the octet rule states that in chemical compounds atoms tend to have the electron configuration of a
noble gas
title: Octet rule text: Octet rule The octet rule is a chemical rule of thumb that reflects observation that atoms of main-group elements tend to combine in such a way that each atom has eight electrons in its valence shell, giving it the same electron configuration as a noble gas. The rule is especially applicable to carbon, nitrogen, oxygen, and the halogens, but also to metals such as sodium or magnesium. The valence electrons can be counted using a Lewis electron dot diagram as shown at the right for carbon dioxide. The electrons shared by the two atoms in a covalent bond are counted title: Octet rule text: attain this configuration in compounds. There are, however, some hypervalent molecules in which the 3d level may play a part in the bonding, although this is controversial (see below). For helium there is no 1p level according to the quantum theory, so that 1s is a closed shell with no p electrons. The atoms before and after helium (H and Li) follow a duet rule and tend to have the same 1s configuration as helium. Some stable molecular radicals (e.g. nitric oxide, NO) obtain octet configurations by means of a three-electron bond which contributes one shared and one unshared electron title: Octet rule text: to the octet of each bonded atom. In NO, the octet on each atom is completed by four electrons from two two-electron bonds, plus a lone pair of non-bonding electrons on that atom alone. The bond order is 2.5, since each two-electron bond counts as one bond while the three-electron bond has only one shared electron and therefore corresponds to a half-bond. Dioxygen is sometimes represented as obeying the octet rule with a double bond (O=O) containing two pairs of shared electrons. However the ground state of this molecule is paramagnetic, indicating the presence of unpaired electrons. Pauling proposed that title: Octet rule text: this molecule actually contains two three-electron bonds and one normal covalent (two-electron) bond. The octet on each atom then consists of two electrons from each three-electron bond, plus the two electrons of the covalent bond, plus one lone pair of non-bonding electrons. Main-group elements in the third and later rows of the periodic table can form hypercoordinate or hypervalent molecules in which the central main-group atom is bonded to more than four other atoms, such as phosphorus pentachloride, PCl, and sulfur hexafluoride, SF. For example, in PCl, if it is supposed that there are five true covalent bonds in which title: Electron configuration text: Electron configuration In atomic physics and quantum chemistry, the electron configuration is the distribution of electrons of an atom or molecule (or other physical structure) in atomic or molecular orbitals. For example, the electron configuration of the neon atom is 1s 2s 2p, using the notation explained below. Electronic configurations describe each electron as moving independently in an orbital, in an average field created by all other orbitals. Mathematically, configurations are described by Slater determinants or configuration state functions. According to the laws of quantum mechanics, for systems with only one electron, a level of energy is associated with each