The electron configuration of an atom, including the arrangement of electrons in different shells or energy levels, is determined by the principles of quantum mechanics. According to the Aufbau principle, which governs the filling of electron shells, electrons occupy the lowest available energy levels before moving to higher energy levels.
The electron shells in an atom are labeled using the numbers 1, 2, 3, and so on, with the first shell being the closest to the nucleus. Each shell has a maximum number of electrons it can hold, based on the formula 2n^2, where n represents the shell number. For example, the first shell (n=1) can hold a maximum of 2 electrons (2 x 1^2 = 2), the second shell (n=2) can hold a maximum of 8 electrons (2 x 2^2 = 8), and so on.
The reason for this specific electron configuration can be explained by the quantum mechanical properties of electrons. Electrons exhibit wave-like behavior and are described by quantum numbers, such as the principal quantum number (n), the azimuthal quantum number (l), the magnetic quantum number (m_l), and the spin quantum number (m_s).
Without delving into too much technical detail, the electron configuration is determined by the interactions between these quantum numbers and the underlying energy levels available to electrons in an atom. The arrangement of electrons follows a pattern to achieve the lowest possible energy state, known as the ground state.
In the case of atoms with electron configurations like 2, 8, 8, and so on, such as neon (10 electrons) or argon (18 electrons), the pattern arises because it allows the atom to achieve a stable, filled electron shell configuration. Filled shells are energetically favorable because they provide increased stability to the atom. In other words, atoms tend to gain, lose, or share electrons to achieve a stable electron configuration resembling the noble gases, which have fully filled electron shells.
It's important to note that the pattern of electron configurations can become more complex for elements beyond the second period (row) of the periodic table. This is due to the interplay of different factors, including electron-electron repulsion, shielding effects, and the varying energy levels of different subshells. However, the basic principles outlined above help explain the general pattern of electron configurations in atoms.