Rule of the octet:
according to Lewis, the elements tend to acquire electrons until possessing eight in the level most exterior of the valence layer. This number coincides with external electrons of the noble gases (except helium).
When two eletronegativos atoms are equal or when if they find joined forming a composition or an element, the model of ionic linking are not applicable. This because the two atoms tend to receive electrons and is practically incapable to yield them.
The model covalente:
The atoms acquire the oitos electrons in its external level for compartilhemento, as it indicates the rule of the octet. The addition of proper electrons more the shared ones, that they form orbital a molecular one, is eight. Then, the addition of electrons in the last level of the valence layer will have that to be eight. The chlorine, for example, possesss seven electrons in the external level and strong attracts situated electrons in the neighborhoods. The hydrogen possesss an only electron and the force with that it attracts electrons is moderate, but the sufficient so that the chlorine cannot take off its electron to it.
Classrooms of linking covalente:
the covalentes linkings can be classified according to number of pairs of electrons shared for the elements.
Linking simple covalente:
the linking covalente is formed by the sharing of a pair of electrons. If the layer of valence of an element is different of another element, its atoms can be joined simultaneously with some neighboring atoms by means of a simple linking. For example: CCL4, PCL3, NF3, where the carbon, the match and the nitrogen act with valence four, three and three, respectively, and if join the monobrave atom number the same.
Double linking covalente:
the number of pairs of electrons shared for two joined atoms is two. The energy necessary to breach a double linking is greater that stops a simple linking, but inferior to the double of this. This means that the second linking is weakker than the first one.
Triple linking covalente:
it is become fullfilled for sharing of three pairs of electrons between two atoms. It is a linking stronger than the pair, but its energy is not the triple of the energy of the simple linking. The third linking is weakker than the simple and double linking.