What is a molecule?
A molecule is the smallest particle of a substance that still retains its chemical properties. It is formed from two or more atoms joined by chemical bonds.
The simplest example is water. Its molecule — written as H₂O — consists of two hydrogen atoms bonded to one oxygen atom. Separate these atoms, and water ceases to be water: you are left with two separate gases, hydrogen and oxygen.
How does a molecule differ from an atom?
This is the most common source of confusion. An atom is a single "building block" — the smallest part of an element. A molecule is a finished structure made of such blocks.
| Atom | Molecule |
|---|---|
| A single particle of an element | Two or more bonded atoms |
| E.g., O — one oxygen atom | E.g., O₂ — the oxygen we breathe |
| Usually does not exist independently | Forms real substances around us |
It is worth noting: the oxygen you breathe is not a single atom, but a molecule composed of two oxygen atoms (O₂). A single oxygen atom is very unstable and immediately combines with something.
Molecule vs. chemical compound
These terms are often confused, but they do not mean the same thing.
- A chemical compound contains atoms of at least two different elements — e.g., water (hydrogen + oxygen).
- A molecule can consist of atoms of the same element — e.g., oxygen (O₂) or nitrogen (N₂).
Hence the conclusion: every chemical compound made of molecules is a molecule, but not every molecule is a chemical compound. O₂ is a molecule, but not a compound — because it is still one element.
How to read a chemical formula?
The formula tells you what a molecule is made of and in what quantity:
- Letters are symbols of elements — H is hydrogen, O is oxygen, C is carbon.
- The subscript number indicates how many atoms of that element are in the molecule.
- No number means one atom.
Example: in CO₂, we have one carbon atom (C, no number) and two oxygen atoms (O with subscript 2). This is carbon dioxide — the gas you exhale.
Why don't atoms combine arbitrarily?
Each element has a specific valency — the number of bonds it can form. Hydrogen forms one bond, oxygen two, nitrogen three, and carbon four. That's why water is H₂O, not H₃O — oxygen only has room for two hydrogen atoms.
Examples of molecules
Here are some compounds you encounter every day. Click to see the description and a fun fact:
Easiest to understand by building yourself
The theory of bonds and valency becomes obvious when you see it in action. On Czasteczki.pl, you connect atoms like building blocks — and if you try to connect them inconsistently with valency, they simply won't bond. The rule is immediately visible, without memorization.
🧪 Build your first molecule See a list of 840 molecules with descriptions Read also: valency of elements