Valency – the most important concept of chemistry. The physical meaning of this concept became clear thanks to development of the doctrine about a chemical bond. Valency of atom is defined by number of covalent communications by which it is connected to other atoms.
Instruction
1. Valent electrons which are least strongly connected with a kernel play a major role in formation of chemical bonds. So call not coupled electrons located on an outer sheath of atom. For this reason it is important to imagine an electronic configuration of the considered element.
2. Electronic configurations of noble gases have the greatest stability. For this reason noble gases under usual conditions are chemically inert, do not react with other elements. Atoms of other elements seek to get the same steady cover at formation of communications.
3. So, valency is an ability of atom to form a certain number of covalent communications with other atoms. It is expressed by a small integer. Number of chemical bonds – a valency measure.
4. To determine valency, it is necessary to understand that the outer electron shell of atom how many not coupled electrons it has is. In the basic and the excited condition of atom the valency can be various.
5. In most cases the highest valency of an element is equal to number of group in Mendeleyev's table in which this element is. But this rule has exceptions. For example, elements of the second period – nitrogen, oxygen and fluorine do not submit to it.
6. So, the highest valency of phosphorus +5. Nitrogen is in the same group, but it cannot show valency, big 4. The outer electron shell of nitrogen contains three not coupled electrons therefore in connections with hydrogen nitrogen trekhvalenten: NH3 ammonia is so formed. At the same time between nitrogen and hydrogen the fourth covalent communication, but already on the donorno-acceptor mechanism, but not on exchange can be formed. The NH4+ ammonium ion is so formed.
7. Atoms of beryllium, a pine forest and carbon have variable valency. It is connected with the fact that electrons can steam out within one power level. The energy spent for a rasparivaniye of electrons with interest is compensated by energy of formation of additional communications.
8. Carbon C if to look at its electronic configuration, dvukhvalenten. But the true valency of carbon is +4. One electron from a 2s-orbital jumps on a free 2p-cell, and now carbon is capable to form not two, but four communications. Tetravalent carbon – fundamentals of organic chemistry.