Formulas

Seven elements exist as diatomic molecules and their formula always carries subscript 2. Writing "N" for nitrogen gas — or "2N", which is a coefficient describing two separate atoms, not the molecule $\text{N}_2$ — puts half the correct atom count on that side. Knowing the word "diatomic" is not enough: it means two atoms of that element bonded in one molecule.

When the question says word equation, only the full names of the reactants and products earn credit: "magnesium + oxygen $\rightarrow$ magnesium oxide". A formula, or a name-and-formula mixture, does not answer the command as set, even when the chemistry is right. The reverse fails too — names written inside what should be a balanced symbol equation.

$\text{CaCO}_3$ contains 3 elements (Ca, C, O) and 5 atoms ($1+1+3$); $\text{H}_2\text{SO}_4$ contains 3 elements and 7 atoms ($2+1+4$). Candidates read the subscript 3 off $\text{CaCO}_3$ as the atom total, or count only the symbols they can see. Elements = how many different symbols; atoms = the subscripts added up, an unwritten subscript counting as 1.

Balance only with coefficients placed in front of whole formulas. Changing a subscript makes a different substance — $\text{H}_2\text{O}$ and $\text{H}_2\text{O}_2$ are not the same compound — and tacking an extra molecule onto one side is not balancing either; adjust the coefficient. Re-check every element at the end: fixing one often unbalances one already correct.

"Including state symbols" means one on every species, reactants and products alike — a correctly balanced equation with two of four states missing still drops the marks for states. A precipitate is (s), not (aq): it is insoluble and has left the solution. A substance dissolved in water is (aq); (l) is reserved for a pure liquid.

$\text{Al}^{3+}$ with $\text{O}^{2-}$ gives $\text{Al}_2\text{O}_3$, not $\text{Al}_3\text{O}_2$. The cation's charge magnitude becomes the anion's subscript and vice versa — swapping them is the standard slip. Check by totalling the charges: $2 \times 3+ = 6+$ balances $3 \times 2- = 6-$. If the two totals do not cancel, the subscripts are the wrong way round.

$\text{NH}_4^+$ with $\text{SO}_4^{2-}$ is $(\text{NH}_4)_2\text{SO}_4$, never "NH42SO4"; $\text{Ca}^{2+}$ with $\text{OH}^-$ is $\text{Ca(OH)}_2$, never "CaOH2". Without brackets the subscript attaches to the last atom of the ion alone, so the formula states a different substance. Bracket any polyatomic ion before giving it a subscript above 1.