Difficulty: Introductory | Prerequisites: Periodic table familiarity, concept of ions and charges.
Binary ionic compounds are the simplest type of ionic compound: one metal, one nonmetal, held together by electrostatic attraction. This is usually the first naming system taught in general chemistry and underpins everything that follows, from polyatomic ions to transition-metal nomenclature. If you can write and balance charges for these, the rest of inorganic naming clicks into place. You should already be comfortable locating metals and nonmetals on the periodic table and know that metals lose electrons (forming cations) while nonmetals gain them (forming anions).
Binary ionic compounds are named by writing the metal name first, then the nonmetal name with its ending changed to "-ide." To write the formula, balance the charges so the compound is electrically neutral. That is the entire system for main-group metals with a single common charge.
Binary compound
A compound made of exactly two elements. In simple terms, "binary" just means two.
Ionic compound
A compound formed by the electrostatic attraction between positively charged cations and negatively charged anions. Think of it as a metal handing electrons to a nonmetal, and the opposite charges holding them together.
Cation
A positively charged ion, formed when an atom loses one or more electrons. Metals form cations. In simple terms, the metal side of the deal.
Anion
A negatively charged ion, formed when an atom gains one or more electrons. Nonmetals form anions. In simple terms, the nonmetal side.
Charge balancing (criss-cross method)
The process of making the total positive charge equal the total negative charge in a formula. The shortcut: take the magnitude of each ion's charge and use it as the subscript on the other ion, then simplify.
"-ide" suffix
The ending applied to the second element in a binary ionic compound name. Chlorine becomes chloride, oxygen becomes oxide, sulfur becomes sulfide, and so on.
Write the name of the metal (the cation) first, exactly as it appears on the periodic table.
Write the name of the nonmetal (the anion) second, but change its ending to "-ide."
Oxygen becomes oxide, sulfur becomes sulfide, iodine becomes iodide, fluorine becomes fluoride.
No prefixes (mono-, di-, tri-) are used. Those are for covalent compounds only.
Example: MgO contains magnesium (Mg²⁺) and oxygen (O²⁻). Name: magnesium oxide.
Identify the metal and look up its charge from its group on the periodic table.
Group 1 metals: +1. Group 2 metals: +2. Aluminium: +3.
Identify the nonmetal and determine its charge.
Group 16 (oxygen family): −2. Group 17 (halogens): −1.
Balance the charges so the compound is neutral. Use the criss-cross method as a shortcut.
Take the magnitude of each ion's charge, use it as the subscript of the other ion, then reduce to the lowest whole-number ratio.
Subscript of 1 is never written.
Sodium and Sulfur: Na⁺ and S²⁻. Criss-cross gives Na₂S₁, written as Na₂S.
Strontium and Oxygen: Sr²⁺ and O²⁻. Both charges are 2, so the ratio is 1:1. Formula: SrO.
Calcium and Iodine: Ca²⁺ and I⁻. Criss-cross gives CaI₂.
Magnesium and Oxygen: Mg²⁺ and O²⁻. Ratio 1:1. Formula: MgO.
Aluminium and Iodine: Al³⁺ and I⁻. Criss-cross gives AlI₃.
Students often add prefixes like "di-" or "tri-" to binary ionic compound names. Those prefixes belong to covalent (molecular) compounds only. Na₂S is "sodium sulfide," not "disodium sulfide."
Students sometimes forget to reduce subscripts after criss-crossing. If both charges are 2 (e.g. Mg²⁺ and O²⁻), the criss-cross gives Mg₂O₂, which must simplify to MgO.
Confusing the order: the metal always comes first in the name and in the formula. Writing "sulfide sodium" or SNa₂ is wrong.
Mixing up "-ide" endings: iodine becomes iodide (not "iodinide"), sulfur becomes sulfide (not "sulfuride").
⚠️ Binary ionic naming is typically the first section tested on a nomenclature exam. Expect 5 to 10 "write the formula" or "name this compound" items.
⚠️ You will need to distinguish these from covalent compounds (which use Greek prefixes) and from transition-metal compounds (which use Roman numerals). The ability to classify a compound correctly is half the battle.
⚠️ Common exam trick: giving you a compound like CaI₂ and asking whether it needs a Roman numeral. It does not, because calcium has only one possible charge (+2). Roman numerals are for metals with variable charges.
True or False: The compound Na₂S is called "disodium sulfide." (Answer: False. No prefixes in ionic naming.)
Fill in the blank: When naming a binary ionic compound, the nonmetal ending changes to ____. (Answer: "-ide.")
True or False: In the formula for aluminium iodide, the subscript on iodine is 3. (Answer: True. Al³⁺ pairs with I⁻, giving AlI₃.)
Fill in the blank: The formula for strontium oxide is ____. (Answer: SrO.)
True or False: You should use Roman numerals when naming calcium iodide. (Answer: False. Calcium has only one common charge, +2.)
Q: Write the formula for the compound formed by calcium and iodine.
A: Ca²⁺ and I⁻. Criss-cross: CaI₂.
Q: Name the compound MgO.
A: Magnesium oxide.
Q: Write the formula for the compound formed by aluminium and iodine.
A: Al³⁺ and I⁻. Criss-cross: AlI₃.
Q: A student writes the formula for sodium sulfide as NaS. What did they do wrong?
A: They forgot to balance the charges. Na is +1, S is −2. Two sodium ions are needed, so the correct formula is Na₂S.
Q: Why does strontium oxide have the formula SrO and not Sr₂O₂?
A: Both ions carry a charge of magnitude 2, so the criss-cross gives Sr₂O₂. This reduces to the simplest ratio, SrO.
This connects directly to transition-metal ionic compounds, where you will add Roman numerals to indicate which charge the metal is using (e.g. Iron (III) Oxide). The charge-balancing method is identical; the only new step is identifying the charge from context.
It also connects to polyatomic-ion compounds (e.g. sodium sulfate, Na₂SO₄), where the anion is a group of atoms rather than a single element, but the naming logic and charge balancing carry over.
Understanding binary ionic formulas is a prerequisite for balancing chemical equations and stoichiometry later in the course.
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