By Karen G. Blaettler
Updated Aug 30, 2022
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Understanding how to write a chemical formula is essential for communicating the composition of any compound. With a clear grasp of basic chemical terminology, you can read and write formulas confidently.
Key Definitions
Accurate language is the backbone of science. Below are essential terms that will help you interpret and create chemical formulas.
- Atom – The smallest unit of an element, comprising a nucleus (protons and neutrons) and surrounding electrons.
- Element – A substance made of one type of atom, identified by a unique symbol on the periodic table.
- Compound – A substance formed when two or more different atoms chemically bond together.
- Molecule – The smallest unit of a compound that retains its chemical properties; molecules are electrically neutral.
- Ions – Atoms or groups of atoms that have gained or lost electrons, resulting in a net positive or negative charge.
- Chemical formula – A concise representation of a substance’s composition, indicating the types and numbers of atoms present.
Element Symbols
Every element has a one‑ or two‑letter symbol derived from its English or Latin name (e.g., Pb from *plumbum* for lead). When a symbol contains two letters, the first is capitalized and the second is lowercase.
Using Numbers in Formulas
Numbers in a formula convey specific information:
- Pre‑symbol numbers indicate how many atoms or molecules of that species are involved (e.g., 2O in C + 2O → CO₂).
- Subscripts show the count of atoms directly following the symbol; absence of a subscript implies one atom (e.g., CO₂ has one C and two O).
- Superscripts denote the charge on an ion (e.g., Mg²⁺, NO₃⁻).
- When a polyatomic ion appears more than once, parentheses enclose the ion and a subscript follows the closing parenthesis (e.g., Mg(NO₃)₂).
Example: In magnesium nitrate, the balanced equation is
Mg²⁺ + 2(NO₃)⁻ → Mg(NO₃)₂, showing that the 2+ charge on Mg balances two 1‑negative nitrate ions to produce a neutral compound.
Latin and Greek Prefixes
Prefixes help identify the number of atoms or ions in a compound:
- mono (1), bi/di (2), tri (3), tetra (4), penta (5), hexa (6), hepta (7)
For instance, carbon monoxide (CO) contains one C and one O, whereas carbon dioxide (CO₂) contains one C and two O atoms.
Common Polyatomic Ions
- hydroxide – OH⁻
- carbonate – CO₃²⁻
- phosphate – PO₄³⁻
- nitrate – NO₃⁻
- sulfate – SO₄²⁻
Practical Formula Writing
When writing a formula, list the cation (positive ion) first, followed by the anion (negative ion).
- Sodium chloride (table salt): NaCl
- Carbon tetrachloride (dry‑cleaning solvent): CCl₄
- Sodium bicarbonate (baking soda): Na₂CO₃ (note: the bicarbonate ion is CO₃²⁻; the formula is Na₂CO₃)
- Dinitrogen heptachloride: N₂Cl₇
- Ammonium hydroxide (NH₃OH): NH₄OH – the ammonium ion (NH₄⁺) and hydroxide ion (OH⁻) combine to form a neutral molecule.
Transition Metals and Roman Numerals
Transition metals often form ions with multiple charges. The charge is indicated in the compound’s name using a Roman numeral:
- CuF₂ is copper(II) fluoride because fluoride is 1⁻; thus copper must be 2⁺.
- FeCl₃ is iron(III) chloride, balancing iron(III) (3⁺) with three chloride ions (1⁻ each).
- SnF₂ (stannous fluoride) and SnF₄ (stannic fluoride) illustrate the use of “stannous” for Sn²⁺ and “stannic” for Sn⁴⁺.
Understanding these conventions ensures your formulas accurately reflect the chemical reality of each compound.
