At your scale, matter looks continuous: a liter of water flows without interruption. But if you could zoom in a billion times, you would see elementary grains: atoms. All of chemistry rests on this idea — matter is made of atoms, and atoms combine to form everything that exists.
The atom in a simple model
An atom has two regions:
- A central nucleus, tiny but containing almost all the mass. The nucleus is made of protons (positive charge) and neutrons (no charge).
- An electron cloud around the nucleus, where electrons (negative charge) circulate. This cloud takes up almost all the atom's volume.
Atoms are electrically neutral: as many protons (+) as electrons (−). The number of protons is the atomic number and it defines the chemical element. An atom with 1 proton is hydrogen (H), with 8 protons it is oxygen (O), with 26 protons it is iron (Fe).
How many elements exist?
To this day, 118 chemical elements are known and listed in the periodic table. Everything you touch, breathe, or eat is built from a small number of them. Your body, for instance, is more than 96 % made of just 4 elements: oxygen, carbon, hydrogen, and nitrogen.
Atoms and molecules
Atoms can stay alone, but more often they bond to form molecules. A water molecule is 2 hydrogen atoms + 1 oxygen atom (formula H₂O). A dioxygen molecule, the gas you breathe, is 2 oxygen atoms (O₂).
Why a simplified model?
The "nucleus + electron cloud" model described here is intentionally simple. In high school, you will learn that electrons do not orbit randomly: they occupy shells further and further from the nucleus, which explains the chemical properties of the elements. In university, the model is refined further by quantum mechanics. But the essential idea is already here: matter is made of atoms, and atoms have an organized internal structure.