The Octet Rule
Inhaltsübersicht
The octet rule is also called the eight-electron rule or noble gas rule. It describes the tendency of all atoms to achieve the noble gas configuration; in other words, to possess eight valence electrons. The valence electrons are the electrons in the outermost shell of an atom. The reason for this tendency is that atoms in this configuration are particularly stable. They do not react with other atoms and do not form bonds. Since the noble gases, with the exception of helium, have eight valence electrons, these are already in their most stable form; the other atoms also strive for this stable noble gas configuration, hence the name noble gas rule.
Note
All atoms of the main groups from the second period onwards strive to have eight valence electrons and thus achieve the noble gas configuration.
Keep in mind that there are exceptions here, too. Hydrogen, for example, also strives for a noble gas configuration, but helium's configuration wants only two valence electrons, not eight. The reason for this is simple: since both atoms are in the first period, they only have one shell, the K shell. This shell only has room for two electrons. Helium, with its two valence electrons in the K shell, is therefore already in its most stable state. There are also some exceptions for atoms in higher periods.
The main group number in the periodic table tells you how many valence electrons an element has. The noble gases are located in the eighth main group and have eight outermost electrons. Hydrogen and the alkali metals are located in the first main group and therefore have only one electron in their outermost shell. For the remaining main groups 2 to 7, the main group number also correlates with the number of valence electrons. You can therefore easily read it off the periodic table.
Strategies
There are various strategies for achieving the noble gas configuration. Atoms can either donate all of their electrons or gain them. This way, they become charged ions. Elements that readily donate their valence electrons are found in the first to third main groups. By donating one or more electrons, they become positively charged cations. For example, sodium donates one outer electron and becomes Na + , calcium donates two outer electrons and becomes Ca2 + , and aluminum donates three electrons from its outermost shell and becomes Al3 + .
In this way, they completely empty their outermost shell, and the shell below receives the desired eight electrons. On the other hand, there are the main group elements in groups five to seven. These prefer to accept electrons, thus filling their outermost shell to eight valence electrons. Chloride, for example, accepts one electron. Since it is in the seventh main group, it only needs one more electron to reach eight valence electrons. It therefore becomes a singly negatively charged anion, Cl - .
Since atoms cannot simply donate or gain electrons at will, they form bonds. A well-known example is sodium chloride, our table salt. This consists of the elements sodium and chloride. Sodium, from the first main group, would like to donate an electron, while chloride, from the seventh main group, strives to gain an electron into its outermost shell. When these two elements form a bond, the chloride takes on the valence electron from sodium, and both elements achieve their desired electron configuration.
Another strategy for fulfilling the octet rule is electron sharing. No valence electrons are actually lost, so no ions are formed, but the elements "share" the valence electrons. This type of bond is called an atomic bond. A well-known example is carbon dioxide. Carbon is four electrons short of the noble gas configuration. It then shares its two valence electrons with two oxygen atoms, which would gladly donate them. This also creates a stable bond.
Remember
- According to the octet rule, the main group elements from the second period onwards strive to have eight valence electrons.
- You can determine how many electrons an element has using its main group number.
- To fulfill the octet rule, elements can form ions, ionic bonds or atomic bonds.
- Main group elements of groups 1-3 readily donate electrons. They become positively charged ions (=cations).
- Elements in main groups 5-7 readily accept electrons. They become negatively charged ions (anions).
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