Names and symbols of elements
Every element has a one or two letter symbol. The first letter is a capital, the second (if any) is small: H, C, O, Na, Cl. Some symbols come from old Latin names: Na (natrium) for sodium, K (kalium) for potassium, Fe (ferrum) for iron, Cu (cuprum) for copper.
To read a formula you must know these symbols. Learn the first 20 elements, and a few common metals, as a start.
Ions and their charges
An ion is an atom, or a group of atoms, that has gained or lost electrons and so carries a charge.
- Cation (positive ion): made when a metal loses electrons. Na⁺, Mg²⁺, Ca²⁺, Al³⁺. Its name is the metal's name (sodium ion).
- Anion (negative ion): made when a non-metal gains electrons. Cl⁻, O²⁻. Its name ends in -ide (chloride, oxide, sulfide).
Charge patterns: metals of group 1 are 1+, group 2 are 2+, aluminium is 3+; halogens are 1−, oxygen and sulfur are 2−.
Writing a formula: the criss-cross rule
A compound has zero total charge. So we pick numbers of ions that make plus and minus equal.
- Write the symbols with charges: Al³⁺ and O²⁻.
- Criss-cross: the number of the charge of one becomes the small number of the other. Al gets 2 and O gets 3.
- Write Al₂O₃. If both small numbers can be divided by the same number, divide (Mg²⁺ and O²⁻ gives MgO, not Mg₂O₂).
- Check: 2 × 3+ = 6+ and 3 × 2− = 6−. Equal, so correct.
The cation goes first in the formula and in the name. A small number 1 is never written.
Polyatomic ions, brackets and common names
Some groups of atoms move together as one ion. They keep their own name and charge:
- hydroxide OH⁻, nitrate NO₃⁻, hydrogencarbonate HCO₃⁻ (1−)
- sulfate SO₄²⁻, carbonate CO₃²⁻ (2−)
- ammonium NH₄⁺ (1+, the common positive group)
If you need more than one of the group, put brackets round it: Ca(OH)₂, Al₂(SO₄)₃, (NH₄)₂SO₄. Without brackets Ca(OH)₂ would wrongly look like CaOH2.
Name endings: -ide for simple non-metal ions and OH⁻, -ate for groups with more oxygen (sulfate, nitrate, carbonate), -ite for groups with less oxygen (sulfite, nitrite).
Metals with more than one charge, and molecules
Some metals can form more than one ion. Iron forms Fe²⁺ and Fe³⁺, copper forms Cu⁺ and Cu²⁺. We show the charge with a Roman numeral in brackets: FeCl₂ is iron(II) chloride, FeCl₃ is iron(III) chloride.
Molecules of two non-metals do not use ion charges. We use prefixes for the numbers of atoms: mono 1, di 2, tri 3, tetra 4, penta 5. So CO is carbon monoxide, CO₂ is carbon dioxide, N₂O₅ is dinitrogen pentoxide. (The first prefix mono is dropped.)
Acids have names you should also know: HCl hydrochloric acid, H₂SO₄ sulfuric acid, HNO₃ nitric acid.
Everyday uses of compounds
NaCl is table salt and keeps food tasty and safe. Ca(OH)₂ (slaked lime) is used in whitewash and in treating acidic soil. CaCO₃ is limestone, chalk and marble. NaHCO₃ is baking soda. NH₄NO₃ and (NH₄)₂SO₄ are fertilisers. MgO is used in antacid tablets. When you read the name on a label, you can now work out which ions are in it.
Try it: take 5 labels from your kitchen or medicine box (salt, baking soda, toothpaste, antacid, fertiliser if possible). Write each compound name and then its formula, and use the 3D builder to check.
Key formulas and definitions
- Total positive charge = total negative charge
- Criss-cross: Al³⁺ and O²⁻ → Al₂O₃
- Brackets for groups: Ca(OH)₂, Al₂(SO₄)₃
- Roman numeral = charge of metal: FeCl₃ is iron(III) chloride
- Prefixes: mono 1, di 2, tri 3, tetra 4, penta 5
Worked examples
1. Write the formula of magnesium chloride.
Mg²⁺ and Cl⁻. Criss-cross gives Mg₁Cl₂, so MgCl₂. Check: 2+ and 2 × 1− = 2−.
2. Write the formula of aluminium oxide.
Al³⁺ and O²⁻. Criss-cross: Al₂O₃. Check: 2 × 3+ = 6+, 3 × 2− = 6−.
3. Write the formula of calcium hydroxide.
Ca²⁺ and OH⁻. We need two OH, so Ca(OH)₂ with brackets.
4. Name Fe₂O₃ and write what the Roman numeral means.
Three O²⁻ give 6−, so the two Fe give 6+, which is 3+ each. Name: iron(III) oxide. The (III) means Fe³⁺.
5. Name N₂O₅ and SO₃.
N₂O₅ is dinitrogen pentoxide (di = 2 nitrogen, penta = 5 oxygen). SO₃ is sulfur trioxide (no mono on the first atom).
6. Write the formula of aluminium sulfate.
Al³⁺ and SO₄²⁻. Criss-cross: Al₂(SO₄)₃, with brackets round the sulfate. Check: 2 × 3+ = 6+ and 3 × 2− = 6−.
Common mistakes
- Forgetting brackets: writing CaOH2 instead of Ca(OH)₂ changes the meaning.
- Changing the small numbers inside a group: sulfate is always SO₄. Never change it to get balance; change the number of groups instead.
- Writing the non-metal first. In ionic compounds the metal (cation) is first in the name and the formula.
- Not reducing the criss-cross numbers: Mg²⁺ and O²⁻ gives MgO, not Mg₂O₂.