The energy levels of electrons in an atom are determined by a combination of factors, including the attractive force between the positively charged nucleus and the negatively charged electrons, as well as the principles of quantum mechanics. The electron shell nearest to the nucleus, often referred to as the "1s" shell, has the lowest energy level among the electron shells.
According to the principles of quantum mechanics, electrons in an atom are described by wave functions, which determine their energy and spatial distribution. The wave function of an electron in an atom can be thought of as a standing wave, and different wave functions correspond to different energy levels.
The energy of an electron is determined by its distance from the nucleus and the effective nuclear charge it experiences. The electrons in the innermost shell are closer to the nucleus and experience a stronger attraction to the positively charged protons in the nucleus. This stronger attraction results in a lower energy level for these electrons.
Additionally, the electron configuration of atoms follows a specific pattern based on the Aufbau principle, which states that electrons fill the lowest energy levels first before occupying higher energy levels. This principle explains why the first electron shell, with its lower energy level, is filled before the subsequent shells.
In summary, the electron shell nearest to the nucleus has the lowest energy because the electrons in this shell are closer to the positively charged nucleus and experience a stronger attraction, as well as following the principles of quantum mechanics and the Aufbau principle.