What Is a Chemical Formula?
When chemists want to describe a substance precisely, they use a chemical formula — a shorthand notation that tells us exactly which elements are present in a compound and how many atoms of each element are in one unit of that substance.
A representation using element symbols and numerical subscripts to show the types and numbers of atoms present in one unit (molecule or formula unit) of a substance.
For example, the chemical formula for water is H₂O. This tells us:
- The elements present are hydrogen (H) and oxygen (O)
- There are 2 hydrogen atoms and 1 oxygen atom in each water molecule
When no subscript is written after a symbol, it is assumed to be 1. So O in H₂O means one oxygen atom — you never write H₂O₁.
Chemical formulas are universal — scientists all over the world use the same notation, making chemistry a truly international language.
Element Symbols and the Periodic Table
Every element has a unique one- or two-letter symbol, taken from the periodic table. These symbols are the building blocks of any chemical formula.
Key rules for element symbols:
- The first letter is always capitalised
- The second letter (if present) is always lowercase
- Symbols often come from the English name (e.g., Carbon → C, Oxygen → O, Nitrogen → N)
- Some symbols come from Latin or other languages (e.g., Iron → Fe from ferrum, Sodium → Na from natrium, Gold → Au from aurum)
Writing CO and Co are completely different! CO is carbon monoxide (a toxic gas), while Co is the element cobalt. Capitalisation matters enormously in chemistry.
Make a personal flashcard set of the most common element symbols in MYP Science: H, He, C, N, O, F, Na, Mg, Al, Si, P, S, Cl, K, Ca, Fe, Cu, Zn, Br, Ag, I, Au, Pb. Knowing these instantly will save you enormous time in exams.
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