6. VALENCY. ХИМИЧЕСКАИЕ FORMULAS. DRAWING UP FORMULAS ON VALENCIES.

Valency is properties of atoms of one chemical element to attach certain number of atoms of another.
This concept has been entered into chemistry in 1853 by the English chemist-organikom Franklend for justification of the quantitative relations of atoms of elements in chemical compounds. Development of the doctrine about valency is to a large extent connected with opening of the Periodic law by D. I. Mendeleyev. It has established connection between the valency of element and its situation in periodic system, the concept about variable valency is entered. The doctrine about structure of atoms and molecules promoted development of the electronic theory of valency.
For calculations, for drawing up formulas of substances неоходимо to know quantitative ratios of atoms of different elements in which they unite. Valency shows to how many atoms of monovalent element atom of this element connects.
Monovalent consider all elements which atoms in two-element connections are always connected with one atom of other element. Example of monovalent element is hydrogen. Therefore it is considered that the valency of element indicates to what hydrogen atoms one atom of the considered element is connected. Например:в HCl chlorine-odnovalenten, in _H2O oxygen-dvukhvalenten, in _NH3 nitrogen-tryokhvalenten.
Hydrogen bonding is known not for all elements, but almost all elements form connections with oxygen O. Oxygen is considered always bivalent.

Table of constant valencies.

Valencies	Elements
I	H, Na, Li, K, Rb, Cs
II	O, Be, Mg, Ca, Sr, Ba, Ra, Zn, Cd
III	B, Al, Ga, In

Using valencies it is possible to make connection formula.
The chemical formula is conditional record of composition of substance by means of chemical signs and indexes.
For example: Water _H2O-formula, where N and O-chemical signs of elements, 2 index which shows number of the atoms of this element which are part of water molecule.
At the name of substances with variable valency its valency which is put in brackets surely is specified. For example, _R2O5-oxide of phosphorus (V).

Drawing up formulas on valencies.

Let's learn to make formulas for binary compounds (consist of two chemical elements). There is rule that the number of valencies at one chemical element is equal to number of valencies at another. Let's consider on the example of formation of the molecule consisting of phosphorus and oxygen.
Let's make according to algorithm:
1. We write down symbols of chemical elements nearby:

2. We put over chemical elements of their valency in the Roman figures, at phosphorus it is equal 5, at oxygen-2.

3. We find least common multiple, that is the smallest number which shares without the rest on 5 and on 2. This number 10. We divide it into valencies of elements 10:5=2, 10:2=5, 2 and 5 will be indexes, respectively at phosphorus and oxygen. We substitute indexes.

It is possible on formula, knowing the valency of one chemical element, to determine the valency of another.

The concept about valency has been entered into chemistry before the atomic structure became known. Now it is established that this property of element is connected with number of external electrons. For many elements maximum valency follows from the provision of these elements in periodic system.
This question will be considered in the following paragraph.

  

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