What are chemical valences

Valence

The valence of an atom indicates how many bonds it can form.
Hydrogen only ever connects to one other atom, the value of hydrogen is therefore 1.
The value is shown as a small line next to the atom symbol:

H-

The valence of other atoms can easily be determined if these atoms can form compounds with hydrogen:
- hydrogen chloride HCl
Hydrogen has the valence 1 (H-) and binds to a single chlorine atom, so the chlorine atom must also be monovalent (Cl-).

- water H2O
The oxygen atom is bound to two hydrogen atoms, since the hydrogen atom has the valence 1, the oxygen atom must have the valence 2.

- methane CH4

The carbon atom is bound to four hydrogen atoms, since the hydrogen atom has the valence 1, the carbon atom must have the valence 4.

Drawing determination of the chemical formulas with the help of the valence

Examples:
- Water, a combination of hydrogen and oxygen
Hydrogen H is in the 1st main group, its valence is 1.
Oxygen O is in the 6th main group, its valence is 2.

Cover letter of the element symbols with the weights:

Establish ties between the valencies:

A valence of the O atom does not yet have a partner, adding a 2nd H atom connects all valences:

In water there are 2 hydrogen atoms for 1 oxygen atom, so the number ratio is as follows:

N(H): N(O) = 2:1

So the chemical formula of water is:

H2O

- Phosphorus oxide, a combination of phosphorus and oxygen
Phosphorus P is in the 3rd main group, its valence is 3.
Oxygen O is in the 6th main group, its valence is 2.

Cover letter of the element symbols with the weights:

Establish ties between the valencies:

A valence of the P atom does not yet have a partner, a second O atom is added and the valences are connected:

A valence of the O atom has no partner yet, a second P atom is added and the valences are connected:

Two valences of the P atom still have no partner, you add a third O atom and you can then connect all valences:

In phosphorus oxide there are 2 phosphorus atoms for 3 oxygen atoms, so the number ratio is as follows:

N(P): N(O) = 2:3

So the chemical formula of phosphorus oxide is:

P2O3