Electrons Configuration
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The Electron Configuration Table presents a detailed information about the residencies of the electrons of the elements of the Periodic Table. Quantum wave mechanics is the theory that established the Electron Configuration Table. Basically, the theory explains the behaviour of subatomic particles like the electrons of atoms and in particular, their residences and motions. Niels Bohr, De Broglie, Werner Heisenberg, Erwin Schrodinger and Paul Dirac were primary developers of the theory.

Quantum wave mechanics posits that there are three regions around the nucleus of an atom that electrons are most likely to reside: shell, subshell and orbitals. A shell is also called the principal quantum number or energy-level of an atom. It is usually denoted by the letter n. For example, n = 1 implies the first shell (shell or energy-level nearest the nucleus of the atom); n = 2 implies the second shell and so on. Energy increases with increasing n. There are n subshells in the nth shell. The first subshell is called the s subshell, the second is called the p subshell, the third is called the d subshell, the fourth is called the f subshell, the fifth the g subshell and so on. There are n2 orbitals in the nth shell. For example, when n = 2 (second shell), the number of subshells in the second shell is 2 and the number of orbitals is 4. The orbitals are the regions where the electrons of an atom are most likely to be found. In general, a shell with principal quantum number n has n subshells. The nth subshell has 2n-1 orbitals which can hold a total of 2(2n-1) electrons. The total number of electrons in the shell is the sum of all the electrons in all the orbitals of the shell's subshells. The total electrons in an orbital type can be expressed as:

nxy where n is the number of the shell (energy-level or principal quantum number) containing the orbital type x, and y is the number of electrons contained in the orbital type x (s, p,d, f, are orbital types).

Periodic Table Subshells See The Periodic Table of Elements to relate element name and atomic number to the electrons configuration
The distribution of an atom's electrons in their respective orbitals is called the electron configuration of the atom. The electron configuration of an atom is usually expressed in an Electron Configuration Table . The following is the Electron Configuration Table for gold:

1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s1

The rule for electron containership in an orbital is given by Pauli exclusion principle: an orbital can hold no more than two electrons and the electrons must be of opposite spin (orientation in magnetic field).

The description of the motions of electrons within their orbitals is given by Heisenberg uncertainty principle: it is impossible to know exactly both the position and momentum of an electron at the same instant.

The interactions of atoms occur when atoms share, or donate, or accept valence electrons. In general, the period of an atom equals its number of shells n and its group (Roman numeral) equals the number of electrons in its valence-shell.

The general form of the valence-shell configuration is:

nsanpb . Where n=is the period of the atom and a + b = number of electrons in its valence-shell. For example, I (Iodine) is at period 5 and group VIIA of the periodic table. So, n = 5 and a + b = 7. Therefore, its valence-shell configuration is: 5s25p5.