 # How to define a chemical formula

The chemical formula is the record made with use of the standard symbols which characterizes structure of a molecule of any substance. For example, a formula all of the known sulfuric acid – H2SO4. It is easily possible to see that each molecule of sulfuric acid contains two atoms of hydrogen, four atoms of oxygen and one atom of sulfur. It is necessary to understand that it is only an empirical formula, it characterizes structure of a molecule, but not its "degree of structure", that is arrangement of atoms relatively each other.

## It is required to you

• - Mendeleyev's table.

## Instruction

1. At first learn the elements which are a part of substance, and their valency. For example: what will be a nitrogen oxide formula? It is obvious that two elements are a part of a molecule of this substance: nitrogen and oxygen. Both of them are gases, that is pronounced nonmetals. So what valency nitrogen and oxygen in this connection have?

2. Remember very important rule: nonmetals have the highest and lowest valencies. The highest corresponds to number of group (in this case, 6 for oxygen and 5 for nitrogen), and the lowest – a difference between 8 and number of group (that is the lowest valency for nitrogen is equal 3, and for oxygen - 2). The only exception of their this rule – fluorine which in all the connections shows one valency equal 1.

3. So what valency – the highest or the lowest nitrogen and oxygen have? One more rule: in connections of two elements, the lowest valency is shown by those which he is in Mendeleyev's Table more to the right and above. It is obvious that in your case it is oxygen. Therefore, in connection with nitrogen oxygen has the valency equal 2. Respectively, nitrogen in this connection has the highest valency equal 5.

4. And now remember determination of valency: it is ability of atom of any element to attach to themselves a certain amount of atoms of other element. Each atom of nitrogen in this connection "attracts" 5 atoms of oxygen, and each atom of oxygen – 2 atoms of nitrogen. What formula of nitrogen oxide? That is, what indexes each element has?

5. One more rule will help to answer this question: the sum of valencies of the elements entering connection has to be equal! What least common multiple for numbers the 2nd 5? Naturally, 10! Having divided it into sizes of valencies of nitrogen and oxygen, you will find indexes and a total formula of connection: N2O5.

Author: «MirrorInfo» Dream Team