Electronic structure is the arrangement of electrons in an atom. The electronic structure of an atom refers not only to the number of electrons that an atom consists but also to their allotment around the nucleus and to their energies. Understanding of the electron configuration of various atoms is useful in knowledge the structure of the periodic table of elements. The concept is also useful for describing the chemical bonds that hold atoms together. Electronic Structure Theory describes the motions of electrons in atoms or molecules. An electron configuration table is a kind of code that explains how many electrons are in each energy level of an atom and how the electrons are arranged inside each energy level. It includes number of information into a tiny space and it takes a little practice to read and understand. Basically, electron configuration describes the manner in which electrons can be distributed in the orbital’s of the atomic or molecular system. Energy is linked with every electron configuration and also in some cases; electrons are able to move from one orbital to another by emission or absorption of a photon.
The first one to predict that the periodicity in the properties of the elements could be demonstrated by the electronic structure of the atom was, a Danish physicist, Niels Bohr, in the year 1923. His suggestion was based on the Bohr’s model, which says that the electron shells were orbits at a permanent distance from the nucleus. The Bohr model, predicted by Niels Bohr, portrays the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus—similar in structure to the solar system, but with electrostatic forces providing attraction, rather than gravity. Later, a British theoretical physicist, E. C. Stoner envisages the shell structure of sulfur. Though, neither Bohr's system nor Stoner's could properly explain the variance in atomic spectra in a magnetic field, called the “Zeeman effect”. In 1926, The Schrodinger equation, formulated in 1927 by Austrian physicist Erwin Schrodinger, explained the atomic orbital’s currently used chemistry books.
In Electronic configuration, each 'sentence' is made up of smaller 'words'. Each row of an electron configuration table will be similar to a sentence. Each 'word' follows the following format: [A Number] [a lowercase letter] [A number in superscript]. The first number is the “energy level”. The lowercase letter is the “sub-shell”. An orbital is a space that can be engaged by up to two electrons. Each type of sublevel holds a different number or orbital’s, and hence, a different number of electrons. The sub-shells are named s, p, d and f. The number of available sub-shells increases as the energy level increases. For instance, the first energy level only contains a ‘s’ sub-shell while the second energy level contains both a ‘s’ sub-shell and a ‘p’ sub-shell. The number in superscript is the “number of electrons in a sub-shell”. Each sub-shell can hold only a certain number of electrons. The ‘s’ sub-shell can hold only 2 electrons, the ‘p’ sub-shell can hold 6, the d sub-shell can hold 10 and the f sub-shell can hold as many as 14. A typical notation to specify the electron configurations of atoms and molecules consists of a sequence of atomic orbital labels (e.g. for phosphorus the sequence 1s, 2s, 2p, 3s, 3p) with the number of electrons allocated to each orbital (or set of orbitals sharing the same label) placed as a superscript. For instance, hydrogen has one electron in the s-orbital of the first shell, so its configuration is written 1s1. Lithium has two electrons in the 1s-subshell and one in the (higher-energy) 2s-subshell, so its configuration is written 1s2 2s1 (pronounced as "one-s-two, two-s-one"). For, Phosphorus with atomic number 15, the electronic configuration is written as: 1s2 2s2 2p6 3s2 3p3. The energy related to an electron is that of its orbital. The energy of a configuration is mostly approximated as the sum of the energy of each electron. The configurations that match up to the lowest electronic energy is called the “ground state”. Any other configuration is an “excited state”.
It should be noted that energy level alters as we go up. For instance, when we go up to the 4th energy level, it becomes 4s first, then 3d. Subsequent to the fourth energy level, one has to move on to the 5th where it follows the order again. This pattern occurs only after the 3rd energy level.