Write The Chemical Formula For Each Compound Described

Ever Tried Naming a Compound and Ended Up with a Soup of Letters?

You've probably been there. Practically speaking, naCl. That's why the problem says something like "sodium reacts with chlorine" and you're supposed to come up with the formula. Easy, right? A chemistry worksheet lands in front of you. But then the next one says "calcium reacts with hydroxide" and suddenly you're staring at the page wondering if it's CaOH, Ca(OH)₂, or something you made up in a panic.

Honestly, writing chemical formulas from compound names trips up more people than it should. And it's not because the rules are impossibly hard — it's because the rules feel arbitrary until you see the why behind them. That said, once the pattern clicks, most formulas start writing themselves. That's what this guide is for.

Below, I'll walk you through how to build a chemical formula for each type of compound you're likely to encounter, with worked examples that go beyond the easy ones. By the end, you should be able to look at a name and sketch out the formula without second-guessing every letter.

What "Writing the Formula" Actually Means

When a question says "write the chemical formula for each compound described," it's giving you a description — sometimes the name, sometimes a short phrase like "magnesium chloride" or "the compound formed between aluminum and oxygen" — and asking you to translate it into the shorthand chemists use. That shorthand uses element symbols, subscripts, and sometimes parentheses to show how many atoms of each element are in one unit of the compound.

The catch: you don't get to decide* the subscripts. Still, they're determined by the charges (for ionic compounds) or by how the atoms bond (for covalent ones). Your job is to read the name, identify the pieces, and arrange them correctly.

There are really only a few major families of compounds you'll run into in most chemistry courses:

• Ionic compounds (metal + nonmetal, or polyatomic ions)
• Covalent compounds (two nonmetals)
• Acids
• Hydrates

Each has its own naming → formula translation rule. Let's go through them.

How to Write Formulas for Ionic Compounds

Binary ionic compounds: one metal, one nonmetal

At its core, the bread and butter. The metal gives up electrons and becomes a positive ion (cation). Even so, the nonmetal accepts electrons and becomes negative (anion). The formula has to balance the total positive charge with the total negative charge, so the overall compound is neutral.

Take "sodium chloride.Now, chlorine (Cl) is in Group 17, so it forms a −1 ion. " Sodium (Na) is in Group 1, so it forms a +1 ion. One of each balances out: NaCl.

Now "calcium chloride." Calcium is Group 2, so it's Ca²⁺. Also, chloride is still Cl⁻. You need two chlorides to balance one calcium: CaCl₂.

And "aluminum oxide.But " Aluminum is Group 13, forming Al³⁺. Oxygen forms O²⁻. To balance, you need two Al³⁺ (total +6) and three O²⁻ (total −6): Al₂O₃.

A shortcut: write the charges, then crisscross them as subscripts (ignoring the signs). Al³⁺ and O²⁻ becomes Al₂O₃. The crisscross is fast, but be careful with the next case. No workaround needed.

Transition metals and the Roman numerals

Transition metals can form more than one possible charge, which is why their names include Roman numerals. "Iron(III) chloride" tells you iron is in the +3 state here. So Fe³⁺ paired with Cl⁻ gives FeCl₃. Simple.

But "iron(II) chloride" is Fe²⁺ with Cl⁻, giving FeCl₂. Different Roman numeral, different formula. This is one of the most common mistakes people make — they write the same formula for both because they ignore the Roman numeral.

Compounds with polyatomic ions

Polyatomic ions are groups of atoms that act as a single charged unit. And things like nitrate (NO₃⁻), sulfate (SO₄²⁻), hydroxide (OH⁻), and ammonium (NH₄⁺). You treat them as one piece.

"Calcium nitrate" — calcium is Ca²⁺, nitrate is NO₃⁻. You need two nitrates per calcium: Ca(NO₃)₂. Practically speaking, notice the parentheses. They're required whenever you have more than one of a polyatomic ion, otherwise the subscript only applies to the closest atom (oxygen, in this case, which would be wrong).

"Sodium hydroxide" — Na⁺ and OH⁻, one of each: NaOH. No parentheses needed because there's only one hydroxide.

"Ammonium phosphate" — ammonium is NH₄⁺ (positive this time), phosphate is PO₄³⁻. You need three ammoniums and one phosphate: (NH₄)₃PO₄. Parentheses around NH₄ because there are three of them.

How to Write Formulas for Covalent Compounds

When two nonmetals bond, they share electrons rather than transferring them. There's no neat charge to balance, so chemists use a different system: Greek prefixes tell you exactly how many atoms of each element are present.

• mono = 1 (usually skipped on the first element)
• di = 2
• tri = 3
• tetra = 4
• penta = 5
• hexa = 6

"Dinitrogen pentoxide" → N₂O₅.

"Sulfur trioxide" → SO₃ (no "mono" on the sulfur because it's the first element listed).

"Carbon tetrachloride" → CCl₄.

"Tetraphosphorus decoxide" → P₄O₁₀. Plus, yes, ten. Yes, it really is called that.

If you found this helpful, you might also enjoy how many times does 11 go into 40 or what is the percent of 12 20.

The biggest trap here: people forget to apply the prefix to both* elements, or they apply "mono" to the first element when it shouldn't be. If the prefix on the first element would be "mono," just leave it off. "Carbon monoxide" is CO, not C₁O — the one is implied.

How to Write Formulas for Acids

Acids deserve their own corner because the name tells you the anion, and the formula is built from that.

Binary acids (hydrogen + one nonmetal)

These use the prefix "hydro-" and the suffix "-ic" in their names.

• "Hydrochloric acid" → anion is chloride (Cl⁻), so the acid is HCl.
• "Hydrobromic acid" → HBr.
• "Hydrosulfuric acid" → H₂S (because sulfide is S²⁻, you need two hydrogens).

Oxyacids (hydrogen + a polyatomic ion containing oxygen)

The naming here depends on the polyatomic ion's suffix.

• If the ion ends in -ate, the acid ends in -ic. "Nitric acid" comes from nitrate (NO₃⁻), so the formula is HNO₃.
• If the ion ends in -ite, the acid ends in -ous. "Nitrous acid" comes from nitrite (NO₂⁻), so the formula is HNO₂.
• Sometimes you also see per-* and hypo-* prefixes for oxyanions, which carry over directly.

Memorize the common polyatomic ions and the rest falls into place. There are only a handful you'll use over and over.

How to Handle Hydrates

A hydrate is an ionic compound with water molecules trapped in its crystal structure. The name includes a prefix telling you how many water molecules per formula unit.

"Copper(II) sulfate pentahydrate" → CuSO₄ with 5 waters: CuSO₄·5H₂O. The middle dot (not a multiplication symbol, just a separator) shows the water is part of the structure but loosely held.

"Calcium chloride dihydrate" → CaCl₂·2H₂O.

The formula of the anhydrous compound follows the usual ionic rules. You just tack on the water at the end with a dot.

Common Mistakes That Trip People Up

Writing formulas looks mechanical, but the mechanical parts hide a few traps. Here are the ones I see most often.

Forgetting parentheses around polyatomic ions

You write Ca(NO₃)₂ as CaNO₃₂ by accident, which would mean calcium bonded to a nitrogen and 32 oxygens. That compound doesn't

exist. Parentheses are your best friend here. If you need more than one polyatomic ion, you must* put them in parentheses with the subscript outside.

Confusing Subscripts and Coefficients

A subscript tells you how many atoms* of the element before it. Still, in a formula like 2H₂O, the "2" is a coefficient (two water molecules), while the "H₂" subscript means each water molecule has two hydrogens. A coefficient tells you how many molecules* or formula units* of the entire compound. They are not interchangeable.

Mixing Up -ous and -ic for Acids and Metals

This is a classic. ferrous (Fe²⁺). And the "-ous" ending means the lower charge or the "hypo-" version. Nitric acid (from nitrate) vs. Remember, the "-ic" ending means the higher charge (for metals) or the normal acid (for oxyacids). Ferric (Fe³⁺) vs. nitrous acid (from nitrite).

Naming Ionic Compounds with Transition Metals

This is the final major piece of the puzzle. Unlike sodium (always Na⁺) or calcium (always Ca²⁺), many transition metals can form ions with different charges. Still, iron, for example, can be Fe²⁺ or Fe³⁺. To specify which one, we use a Roman numeral in parentheses right after the metal's name.

• FeCl₂: Iron has a +2 charge to balance two chlorides (each Cl⁻). So, it's iron(II) chloride.
• FeCl₃: Iron has a +3 charge to balance three chlorides. So, it's iron(III) chloride.

The "old" system of -ous and -ic is still used but less commonly in modern IUPAC nomenclature. Practically speaking, "Ferrous chloride" is the same as iron(II) chloride, and "ferric chloride" is iron(III) chloride. Stick with the Roman numeral system for clarity.

The Stock System vs. The Classical System

• Stock System (Roman Numerals): Unambiguous. Used for all metals with variable charge. Example: copper(I) oxide (Cu₂O) and copper(II) oxide (CuO).
• Classical System (-ous/-ic): Based on Latin roots. Cuprous oxide (Cu₂O) and cupric oxide (CuO). It's less precise and can be confusing, but you'll still encounter it.

Conclusion

Mastering chemical nomenclature isn't about memorizing a million random names; it's about learning a consistent set of logical rules. By breaking it down—first for molecular compounds with prefixes, then for ionic compounds with fixed charges, followed by the tricky acids, hydrates, and finally transition metals with their variable charges—you build a reliable framework. The key is to practice applying these rules systematically, paying close attention to the traps of polyatomic ion parentheses, the meaning of subscripts, and the crucial Roman numerals that specify a metal's charge. With this foundation, you can confidently decode and construct the language of chemistry.