Atoms: The Alphabet of Matter
Everything you can touch is spelled with just 118 letters. Each letter is an atom — and three tiny particles decide which one.
Three particles, endless matter
Cut any piece of matter smaller and smaller and you eventually hit a limit: the atom. Atoms themselves are built from just three ingredients. In the centre sits a tiny, dense nucleus containing protons (positively charged) and neutrons (no charge). Around it buzz electrons (negatively charged), so light that the nucleus holds more than 99.9% of the atom’s mass while filling almost none of its space. If an atom were a football stadium, the nucleus would be a pea on the centre spot — the rest is electron territory and emptiness.
The proton count is the element
Here is the single most important sentence in chemistry: the number of protons decides which element an atom is. Six protons? Carbon — always, everywhere in the universe. Seven? Nitrogen. Seventy-nine? Gold. This number is the atomic number Z, and it is why the periodic table is numbered: it simply counts protons from 1 (hydrogen) to 118 (oganesson).
Neutrons, by contrast, are negotiable. Carbon usually carries 6 of them (carbon-12), but some carbon atoms carry 7 or 8. Same element, different mass — these siblings are called isotopes. Most are perfectly stable; some, like carbon-14, slowly decay, which is exactly what makes radiocarbon dating work.
Ions: when the bookkeeping doesn’t balance
A neutral atom has exactly as many electrons as protons. But electrons are the atom’s loose change — they can be lost or gained. An atom that has lost electrons is positively charged (a cation); one that has gained electrons is negative (an anion). Note what did not change: the proton count. A sodium ion is still sodium. Ions are not exotic — the salt on your table is nothing but Na⁺ and Cl⁻ ions holding hands.
A single water droplet contains about 10²¹ molecules — a thousand billion billion. If you could count one per second you’d need thirty thousand times the age of the universe. Chemistry never handles atoms one at a time; it handles them in armies. (Unit 2 gives that army a name: the mole.)
In the lab below you are the architect: set the particle counts and watch the identity, mass and charge of your atom follow. One slider matters more than the others — find out which.
⚗️ Lab — The Atom Builder
Assemble atoms particle by particle and watch the name plate react.
- Slide protons up and down — the element name changes with every step.
- Add neutrons: the mass number A climbs, the element stays put (isotopes!).
- Remove an electron from a neutral atom — you just made a cation.