Noble gas configuration

The noble gas configuration (rarely also noble gas state ) denotes an electron configuration of an atom or an ion that corresponds to the electron configuration of the noble gas of the respective period or the previous period. Noble gas configurations are energetically particularly stable, so that many chemical reactions proceed in such a way that noble gas configurations are formed or maintained. That is the statement of the noble gas rule . According to this, hydrogen atoms strive for the configuration of helium with two electrons. Apart from these elements of the first period, a configuration with 8 valence electrons is energetically favorable for most main group elements . That is what the octet rule says . For subgroup elements which instead applies 18-electron rule .

Atoms or ions with a noble gas configuration are particularly stable and have little tendency to release or accept electrons.

Reaching the noble gas configuration

Noble gases

The noble gases already have their noble gas configuration in their elementary state, they are therefore monatomic in their elementary state and only form noble gas compounds in exceptional cases .

Noble gas state in ions

This noble gas configuration can also be achieved by an atom by accepting or releasing electrons , which makes it a charged ion . This creates a chemical bond with an ionic bond with the partner from whom the absorbed electrons originate or to whom they were given. So many electrons are accepted or released until the noble gas configuration is reached, i.e. H. until all electron shells are completely occupied with electrons.

Example:

Magnesium releases two electrons and becomes a doubly positively charged magnesium ion, which with 10 electrons (8 valence electrons) reaches the electron configuration of neon :

${\displaystyle \mathrm {{Mg}\longrightarrow {Mg}^{2+}+2\ {e}^{-}} }$

For example, if these two electrons are given to an oxygen atom that lacks two electrons for the noble gas configuration in the elementary state, it becomes an oxygen ion and also reaches the electron configuration of neon with 10 electrons (8 valence electrons):

${\displaystyle \mathrm {{O}+2\ {e}^{-}\longrightarrow {O}^{2-}} }$

This is how the chemical compound MgO ( magnesium oxide ) is created with a strong release of energy in the form of heat (i.e. an exothermic reaction ):

${\displaystyle \mathrm {{Mg}^{2+}+{O}^{2-}\longrightarrow {MgO}} }$

It is held together by the two strong positive and negative charges and is extremely stable due to the noble gas configuration achieved by each of the two atoms.

Because elemental oxygen occurs as a diatomic molecule, the overall reaction equation is correctly formulated with two atoms each of Mg and O (see stoichiometry ):

${\displaystyle \mathrm {2\ Mg+O_{2}} \longrightarrow \mathrm {2\ MgO} }$

Often no charges are represented in such summarizing equations, so that the exact proportions can be derived from them, but no direct indications are evident for the causes of the reaction process, the energetic behavior or the achievement of the noble gas configuration.

Example chlorine and chloride :

	Number of protons	Number of electrons	Valence electrons
Atom Cl	17th	17th	7th
Ion Cl - in the noble gas state	17th	18th	8th
Atom of the noble gas argon in the noble gas state	18th	18th	8th

Noble gas state in molecules

When binding in molecules , the noble gas state of the atoms involved is achieved in that electron pairs of atoms bound to one another belong to both atoms together. These electron pairs belonging to both atoms are binding and are calculated twice when considering the electron configurations of the atoms involved and, in this sense, are used jointly to achieve the noble gas configuration. For example, in the hydrogen molecule H _2, every hydrogen atom has the helium configuration, since in the molecule H — H both the 'left' and the 'right' atom can count two electrons in its electron shell.

Noble gas state in metals

When the metal is bonded, all metal atoms involved give off electrons. The remaining positively charged metal ions are also called “atomic cores”. They are embedded in an electron gas formed from the released electrons , which holds the metal lattice together and prevents the atomic cores from repelling one another. In sodium metal , every sodium atom has given up a valence electron to the electron gas; it thus achieves the electron configuration of neon . Magnesium , which is next to sodium in the periodic table, has to give off two valence electrons to the electron gas of the metal lattice in order to achieve the configuration of neon, aluminum three. With the increasing charge of the atomic cores, the strong increase in the lattice energy and the significant decrease in the metal atomic radii in the series sodium - magnesium - aluminum can be explained.

Individual evidence

1. ^ AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 102nd edition. Walter de Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 , p. 114.