Follow the idea
At the scale of an atom, almost all the mass is concentrated in a tiny nucleus. Electrons occupy quantum states around it rather than little planetary tracks. The atomic number counts protons and identifies the element. Think of the periodic table as a map whose address is the proton count. The nucleus contains protons and neutrons bound by the strong nuclear interaction. Nuclear size, binding and stability depend on the balance of several physical effects. Nuclear structure explains isotopes, radioactivity, nuclear energy and much of the origin of the chemical elements. Electron configuration describes how electrons occupy available atomic states. This arrangement is central to periodic trends and helps explain why elements in the same group often react in related ways. The outer electrons are especially useful when you want to anticipate chemical behaviour.
Primary references
IUPAC Gold Book · NIST · BIPM