Video narration
How do separate energy levels in atoms become a broad range in a solid? Imagine one traffic lane widening into many side-by-side lanes: one option becoming many close options. That comparison helps you picture the change, but it isn't the cause. Start with identical atoms far apart. In each, electrons occupy distinct allowed energies, like separate steps. Bring those atoms close, and their electrons interact. Here's the shift: one atomic energy level splits into many nearby allowed energies for the whole group. With an enormous number of atoms, the allowed energies sit so close together that they form a dense range. This is an energy band: many closely spaced energies electrons are allowed to have. Between bands, a band gap is a range with no allowed energies. So in a solid, crowding turns each isolated atomic level into a band of allowed energies.