Atoms: The Building Blocks

Everything you can touch — this screen, the air, you — is built from atoms. They’re astonishingly small: about a hundred million of them side by side would span a single centimetre. Yet they have parts, and understanding those parts unlocks the whole of chemistry.

Three particles

An atom is made of just three kinds of particle:

• Protons — positively charged, packed into the centre.
• Neutrons — no charge, also in the centre.
• Electrons — negatively charged, tiny and fast, buzzing around the outside.

The protons and neutrons huddle together in a dense core called the nucleus. The electrons occupy the space around it — nearly all of an atom is empty space.

The atomic number is the element

Here’s the key idea: what makes an element an element is its number of protons. That count is called the atomic number.

• 1 proton → hydrogen.
• 6 protons → carbon.
• 8 protons → oxygen.

Change the number of protons and you literally change which element you have. This is why the periodic table is ordered by atomic number — it’s the atom’s identity badge.

Mass number counts the heavy bits

Electrons are so light they barely count. Almost all of an atom’s mass comes from its protons and neutrons. Add those two together and you get the mass number:

mass number = protons + neutrons

Atoms of the same element always have the same number of protons, but they can have different numbers of neutrons. Those variants are called isotopes — carbon-12 and carbon-14 are both carbon (6 protons), but carbon-14 has two extra neutrons.

Build an atom yourself

Use the +/− buttons to add and remove particles. Watch how the element name changes the moment you change the proton count, and how removing an electron turns the atom into a charged ion.

Electrons live in shells

Electrons don’t orbit at just any distance. They occupy shells — layers around the nucleus, each holding a limited number of electrons (2 in the first, then 8, and so on for the light elements). The simple Bohr model below draws those shells as rings. It’s not the full quantum-mechanical picture, but it’s a brilliant first sketch — and the outermost shell is what drives an atom’s chemistry.

The lone electron in sodium’s outer shell is exactly why it’s so eager to react — we’ll see that story in the periodic table and again in bonding.

Check yourself

Next: the periodic table — the map that arranges all the elements so their patterns leap out.