix hydro- and the suffix -ic acid.

Hydroselenic acid

Calcium is a metal. This is an ionic compound. The other element is nitrogen, but since it's combined with the metal, it must be the nitride ion.

Calcium nitride

Note: The charge on the calcium ion is +2; the charge on the nitride ion is -3. Make sure you understand why these elements have these charges in simple ionic compounds, and why they are combined, in this compound, in a 3:2 ratio.

Iron is a metal: ionic compound. The other thing is the ClO3 group, which you should recognize as the chlorate ion, ClO₃^-. Since the chlorate ion has a -1 charge, there is a total negative charge of -2 in this compound. The net charge must be zero, so the iron must have a charge of +2.

Iron (II) chlorate

Zinc is a metal, so it's an ionic compound. Zinc always has a charge of +2 when it's in a compound, so we don't have to use the Stock system. The other thing is the PO₃ group, which, like all other phosphate oxyanions has a charge of -3. This isn't phosphate, PO₄^3-, though. It has one fewer oxygen atom than phosphate, so it is phosphite.

Zinc phosphite

Magnesium is a metal. The compound is ionic. The other thing is OH, which should be recognizable as the hydroxide ion OH^- . It's also nice to see that the charges balance , even though magnesium doesn't need the Stock systmem.

Magnesium hydroxide

Starts with hydrogen: acid. Only one other element: binary acid. This is the acid of the phosphide ion, P^3-. Notice how it has three hydrogen ions. Nice.

Hydrophosphoric acid

Note: if this is in aqueous solution, it's called hydrophosphoric acid. In it's natural molecular state, it's a gas, and is called trihydrogen phosphide. Even more commonly, this compound goes by the name, "phosphine." These common names are not required, but are nice to know.

Chromium is a metal: Ionic. Other element in this binary ionic compound is phosphorus. Actually, since this is combined with chromium, it is the phosphide ion, an anion with a charge of -3. Since there's only one of each atom, the charge on the cobalt ion must be +3.

Chromium (III) phosphide

Two nonmetals. Use prefixes for this binary covalent (molecular) compound.

Carbon tetrabromide

Two nonmetals. Covalent. Use prefixes.

Triphosphorus heptoxide

Potassium is a metal: ionic compound. The other thing is hydrogen. Since hydrogen is combined with the metal, it takes on a negative charge. It's the hydride ion, with a charge of -1.

Potassium hydride.

Starts with H. Must be an acid. Acid of what? Well, there are two H⁺ ions, so the other part must have a charge of -2. This is nice, because the sulfate ion has a -2 charge, as does sulfite. Since this is an acid of the sulfite ion, its name must end in -ous.

Sulfurous acid

There is a prefix here (di-). That means it's a covalent (molecular) compound. The formula comes directly from the name.

GeS₂

Note: The -ide ending is seen in both types of binary compounds: ionic and covalent. When you see a prefix, it's covalent (between two nonmetals ...or, sometimes, semimetal and nonmetal).

Gallium is one of those metals that always has the same charge, +3. It's combined with oxygen,which is in the form of an oxide ion (charge of -2). There aren't any prefixes here to help you out because it's an ionic compound, and they don't have prefixes because you can get the formula from the charges on the ions.

Ga₂O₃

Calcium is a metal that, when combined in an ionic compound, has a +2 charge. This is an ionic compound because the calcium is combined with arsenide (the -ide tells you that it's the simple ion of the arsenic atom). Since arsenic is in Group 5A along with nitrogen and phosphorus, it will tend to gain three electrons when it forms an ion, giving it a -3 charge. Again, the formula comes from the charges, which must add up to zero.

Ca₃As₂

This one is easy. Chromium (III) is the Cr³⁺ ion. The Roman numeral tells you what the charge on the metal is. Fluoride is the simple ion of the fluorine atom, which is in Group VIIA. Fluorine, when it's combined in ionic compounds, tends to gain one electron to become isoelectronic with neon, a stable noble gas. Therefore the fluoride ion has a -1 charge. Again, the charges on all the ions in the compound must sum to zero.

CrF₂

Okay, it's an acid, meaning that it's formula will be like an ionic compound, with H⁺ as the positive ion (the cation). The name phosphoric means that this is an acid in which the negative ion (anion) is phosphate. Phosphate is PO₄^3-. Since phosphate has a -3 charge, the compound needs three hydrogen atoms to balance the charge.

H₃PO₄

This one is just like calcium arsenide, because Be is in the same family as Ca, and N is in the same family as As.

Be₃N₂.

Scandium (III) means Sc³⁺. Sulfide is the simple ion of sulfur, an atom in Group VIA, which, just like oxygen, tends to pick up two electrons when it combines with metals in ionic compounds, forming a sulfide (S^2-) ion. As always for ionic compounds, the charges must balance.

Sc₂S₃

Metal-nonmetal: ionic. Potassium ion is K⁺ because this element just needs to lose it's single outer shell electron (4s¹) to become isoelectronic with the noble gas, argon. Phosphide is the simple ion of phosphorus, P^3-.

K₃P

Acids are like ionic compounds in which H+ is the positive ion. The prefix/suffix combination of hydro-...-ic indicates that this is a simple (binary) acid of the bromide ion. Since the bromide ion, like fluoride and chloride, has a charge of -1, there is just one hydrogen in the chemical formula for the acid.

HBr

Prefix: covalent compound. The prefixes tell you how many of each element are in the compound.

OF₂

Metal-nonmetal: ionic compound. Lithium ion has a charge of +1 because it just needs to lose it's single outer shell electron (2s¹) to become isoelectronic with the noble gas, helium. Oxide is the simple ion of the oxygen atom, which tends to gain two 2p electrons to become isoelectronic with neon when it combines with a metal in an ionic compound. Oxide, therefore, is O^2-. Make sure the charges balance in the chemical formula.

Li₂O

Acid: H⁺ and an anion. What's the anion? Carbonic means that the anion (negatively charged ion) is carbonate, CO₃^2-. Since carbonate has a charge of +2, the acid must have two hydrogen ions in its formula.

H₂CO₃

Another acid. H⁺ and an anion. This time, the anion is sulfite. (-ous acids come from -ite ions) What's sulfite? Well, you should know sulfate is SO₄^2-, so sulfite is the same thing, but with one fewer oxygen atom, SO₃^2-. Balance the charges on the ions to get the formula.

H₂SO₃

Two nonmetals: binary covalent compound. The prefixes tell you how many of each atom are present in the compound.

B₂O₃

Cobalt (II) is Co²⁺. Nitrate is NO₃^-. Make sure the net charge on the compound is zero.

Co(NO₃)₂