In atomic physics, the terms "shell," "subshell," and "orbital" are used to describe the arrangement of electrons in an atom. These concepts can be explained using both Bohr's model of the atom and the more accurate quantum mechanical model.
Shell: In Bohr's model, electrons are arranged in specific energy levels or shells around the nucleus. Each shell is represented by a whole number (n) and corresponds to a particular energy level. The shells are labeled as K, L, M, N, and so on, with K being the innermost shell and the one closest to the nucleus. The higher the value of n, the farther the shell is from the nucleus and the higher its energy level. Each shell has a maximum number of electrons it can hold, given by 2n^2.
Subshell: Within each shell, there are subshells, also known as energy sublevels or suborbitals. Subshells are represented by a letter (s, p, d, f, and so on) and are associated with different shapes and orientations of the electron clouds. The number of subshells within a shell is equal to the value of n. For example, the second shell (n=2) has two subshells: the s subshell and the p subshell. The third shell (n=3) has three subshells: s, p, and d.
Orbital: An orbital refers to a specific region of space within a subshell where an electron is most likely to be found. Each orbital can accommodate a maximum of two electrons. The s subshell has one spherical-shaped orbital, the p subshell has three dumbbell-shaped orbitals, and the d subshell has five complex-shaped orbitals. The number of orbitals within a subshell depends on its type: s subshell has 1 orbital, p subshell has 3 orbitals, d subshell has 5 orbitals, and f subshell has 7 orbitals.
To summarize, in Bohr's model of the atom, electrons occupy specific shells with discrete energy levels. Each shell consists of subshells, which are further divided into orbitals. The shells are designated by whole numbers (n), subshells by letters (s, p, d, f), and orbitals by numbers and letters (e.g., 2s, 2p). The arrangement of electrons within these shells, subshells, and orbitals determines the electronic structure of an atom. However, it's important to note that the quantum mechanical model provides a more accurate description of electron behavior, incorporating wave-particle duality and other quantum principles.