Table salt, the road salt on an Ontario sidewalk in January, the zinc oxide in mineral sunscreen and the reddish-brown rust on an old bike are all ionic compounds. Every one of them has a name and a chemical formula, and chemists everywhere use the same rules to write them, so that "iron(III) oxide" means exactly one substance in Toronto, Tokyo or Nairobi. This lesson teaches those rules. Once you can read and write formulas, you can write chemical equations, and the rest of this unit is built on equations. The good news is that the rules come from one simple idea: positive and negative charges must cancel out.
Ions: atoms that have gained or lost electrons
An atom is neutral because it has equal numbers of protons (positive) and electrons (negative). When an atom gains or loses electrons, it becomes an ion, a particle with an electric charge. Atoms tend to gain or lose just enough electrons to end up with the same number of electrons as the nearest noble gas, which is a very stable arrangement. Metals lose their few valence electrons and become positive ions called cations. Non-metals gain electrons and become negative ions called anions. A cation keeps the element's name (a sodium atom becomes a sodium ion), while a simple anion changes its ending to -ide (a chlorine atom becomes a chloride ion).
| Group | Electrons gained or lost | Ion charge | Examples (ion name) |
|---|---|---|---|
| 1 (alkali metals) | loses 1 | 1+ | Li⁺ lithium, Na⁺ sodium, K⁺ potassium |
| 2 (alkaline earth metals) | loses 2 | 2+ | Mg²⁺ magnesium, Ca²⁺ calcium, Ba²⁺ barium |
| 13 | loses 3 | 3+ | Al³⁺ aluminum |
| 15 | gains 3 | 3− | N³⁻ nitride, P³⁻ phosphide |
| 16 | gains 2 | 2− | O²⁻ oxide, S²⁻ sulfide |
| 17 (halogens) | gains 1 | 1− | F⁻ fluoride, Cl⁻ chloride, Br⁻ bromide, I⁻ iodide |
| 18 (noble gases) | none | no ions | already stable |
Writing a formula: the charges must add to zero
An ionic compound as a whole has no charge, so the total positive charge must exactly cancel the total negative charge. The formula shows the smallest whole-number ratio of ions that does this. The positive ion (here, the metal) is always written first and the negative ion (the non-metal) second. Subscripts (the small numbers after a symbol) tell how many of that ion are in the ratio; a subscript of 1 is never written.
- Write the symbol and charge of each ion, metal first. Example, aluminum oxide: and .
- Criss-cross: the size of the metal's charge becomes the non-metal's subscript, and the size of the non-metal's charge becomes the metal's subscript. Here that gives .
- Reduce the subscripts to the lowest ratio if they share a common factor (for example becomes MgO).
- Check: (number of cations × cation charge) + (number of anions × anion charge) must equal zero.
Metals with more than one charge: Roman numerals
Many transition metals, and a few other metals such as tin and lead (group 14), can form more than one ion. Iron, for example, can lose two electrons () or three (), so "iron oxide" could mean FeO or , two different substances with different properties. These are called multivalent metals. The IUPAC name puts the ion's charge in Roman numerals in brackets right after the metal: iron(II) oxide is FeO and iron(III) oxide is , the main part of rust. The Roman numeral is the charge on one metal ion, not the number of metal atoms in the formula. Older names you may still see on labels use endings instead: ferrous for iron(II), ferric for iron(III), cuprous for copper(I) and cupric for copper(II).
| Metal | Ions | IUPAC ion names | Example compounds |
|---|---|---|---|
| Iron | Fe²⁺, Fe³⁺ | iron(II), iron(III) | FeCl₂ iron(II) chloride; Fe₂O₃ iron(III) oxide |
| Copper | Cu⁺, Cu²⁺ | copper(I), copper(II) | Cu₂O copper(I) oxide; CuO copper(II) oxide |
| Lead | Pb²⁺, Pb⁴⁺ | lead(II), lead(IV) | PbO lead(II) oxide; PbO₂ lead(IV) oxide |
| Tin | Sn²⁺, Sn⁴⁺ | tin(II), tin(IV) | SnCl₂ tin(II) chloride; SnO₂ tin(IV) oxide |
Ionic compounds share a set of properties that come from the strong attraction between oppositely charged ions packed into a crystal. They are solids at room temperature with high melting points (sodium chloride melts at about 801 °C), many dissolve in water, and they conduct electricity when melted or dissolved, because then the ions are free to move. Everyday examples include sodium chloride (table salt), calcium chloride (a de-icing salt), sodium fluoride (in many toothpastes), zinc oxide (in mineral sunscreen) and magnesium oxide (used in some antacids and heat-resistant materials).