atomic structure
Everything you can touch is built from atoms, and each atom is almost entirely empty space with a tiny, dense nucleus at its centre and a faint cloud of electrons around it. A friendly cartoon draws it like a miniature solar system: the nucleus is the sun, the electrons are planets circling it. This picture answers the most basic question in materials science: what is a material made of at the smallest level that still carries chemical identity?
The nucleus holds protons (positive charge) and neutrons (no charge); it is minuscule but carries almost all the mass. The number of protons is the atomic number Z, and it alone names the element: 1 proton is hydrogen, 6 is carbon, 26 is iron. In a neutral atom the number of electrons (negative charge) equals the number of protons, and those electrons occupy shells at fixed energy levels. The scale gap is enormous: a nucleus is about 10^-15 m across while the whole atom is about 10^-10 m (0.1 nm), so the atom is roughly 100,000 times wider than its own core.
Why does this matter for materials? Because bonding and chemistry happen almost entirely in the outermost electrons. The nucleus simply sets how many electrons there are and how tightly they are held. Get the atomic structure right and you have the root cause of every property that follows: stiffness, melting point, colour, whether a solid conducts or insulates.
A carbon atom has 6 protons and (usually) 6 neutrons in its nucleus, surrounded by 6 electrons. Iron has 26 protons and 26 electrons. Change the proton count and you change the element itself; change only the neutron count and you get an isotope of the same element.
The proton count fixes the element; electrons do the bonding.
The solar-system picture is a useful cartoon but wrong in detail: electrons do not travel in neat circular orbits. Quantum mechanics describes them as fuzzy probability clouds (orbitals), which is why the next terms talk about shells and configurations rather than paths.