How to define a formula in chemistry

How to define a formula in chemistry

The chemical formula is the symbol written by means of certain symbols and characterizing composition of any substance. By means of a chemical formula you can see what atoms of elements and in what quantities are a part of any given molecule. It is very important to make and write down chemical formulas correctly. Without it out of the question about studying chemistry as with their help the nomenclature of substances and also the equations of chemical reactions is formed.


1. Let's say you should define what formula at sulfur oxide. Follows from the name of substance that each its molecule consists only of two elements: oxygen (O) and sulfur (S). The structure of a molecule depends on the size of valency of each of these elements, that is on what quantity of chemical bonds atom of an element is capable to form with other atoms.

2. Oxygen in a usual state – gas, sulfur – solid substance. Both of these elements have pronounced nonmetallic properties. Therefore, both of them submit to the rule: each nonmetal has the highest valency corresponding to number of group of the table of Mendeleyev in which it is located, and the lowest, corresponding to the rest from subtraction of number of this group of eight. That is as both oxygen, and sulfur are located in the 6th group of the table of Mendeleyev, their highest valency is equal 6, and the lowest - 2.

3. Now it is necessary to define what of these valencies has oxygen and what – sulfur. It is impossible that both of these elements in connection had at the same time either the highest, or lowest valency. Now one more rule comes into effect: "At compound of two nonmetals that from them which is closer to the top right corner of the table of Mendeleyev has the lowest indicator of valency". Once again look in the table. You see that oxygen is located above sulfur, therefore, it is closer to the top right corner. Thus, in connection with sulfur it will have the lowest valency equal 2. And sulfur, respectively, has the highest valency equal 6.

4. There is the last step. What indexes will each of these elements have? It is known that the works of sizes of valencies of elements increased by their indexes have to coincide in number. Sulfur valency three times more than the valency of oxygen, therefore, the index of oxygen has to be three times more than the index of sulfur. From here follows: SO3 connection formula.

Author: «MirrorInfo» Dream Team