How to determine valency?

Valence is the property of atoms of elements to attach a certain number of atoms of other elements.
There are elements whose valency does not change in compounds. Such elements are said to have constant valence. If the valency of an element can be different, these are elements with variable valency.
The highest valence of an element numerically coincides with the serial number of the group of the Periodic System in which it is located. For example, carbon is in group IV, its highest valency is IV.
There are three exceptions:
• nitrogen – is in group V, but its highest valency is IV;
• oxygen – is in group VI, but its highest valence is II;
• fluorine – is in the VII group, but its highest valency is I.
Based on the fact that all the elements are located in eight groups of the Periodic System, valency can take values ​​from I to VIII.
Formulating substances using valence
To compose formulas of substances using valence, you can use a certain algorithm:
1. Write down the chemical formulas of the elements
2.Above, above the symbols of the elements, write down the value of their valence.
3. Find the smallest common multiple of valence values, write it down at the top.
4. Divide the smallest common multiple by the values ​​of the valencies of the elements – these are indices expressing the number of atoms.