Inside the atom

Atoms contain a nucleus of positively charged protons and, in most isotopes, neutral neutrons. Ordinary hydrogen has one proton and no neutron. Electrons have negative charge; a neutral atom has equal numbers of electrons and protons.

The solar-system picture is only a rough historical analogy: electrons occupy quantum states, not planet-like orbits. How electrons fill those states and interact in a solid determines whether charge can move easily.

In metals those loose outer electrons break free and wander from atom to atom, forming a roaming "sea" of free electrons. Apply a voltage and the whole sea drifts in one direction — that drift is your electric current.