In the absence of spin, these levels are split by valley-orbit terms and, in increasing
energy, are labelled according to 1s:A (one level), 1s:T2 (three levels) and 1s:E (two levels).
This notation refers to the electron configuration of an atom. The number before the letter (1s) represents the energy level, and the letter (A or E) represents the sublevel. In this case, 1s:A means that there is one electron in the 1s sublevel with an A spin orientation, while 1s:E means there is one electron in the 1s sublevel with an E spin orientation.
This notation is determined by the Aufbau principle, which states that electrons fill the lowest energy levels and sublevels first before moving to higher ones. The order of filling follows the diagonal rule, where electrons fill sublevels in the order of increasing energy.
This notation is important because it provides information about the arrangement of electrons in an atom, which determines its chemical and physical properties. It also follows the periodic trends of the elements, allowing for predictions about an element's reactivity and behavior.
Other notations used in chemistry include the Lewis dot structure, which shows the valence electrons of an atom, and the structural formula, which shows the arrangement of atoms in a molecule. Molecular orbital notation is also used to show the distribution of electrons in a molecule.
This notation can be used in practical applications such as predicting the reactivity of elements, understanding molecular bonding and structure, and determining the electronic properties of materials. It is also used in spectroscopy to interpret and analyze the energy levels and transitions of atoms and molecules.