Question

How do you determine the which charge of a periodic element to use in a chemistry equation using stoichiometry?

Answer 1

Short answer: use the element’s usual ionic charge (oxidation state) — for main-group elements you can read it from the periodic table (group 1 → +1, group 2 → +2, group 13 → +3, group 15 → −3, group 16 → −2, group 17 → −1). Then combine ions so total charge = 0. For transition metals you must be told (or determine) the charge because many have multiple charges.

Step-by-step procedure
1. Identify whether the element will be ionic or covalent in the compound. Main-group metals form cations; nonmetals usually form anions; many compounds between nonmetals are covalent (no ionic charges).
2. For main-group elements, get the common ionic charge:
- Group 1 → +1
- Group 2 → +2
- Group 13 → +3
- Group 14 → usually ±4 (often covalent)
- Group 15 → −3
- Group 16 → −2 (oxygen usually −2)
- Group 17 → −1 (halides)
- Group 18 → usually neutral (noble gases)
3. For transition metals, use the charge given in the problem or determine it from other ions present (or from the compound’s name, e.g., iron(III) = Fe3+).
4. Write the ions with their charges and combine them so the algebraic sum of charges = 0. Use the criss-cross method if helpful: take magnitudes of charges as subscripts for the other ion, then reduce to lowest terms.
5. Use the balanced formula for stoichiometric calculations (moles, mass, etc.).

Examples
- Na and Cl: Na is +1, Cl is −1 → NaCl.
- Mg and O: Mg2+ and O2− → MgO (charges cancel 1:1).
- Al and O: Al3+ and O2− → criss-cross gives Al2O3 (2 Al3+ = +6 and 3 O2− = −6).
- Ca2+ and NO3− → Ca(NO3)2 (balance charges; NO3− is a polyatomic ion).
- Fe3+ and S2− → Fe2S3.

Notes and tips
- Memorize common polyatomic ions (NO3−, SO4 2−, OH−, CO3 2−, NH4+).
- If a transition metal’s charge is not given, look for other ions in the formula or the compound name.
- For redox problems, determine oxidation numbers using standard rules if you need electrons transferred rather than simple ionic charges.
- Always check that the final formula is electrically neutral.

If you have a specific reaction or element in mind, give it and I’ll show you the exact steps.