the atom
Everything you can see, hold, or breathe is built from atoms, the smallest pieces of an element that still count as that element. Split a bar of gold in half, then half again, and again, and after about thirty such cuts you reach a single gold atom, and cutting once more destroys the very idea of gold. A useful cartoon draws the atom as a miniature solar system: a dense sun at the centre with tiny planets circling it.
The dense centre is the nucleus, which packs protons (positive charge) and neutrons (no charge); it is minuscule yet carries almost all the mass. Around it, at fixed energy levels called shells, sits a cloud of electrons (negative charge). The number of protons is the atomic number Z and it alone names the element: 1 is hydrogen, 6 is carbon, 13 is aluminium, 26 is iron. A neutral atom has as many electrons as protons. The scales are startling: the nucleus is about 10^-15 m across while the whole atom is about 10^-10 m (0.1 nm, one angstrom), so the atom is roughly 100,000 times wider than its own core and is almost entirely empty space.
For the structure of materials one thing about the atom matters most: bonding is done by the outermost electrons, while the nucleus merely sets how many electrons there are and how tightly it holds them. Everything downstream, how atoms grip each other, how close they sit, which crystal they build, is decided at this outer edge. Get the atom right and you hold the root cause of every property that follows.
One gram of carbon contains about 5 x 10^22 atoms, fifty thousand billion billion of them. Line up ten hydrogen atoms and they span about one nanometre. That is why we never see individual atoms with the naked eye; it took the scanning tunnelling microscope in 1981 to image them directly.
Atoms are unimaginably small and numerous; the proton count fixes the element.
The solar-system picture is a helpful cartoon but wrong in detail: electrons do not orbit in neat circles. Quantum mechanics describes them as fuzzy probability clouds (orbitals), which is why we speak of shells and likelihoods rather than tidy planetary paths.