Noble gas rule
The noble gas rule states that the atoms of other elements strive for the same number of electrons as with a noble gas ( noble gas configuration ).
General laws
Almost all noble gases have eight electrons on the valence shell . The only exception is helium, which has two electrons. These electron configurations are so stable that most noble gases do not enter into a chemical reaction . This means that under standard conditions , noble gases only occur atomically and not molecularly . Lewis and Kossel (1916) developed the noble gas rule with this in mind:
- "Atoms of elements other than the noble gases can reach the noble gas configuration and thus fulfill the noble gas rule by entering into chemical reactions and thereby completely accepting or releasing electrons ( ionic bond ) or using them together with other atoms ( molecular bond or covalent bond )."
Laws of the Periodic Table of the Elements
In the first period of the periodic table , the noble gas helium has the electron configuration 1s 2 . Hydrogen can formally achieve the noble gas configuration of helium by absorbing an electron, i.e. ionization to form a negatively charged hydride ion (cf. metal hydrides ) or by forming an electron pair bond. Lithium , beryllium and boron can also get the electron configuration of helium through the release of electrons ( oxidation to the correspondingly charged cations ).
The other elements of the second period usually reach the noble gas configuration by accepting electrons ( reduction ). This gives you the electron configuration of neon (1s 2 2s 2 2p 6 ).
The noble gas configuration is also present for ions and atoms in compounds in all subsequent periods if eight electrons (s 2 p 6 ) are formally present or assigned in the outermost shell , which results in the electron configuration of a noble gas.
Comparison with the octet rule
In many compounds, especially in organic chemistry , the noble gas rule is the same as the octet rule . This states that atoms strive for a total of eight outer electrons (including binding electrons and non-binding electrons). Most of these connections are relatively stable. As shown above, however, it does not apply to the first period (two electrons), from the third period no longer exclusively and from the fourth period only rarely. From the third period onwards, the use of d orbitals can result in stable electron configurations with more than eight external, bond or free electrons. It is therefore advantageous to apply the noble gas rule not according to periods but according to groups. The octet rule applies to the main groups, as long as it is not an electron deficiency connection or a case of the relativistic effect . The eighteen-electron rule applies to the subgroups , the other possible interpretation of the noble gas rule. This can be seen in complex compounds such as ferrocene or nickel tetracarbonyl , in which the central metal atom reaches the electronic configuration of krypton .
Scope
The question of when to strive for two, eight or eighteen outer electrons can be recognized by the affiliation to the periods or groups. The two-electron rule only applies to the first period and the first metals of the second period. Most other main group elements aim to achieve the octet. In the subgroups, the eighteen-electron rule usually applies, although the noble gas rule is valid for more compounds than the octet rule.
Non-metals are compounds that (formally) exceed the octet. These include compounds of fluorides with elements of the 5th , 6th and 7th main group . Exceptions also occur with main group metals . An example of this is lead (II) oxide. Electron deficiency bonds are also possible. Typical examples are the boron hydrocarbons (see diborane , boranes ). Above all, however, these occur with electropositive (electron-poor) transition metals as well as the lanthanoids and actinides . The exceeding of the octet and the falling below the octet can in many cases be explained by the formulation of multi-center bonds . The 18-electron rule often applies to compounds of complexes of transition metals . There are exceptions to all of the rules mentioned here, especially with the subgroup metals .