Explain that electrons occupy the lowest available energy levels or innermost shells first.

Electrons in an atom are arranged in energy levels or shells around the nucleus. According to the principle of energy minimization, electrons will fill the lowest available energy levels first before occupying higher ones. This arrangement is crucial for understanding the chemical properties of elements, as the distribution of electrons influences how atoms interact and bond with each other. For example, in sodium (Na), the electronic structure is represented as 2, 8, 1, indicating that the first shell has 2 electrons, the second has 8, and the outermost shell has 1 electron.