<P> The energy required to transfer an electron from a sodium atom to a chlorine atom (the difference of the 1st ionization energy of sodium and the electron affinity of chlorine) is small: + 495.8 − 328.8 = + 167 kJ mol . This energy is easily offset by the lattice energy of sodium chloride: − 787.3 kJ mol . This completes the explanation of the octet rule in this case . </P> <P> In the late 19th century it was known that coordination compounds (formerly called "molecular compounds") were formed by the combination of atoms or molecules in such a manner that the valencies of the atoms involved apparently became satisfied . In 1893, Alfred Werner showed that the number of atoms or groups associated with a central atom (the "coordination number") is often 4 or 6; other coordination numbers up to a maximum of 8 were known, but less frequent . In 1904 Richard Abegg was one of the first to extend the concept of coordination number to a concept of valence in which he distinguished atoms as electron donors or acceptors, leading to positive and negative valence states that greatly resemble the modern concept of oxidation states . Abegg noted that the difference between the maximum positive and negative valences of an element under his model is frequently eight . In 1916, Gilbert N. Lewis referred to this insight as Abegg's rule and used it to help formulate his cubical atom model and the "rule of eight", which began to distinguish between valence and valence electrons . In 1919 Irving Langmuir refined these concepts further and renamed them the "cubical octet atom" and "octet theory". The "octet theory" evolved into what is now known as the "octet rule". </P> <P> The quantum theory of the atom explains the eight electrons as a closed shell with an s p electron configuration . A closed - shell configuration is one in which low - lying energy levels are full and higher energy levels are empty . For example, the neon atom ground state has a full n = 2 shell (2s 2p) and an empty n = 3 shell . According to the octet rule, the atoms immediately before and after neon in the periodic table (i.e. C, N, O, F, Na, Mg and Al), tend to attain a similar configuration by gaining, losing, or sharing electrons . </P> <P> The argon atom has an analogous 3s 3p configuration . There is also an empty 3d level, but it is at considerably higher energy than 3s and 3p (unlike in the hydrogen atom), so that 3s 3p is still considered a closed shell for chemical purposes . The atoms immediately before and after argon tend to 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). </P>

How does the octet rule explain the formation of a potassium ion