Write chemical formulas by balancing ionic charges (including polyatomic ions), and name ionic and simple covalent compounds using the standard rules (MA STE HS-PS1-2 support, formulas and naming).

What this topic is asking

To write reactions and run calculations you first need to write formulas correctly and name compounds. Massachusetts high school chemistry expects you to build an ionic formula by balancing charges, handle common polyatomic ions, and name both ionic and simple covalent compounds with the standard rules. This is a skill topic: the rules are short, but they must be applied carefully, because a wrong formula derails every later calculation.

Writing ionic formulas

For calcium chloride, $\text{Ca}^{2+}$ and $\text{Cl}^-$: the calcium charge of 2 becomes the chlorine subscript, and the chlorine charge of 1 becomes the calcium subscript (a subscript of 1 is not written), giving $\text{CaCl}_2$. For aluminum oxide, $\text{Al}^{3+}$ and $\text{O}^{2-}$ cross over to $\text{Al}_2\text{O}_3$. Always check that the total charges balance: in $\text{Al}_2\text{O}_3$, two aluminum give $6+$ and three oxygen give $6-$.

The charge of a main-group ion comes from its group, as set up in electron arrangement and valence electrons: group 1 is $1+$, group 2 is $2+$, group 13 is $3+$, group 15 is $3-$, group 16 is $2-$, group 17 is $1-$.

Polyatomic ions

A polyatomic ion is a charged group of covalently bonded atoms that travels as a unit. The common ones to recognize are the nitrate $\text{NO}_3^-$, sulfate $\text{SO}_4^{2-}$, carbonate $\text{CO}_3^{2-}$, hydroxide $\text{OH}^-$, ammonium $\text{NH}_4^+$, and phosphate $\text{PO}_4^{3-}$. Balance their charge the same way as a simple ion, but if you need more than one of a polyatomic ion, enclose it in brackets with the subscript outside: calcium nitrate is $\text{Ca(NO}_3)_2$, because one $\text{Ca}^{2+}$ needs two $\text{NO}_3^-$. Never change the subscripts inside a polyatomic ion; only the bracket subscript changes.

Naming ionic compounds

The rule for an ionic compound (metal plus nonmetal, or with a polyatomic ion):

1. Name the metal (cation) first, unchanged.
2. Name the nonmetal (anion) second, changing its ending to -ide (chlorine becomes chloride, oxygen becomes oxide). If the anion is a polyatomic ion, use its name (nitrate, sulfate).
3. For a multivalent metal (a transition metal that can have more than one charge, such as iron or copper), show the charge with a Roman numeral: $\text{FeCl}_2$ is iron(II) chloride and $\text{FeCl}_3$ is iron(III) chloride.

Ionic names never use prefixes like di- or tri-; the formula is fixed by charge balance, so the count is already implied.

Naming covalent compounds

For a covalent compound (two nonmetals), use Greek prefixes to show how many of each atom there are:

• mono = 1, di = 2, tri = 3, tetra = 4, penta = 5.

So $\text{CO}$ is carbon monoxide, $\text{CO}_2$ is carbon dioxide, and $\text{N}_2\text{O}_4$ is dinitrogen tetroxide. The prefix mono- is usually dropped on the first element (it is carbon dioxide, not monocarbon dioxide). The second element still takes the -ide ending. Prefixes are needed here precisely because two nonmetals can combine in several ratios, unlike a charge-fixed ionic compound.

Try this

Q1. Write the formula for sodium oxide, from $\text{Na}^+$ and $\text{O}^{2-}$. [1]

• Cue. $\text{Na}_2\text{O}$ (two sodium ions balance one oxide).

Q2. Name the compound $\text{N}_2\text{O}_3$. [1]

• Cue. Dinitrogen trioxide (covalent, so use prefixes di- and tri-).